Headphone Amp => Project finished (schematic / PCB / BOM available) - Page 1
Forum section: electronics / projects

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:27:23 am
content: I've got one of those cheap Behringer HA400 4 channel headphone amp, but it has several issues (crackling pots, jacks are a bit loose and a horrible design decision: one coupling capacitor at the ground connection of the headphone instead of one at each channel, that means the headphone is not grounded and TRS jacks with exposed ground (basically all that are made of metal) cause serous noise/humming). While there are a few mods out there to improve sound quality I thought: Why not build a new one that fits into that case.I looked for headphone amp projects in the net, but none of them seems to fit my goals, I don't plan to build an super-ultra-performing audiophoolery amp with all the best possible parts, butSingle 12 Volt supplyFlawless sound (no audible noise/distortion etc.)AffordableAbility to drive headphones with reasonable low impedanceSmall design to fit in the existing case (pots and jacks take up most space at the top side of the PCB, SMD components at the bottom rather uncritical)Input ground = Output groundPot problems (bad taper connection) must not result in loader output signal (ear safety)I started a circuit (just one stereo channel yet), but while I have done op amp projects before, never in this configuration, so I have a few questions:Is the coupling at the input and output done right and sufficient? Do I need coupling between the input and output op amp?Is the volume control working in this configuration? I have some serious doubts, because of the inverting amp and how to calculate gainCoupling of pot ground a good idea?Feel free to make any suggestion to fix/improve the design.Best regardsStefanEdit:The project is finished, all data for building the amp is available inthis post.

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 09:45:26 am
content: Okay, so it looks like you are using inverting stages to avoid signal injection into the virtual ground. I suppose that's some method to that madness.You have noticed that you have nowhere to connect the volume pot to - it should go to Vcc/2, but hello signal injection and channel crosstalkWell, I guess you can AC couple it to ground with separate capacitors like here, but then the second stage amplifies the difference between C7/C8 voltage and Vcc/2. Time constants need to be tweaked (matched equal?) so it raises at about the same rate as Vcc/2 to reach proper operating conditions ASAP. There will be some extra DC on the second stage output in the meantime, but the amp is AC coupled throughout so there are startup transients everywhere anyway.I wonder if it would work to place the volume pot in place of R5/R6?470µF is barely enough for 16Ω with -3dB at 20Hz. The input time constant is similar, I see.Those CMOS opamps are R2R on the outputs but probably not on the inputs. You could maximize dynamic range by giving the second stage a bit of closed loop gain. It's already working at a noise gain of two, so distortion increase will not be severe.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 11:16:39 am
content: Quote from: magic on February 09, 2022, 09:45:26 amI'm open to other suggestions, but for me that seemed to be simple & easyQuote from: magic on February 09, 2022, 09:45:26 amStartup transients don't worry me as long as stable operation is reached within a few seconds. What bugs me is how to calculate the gain of the second stage, do I have to consider the pot somehow? Gain is -R9/R7, but the pot is in series with R7...Quote from: magic on February 09, 2022, 09:45:26 amTwo problems: This is the input stage, I've got 4 output stages with a volume pot each (only one is drawn), so pot must be after the input stage and for ear safety: A bad taper connection must not increase volume, this would be the case.Quote from: magic on February 09, 2022, 09:45:26 amI didn't consider 16Ω "reasonable low impedance", but rather 50Ω. But you're right for the input, I changed the Rs again after calculating the input cap, but using 33k/68k at the op amp should fix that.Quote from: magic on February 09, 2022, 09:45:26 amMy assumption was, if I amplify in the first stage and I reduce volume with a pot afterwards I also reduce the noise again, while reducing the volume and amplifying afterwards also amplifies the noise.

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 11:40:37 am
content: Well, the easy way is with dual supplies. A halfway-there solution which could perhaps work acceptably is to have one virtual ground shared by all inputs and outputs and DC couple everything - in such case, ground ripple doesn't exist as far as the signal is concerned, there is only a simultaneous and equal ripple on both supplies which will be rejected a great deal by PSRR of the opamps. One word: CMOYYou are right about pot disconnection faults. Regarding R7, obviously the pot needs to be accounted for (its output resistance equals parallel resistance of the track segments above/below the wiper, 12.5kΩ worst case) and this means your volume control basically doesn't work as-is.For the record, 16Ω headphones definitely are a thing, although it's mainly earphones and earbuds which are like that.You are correct about amplification of volume pot noise by closed loop gain of the second stage, but something like 1.5x is not much (3dB) and enables more output swing before clipping. Which limitation is more important will depend on the headphones used, you can always make the circuit tweakable by swapping resistors in the final build.Resistors haveJohnson noisewhich will exceed noise of those opamps above some 10kΩ. I'm not entirely sure when noise becomes audible in headphones (and it surely varies from HP to HP).

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 01:44:05 pm
content: Quote from: magic on February 09, 2022, 11:40:37 amYou mean no input/output capacitors, but rather connect the input/output ground to the virtual ground? CMOY projects usually are just one non-inverting op amp, but I need 4 outputs with a volume control each.Quote from: magic on February 09, 2022, 11:40:37 amThat's what I thought, all solutions I can find are in way that bad taper connection increases volume.Quote from: magic on February 09, 2022, 11:40:37 amYes, but I don't care about them.I made another attempt to fix the pot issues by using non-inverting configuration and using AC coupling everywhere (most pots prefer AC anyway) and removed the need for virtual ground in this circuit completely. I also used AC coupling in the feedback loops, did I calculate the capaictors correctly for -3dB at ~1 Hz or is something else to consider? Usually ceramic caps in the signal path are considered bad, is there something wrong with using them here? Or should I go for tantalums or normal electrolytics?Is my choice of op amps good? They are reasonable priced (not cheap, but 2€ for a double op amp is still ok). I found the OPA1688 as a headphone amp is given as an example application in the datasheet and the OPA1652 is used as an input stage of a headphone amp in its datasheet (although in a very different configuration)

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 02:41:26 pm
content: I wonder if it would be possible to reduce component count by creating a single buffered Vcc/2 virtual ground (or one for left, one for right) for all those pots and feedback networks. They don't load the ground too much so an opamp would do - does OPA1654 exist by any chance?The signal would be AC coupled once at the input and once at the output (per headphone). There is still the safety of headphone AC coupling, but without a million capacitors sprinkled all over the board.CMoy could be scaled to multiple stages and outputs, no problem. But it is DC coupled throughout and the reliance on PSRR is perhaps a bit how ya doin' if it can be avoided.Time constants seem right near 1Hz, but this feedback divider with 23.5kΩ output impedance is noisier than before. Maybe it would be possible to reduce it using the buffered virtual ground approach - no need for large caps in such case. OTOH, biasing resistors in front of noninverting inputs can be high in value because their noise is shunted to the signal source by the corresponding coupling capacitor. If you give up the 1.5x second stage gain, you won't need a feedback capacitor there.The problem with X7R and similar ceramics is that their capacitance varies with applied voltage which could increase THD if there is meaningful AC voltage across the cap and they are somewhat microphonic too (capacitance varies with mechanical vibration).The opamps seem good enough. They are rated for audio use and I AFAIK there are commercial products where they are employed to a good effect. They are CMOS so no big problems with input current and current noise. I'm sure they have a rise in low frequency noise, but Fletcher–Munson works in their favor.

author: Bassman59
author_rank: Super Contributor
timestamp: February 09, 2022, 03:29:11 pm
content: For your power supply, why not use say a 16 VAC wall wart and the standard rectifier followed by linear regulators to generate bipolar supply rails?

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 04:08:12 pm
content: Quote from: magic on February 09, 2022, 02:41:26 pmYes it does exist, but total component count is not the most important aspect, but place on the top side of the PCB (very limited number of electrolytic caps). For the 1µF I can use SMD PET foil capacitors (unfortunately quite expensive). You're suggestion is to remove C7 and C15 and connect the resistors to virtual ground, replace R3/R11 with one resistor to virtual ground, remove C17/C19 and R18/R20 completely and connect the POT to virtual ground? The load would be: 1x 100k (input bias) + 1x 4k7 (input stage) + 4x 1k5 (output stages) + 4x 20k pots, that would make 320 Ohms, at 6 Volt maximum that's almost 20mA, but a OPA165x can drive 30mA, should be fine or is my calculation wrong?Quote from: magic on February 09, 2022, 02:41:26 pmIn the input stage I could use 4k7 resistors and a 33µF tantalum. I would give up the gain, but I the datasheet doesn't tell if the op amp is unity gain stable, what I could find:QuoteI'm not sure if that can happen in an audio application where input signal frequency is maximum at around 20 kHz.Quote from: magic on February 09, 2022, 02:41:26 pmI knew about the disadvantages of X7Rs, I wasn't sure if they matter a lot here, but tantalum are available at even higher capacities (at higher cost). The only bad thing about the op amps is, they doesn't seem to be in stock at neither Mouser, DigiKey nor Farnell... Any good alternative for the 1652/1654?Quote from: Bassman59 on February 09, 2022, 03:29:11 pmDual rail wall wart? Never seen that. Creating a virtual ground from a single supply is what magic has already suggested

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 04:58:51 pm
content: No, the suggestion is to build a true dual supply by half-wave rectification of AC rather than splitting DC. Like in Objective2.https://nwavguy.blogspot.com/2011/08/o2-details.htmlAvailability of fancy modern chips can be problematic because there is a global shortage of all sorts of components right now. Go to TI store and see if you can get them from there or if there is any information if they will be available anytime soon.The NJM4556A used in O2 could be an alternative output stage - they are old, not that popular anymore, there is probably a stock somewhere. They are available in SMD (DMP8 package, a bit larger than SO8).But they have a nasty trap, which is phase reversal. If the negative supply disappears (somebody plugged in a positive DC wall wart) and their input is grounded, the output goes up to the positive railIt would help to use some modern positive regulator with shutdown and rig a small circuit which turns it off when the negative rail is absent. NwAvGuy tried to deal with it using that complex "power management" circuit with LM2903 comparator and MOSFETs.And I wouldn't run it from 16V ACAlmost all of that voltage will only serve to heat up the voltage regulators (if used) and/or opamps.editSorry, it's the 1652 which is missing. I don't know, OPA1642 but it may be unavailable too. Otherwise, older OPA2134 if the input has to be AC coupled with a small capacitor, or NE5532 if you could use a larger coupling cap (elco?) or DC couple the input (one of the split rail / dual rail options).Quote from: stefan_o on February 09, 2022, 04:08:12 pmIt seems you are about correct, and it's frankly more than I expected. It seems that second stage gain networks are mostly to blame, their resistance could be increased a little.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 06:11:08 pm
content: Quote from: magic on February 09, 2022, 04:58:51 pmSuggestion from Bassman59 or you're suggestion? There is not much room for rectifiers/regulators and half-wave rectification probably needs quite some big caps to have smooth output. This is probably a good option if you need more current, but I think the op-amp virtual-ground idea is easier in this case.Quote from: magic on February 09, 2022, 04:58:51 pmAvailability is a problem with basically everything right now. Luckily, the OPA1644 is available, it seems to be the higher-priced higher-performing version of the OPA1654I redisgned the circuit as I understood you're proposal, I also swapped the Darlington transistor in the capacitance multiplier for a Sziklai pair, not sure if my transistor choice is good (high-hfe, low-noise and enough current).

author: magic
author_rank: Super Contributor
timestamp: February 09, 2022, 10:15:38 pm
content: I think every power supply option has been suggested by now, except for switchersLet's analyze them. There are 8 channels, ~60mA peak each, 500mA peak total, 350mA RMS max.The one I mentioned was CMoy-style passive RC rail splitter. For a perhaps manageable 2Vpp ripple in the worst case (20Hz) we need 2Ω ground impedance. We get that with 4mF of capacitance, which is a 2mF bank of capacitors per rail. Not a single coupling cap anywhere. Instant turn-on with no transients. If two channels fault to opposite rails, 60mA flows through corresponding HP transducers in series - potential problem here. With one fault (or faults to the same rail) little happens, because the ground will soon "fill up" with charge.Bassman59 suggested O2-style dual rails generated from AC. O2 is designed for 300mA peak and has as much capacitors as you can see. For 500mA, capacitance could be scaled up or it could be the same, and rail voltage decreased instead (from O2's ±12V), permitting more pre-regulation ripple, which is a more efficient use of input capacitors. Instant turn-on, some transients possible if the rails don't come up synchronously, depending on opamps. No protection from failed channels at all.You suggested single supply and AC coupling of inputs and outputs. Slow turn-on with unavoidable transients, but no possibility of long term DC on any output. With the same amount of capacitance distributed across 8 channels you get 16Ω reactance in series with each channel and same 2Vpp across each coupling cap in the worst case of full current at 20Hz. A remaining question is how to "ground" feedback dividers and pots. Possible answers: low power active ground or several additional capacitors.There is one more option: a high power active ground for everything, including headphones. Which means some power amp which generates a voltage of Vcc/2 and can sink or source 0.5A of current. Probably not worth the hassle and it basically doubles power dissipation, but then the amp could be built with almost no capacitors at all.Quote from: stefan_o on February 09, 2022, 06:11:08 pmIt's JFET (i.e. bipolar with JFET inputs) rather than CMOS, but I guess they are functionally similar for our purposes. Both amplify audio and both have no annoying input bias currentsQuote from: stefan_o on February 09, 2022, 06:11:08 pmYes, that's what I meant. Whether it's a good idea is another question. Any comments from others?One disadvantage I see now: such active ground is no longer noiseless, its noise is exactly the 5nV/rtHz from the ground opamp. This will be added to the signal, just like noise of the 1st stage. I think I will have to do some experiments on audibility of noise in headphones, because I'm curious how much is too much. That being said, O2 is said to be noiseless even at 7x 1st stage gain and max volume, while you have only 3x gain. You should be safe, I think.I would be tempted to try 100nF/1MΩ or even 10nF/10MΩ input coupling - not sure if anything could go wrong with that? Gotta recover some of that money dumped on JFET opampsBTW, if SMD caps are expensive, THT may be cheaper, and can still be soldered manually SMD-style with a bit of creativity.Second stage feedback resistors could be increased to relieve the ground amp since there is four of them. All resistors could be optimized with "noise efficiency" in mind - not too much, but not too little either. Maybe pots could be reduced to 20kΩ.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 09, 2022, 11:22:19 pm
content: Quote from: magic on February 09, 2022, 10:15:38 pmNoise that's not audible is not a problem. Maybe change the second stage to unity gain, so no ground in that feedback loop? Could you see any problem with that in the datasheet? Would a small capacitor (100nF) at the output of the ground op-amps help?Quote from: magic on February 09, 2022, 10:15:38 pmIs there any advantage over 1µF/100kΩ? The two 1µ caps at the input I can fit at the top side, they are not the problem. For the 470µF out caps it's a lot trickier, I cannot find any with 5mm diameter, but I can bend them over the output jacks.Quote from: magic on February 09, 2022, 10:15:38 pmPots turned out to be somewhat problematic. If at all, only Mouser has one fitting type in stock (the exact part number is not listed in the datasheet and google only turns up Mouser again, so I'm not sure it's the correct size). That would be 5kΩ, a 10kΩ variant should be available again in May. Other options only if I order A LOT (as I'm not planning mass production, 5kΩ and 10kΩ are the options). But even four 5kΩ pots need less than 5mA, so no problem for the op amps (the input stage and the ground ones).What other resistors could/should be optimized?

author: magic
author_rank: Super Contributor
timestamp: February 10, 2022, 10:08:38 am
content: I performed possibly the most important experiment of 21st century audio scienceI pulled out my most sensitive IEMs which easily pick up hiss from many integrated soundcards and laptops; their sensitivity measures 120~130dBSPL per 1V RMS, depending on whom you ask.I plugged them into my Objective2 and tried a few opamps in the gain stage. Volume was set to maximum and the output was NJM4556A each time.LM358: ~40nV/rtHz IIRC. Distinct hiss at 2.5x gain, faint hiss at 1x gain. This is the dodgiest and noisiest opamp know to mankind.TL072: 18nV/rtHz @ 1kHz. Very faint at 2.5x gain, barely audible at 1x gain.NE5532: 5nV/rtHz @ 1kHz. Can't hear a damn thing.So it looks like getting down to 20nV/rtHz is the point of diminishing returns and 10nV/rtHz (another 6dB down) is probably safe overkill. Maybe it would be different in the middle of the night, but my room was already pretty quiet and those earplugs isolate well.This is if you care about IEMs, because full size cans should easily take 20dB more noise if those published sensitivity measurements are worth anything.

author: magic
author_rank: Super Contributor
timestamp: February 10, 2022, 10:50:37 am
content: Quote from: stefan_o on February 09, 2022, 11:22:19 pmClosed loop output impedance of an opamp is <1Ω. The rest is a matter of time constantsIt's hopeless. The opamp will simply charge or discharge the capacitor as it pleases. Or capacitive loading may make it oscillate.BTW, how stable are those 1688 with capacitive loads like headphone cables? Those chips are advertised for HP amplification, but it doesn't mean that they were designed for it from the beginning. Most of those OPA16xx series are rebranded general purpose opamps that somebody at TI realized are suitable for audio and produced a nice datasheet and maybe made a small design tweak here and there to make them better. At DIY Audio there is a TI employee who admits it quite openly.On the first page of the datasheet it is shown working at noise gain of ~2. I would search DIY Audio to see how people are dealing with this chip, I'm sure somebody tried it. Or hell, just ask in this thread, john124c is doing it for the money:https://www.diyaudio.com/community/threads/new-audio-op-amp-opa1688.280113ESL of your coupling electrolytics may help a bit. You can create PCB footprints for gain resistors if space is not a problem and then see what happens.Quote from: stefan_o on February 09, 2022, 11:22:19 pmSize and cost.Quote from: stefan_o on February 09, 2022, 11:22:19 pmAll of themIt's pretty simple.1. Thevenin equivalence works for noise too. Your second stage divider makes the same Johnson noise as one 500Ω resistor.2. I linked a noise calculator before. Set bandwidth to 1Hz to get noise density in µV/rtHz which is directly comparable with specs in opamp datasheets.3. Noise from uncorrelated sources (and they are uncorrelated) adds as RMS sums: sqrt(a²+b²+c²+...).4. Two equal noise sources become 3dB / 1.4x louder together.5. Small noise added to big noise barely makes a difference.6. Try not to exceed 20nV/rtHz at the final output.If you are lazy, SPICE can do those calculations for you.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 10, 2022, 01:08:01 pm
content: Have you considered a PGA chip instead of potentiometers?

author: stefan_o
author_rank: Regular Contributor
timestamp: February 10, 2022, 09:49:12 pm
content: Quote from: magic on February 10, 2022, 10:50:37 amYes, that's a good idea. I tried both variants in LTSpice, noise increases similar to the gain of the second stageQuote from: magic on February 10, 2022, 10:50:37 amLTSpice simulation shows minimal lower noise with larger cap/lower resistor (I modeled the source with a series resistance of 100Ω)Quote from: magic on February 10, 2022, 10:50:37 amAh ok, so just calculate and check, I thought there might be a way to calculate the optimum values. I used LTSpice, results show a peak of about 34 nV/rtHz at ~8Hz (50% volume). At maximum volume it reaches 20 nV/rtHz at around 240 Hz with a asymptote at almost 18 nV/rtHz (for 50% volume it's 52 Hz and 14 nV/rtHz).I tried messing around with the resistor values, the only ones that I could find with a significant influence is the pot and the feedback resistors in the input stage (for both = lower is better). But while larger values have a big influence, smaller values only give minimal improvement. 4k7 feedback resistors and 5k pots seem to be all you need (in fact 10k doesn't make a huge difference, but 50k does).I used the PSpice models from TI and copied them into LTSpice. I didn't use any special resistor/capacitor model, do I need that for noise calculation of resistors?Quote from: NiHaoMike on February 10, 2022, 01:08:01 pmNo, I would like to keep the design simple and make it a drop-in replacement for the current pcb.

author: magic
author_rank: Super Contributor
timestamp: February 10, 2022, 10:35:08 pm
content: Resistors technically have some low frequency "excess noise" besides Johnson noise, but I think it's rare for audio people to worry about it. LTspice models Johnson noise, default simulation temperature is 300K or ~27°C.Those TI opamp models in LTspice could potentially be wrong about noise. LTspice has built-in noise models of LT's opamps which tend to be consistent with datasheets, but I have never used foreign models like that. It's easy to test them: run the model alone at unity gain, see what happens.I thought that maybe going 2k2 in the input stage would bring some improvement because that's amplified by the stage's gain, and of course output stage resistors could be increased a bit. Regarding volume pots, it doesn't look like you have much choiceI forgot to point it out before, but the virtual grounds could use more filtering at lower frequencies. Otherwise, supply voltage ripple will couple into them. Increase R and/or C for time constant below audio band. Besides, their time constant determines start-up behavior (capacitor charging transients) of the whole circuit. Now that you are in SPICE, it would be a good idea to run transient sim of supply ramp-up.Beware that many common sources are 2V RMS, or 5.66V peak after 1st stage gain. If you don't have enough supply to make it happen, you will have weird intermittent clipping of peaks even with the volume turned down, driving you nuts until you realize what happens and turn the volume down on the source. It seems that 12V supply with that cap multiplier could be marginal. Not sure if the multiplier is really needed.BTW, do you have any sketch of the original circuit and what was actually wrong with it?editWinnerI downloaded TI's PSpice model and it works in LTspice, including spectral noise density. Thanks for the heads-up.FailThey don't model current noise density, just assume it's flat. Even for OPA134 where it's documented in the datasheet.But you don't need to worry about it here, it's zero for any practical audio circuit.edit editIt stops being flat at 1MHz, but neither OPA134 nor OPA604 are consistent with Burr-Brown datasheet so I wouldn't trust any TI model with that.

author: AaronD
author_rank: Frequent Contributor
timestamp: February 11, 2022, 12:27:48 am
content: Coming in late to this game, but about the power supply:Older Alesis gear likes to use a single 9VAC wall wart with a capacitive doubler for both rails. Minimum 2 caps and 2 diodes per rail. Here's my version of it, minus some noise-suppression caps around the diodes, and with some extra filtering before the regulators.Because the input of that is an isolated transformer, you can connect any one point of that circuit to anything you want, and it's perfectly fine. Usually, that point is the regulators' center tap to the signal reference, which gives you the standard 15-0-15V split supply for analog processing. (30V total with a grounded center tap)For the amp itself, you might have a look at this:https://www.allen-heath.com/media/GL3300-SCHEMATICS.pdfThat's the full schematic of a large-format analog mixing desk, with the engineer's headphone amp on page 19. It's essentially an opamp, but with the output current boosted by some discrete transistors that are configured as emitter-followers, which are themselves included in the opamp's feedback path so that it automatically corrects for their distortion. The whole thing runs from the same 15-0-15V rails that power the rest of the console, and is incidentally the *only* power amplifier in the entire product. All other outputs are line-level and definitely not intended to drive speakers directly, not even headphones.

author: BrianHG
author_rank: Super Contributor
timestamp: February 11, 2022, 01:07:56 am
content: Have you considered TI's TPA6120A2 high performance headphone opamp + if you want to run from a single supply, it's paired split rail TPS65135 dual converter inverting tracking switching power supply?It's designed for as low as a 16 ohm load and can go up to +/-15v for high impedance headphones.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 11, 2022, 01:56:16 am
content: Quote from: magic on February 10, 2022, 10:35:08 pmI tested that: Bang on the values in the datasheet (for 1 kHz), noise decreases with frequency, they are causing the peak.Quote from: magic on February 10, 2022, 10:35:08 pmWill result in a change of about 0.5 nV/rtHzQuote from: magic on February 10, 2022, 10:35:08 pmI checked different values, no influence on the noise, but I didn't model power supply noise.Quote from: magic on February 10, 2022, 10:35:08 pmThat's a good point, I changed both stages to a gain of 1.5, noise is minimal increased, but still below 20nV/rtHz at 1kHz. I also modelled the sziklai pair (the transistors had the model included in the datasheet), I was able to get 65dB attenuation (simulated 12 V DC with .2 V AC) after tuning the resistor values with voltage drop of about 1 V, introduced noise is at .7nV/rtHz, decreases after 10kHz down to .07nV/rtHz. A good result or are there better options for supply filtering?I will have to model that all together. Is there a simple way to add noise to a voltage source in LTSpice?Quote from: magic on February 10, 2022, 10:35:08 pmI just reverse engineered it (see attached circuit only one second stage shown, but not a single part missing) and it's worse than I thought. Issues in circuit design:Different ground potential for each headphoneCan only drive high-impedance headphonesA bit noisy at high gain (not a big issue, because no use for high gain if input signal is normal volume)The build quality is another issue. But the case is quite nice, just two folded metal parts.Quote from: magic on February 10, 2022, 10:08:38 amSo with a result between 10 and 20nV/rtHz I should be more than safe.Quote from: BrianHG on February 11, 2022, 01:07:56 amConsidered yes, but decided against: It isrecommend to pair it with another special opamp to create a composite op-ampto prevent DC offset and instable behavior. For this project this is a bit overkill.Quote from: AaronD on February 11, 2022, 12:27:48 amVery interesting design, because I still have some 9V AC wall warts. But limited space does not allow this solution here.Quote from: AaronD on February 11, 2022, 12:27:48 amI used a GL 2400 quite regularly, I remember the headphones being somewhat noisy (doesn't matter at all in a live/concert environment, but when you're alone it was noticeable). Not sure if the GL2400 uses the same design, I would have to compare the schematics.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 11, 2022, 02:42:28 am
content: Quote from: stefan_o on February 11, 2022, 01:56:16 amInstead of cascading two opamps which makes it harder to keep them stable, you can use a second, slow opamp to actively correct the offset. See the OpenDAC HD schematic for an example:https://github.com/NiHaoMike/OpenDAC-HD/blob/master/OpenDAC_sch_ana.png

author: AaronD
author_rank: Frequent Contributor
timestamp: February 11, 2022, 04:33:08 am
content: Quote from: stefan_o on February 11, 2022, 01:56:16 amAlesis used much smaller caps than I have there. I was going for a more of a standalone, general-use thing, whereas Alesis had it on the same PCB with a circuit that they knew the exact power requirement of. But since you're running a power amp from it, you'd probably want some big caps there too, like I did.Something else I've done is to buy two 15VDC wall warts (or 18V wall warts and 15V regulators) and put their connections in series on my PCB. Call the center tap "GND", and just go with it. The wall warts' safety isolation from the mains makes that work.Quote from: stefan_o on February 11, 2022, 01:56:16 amLooks like the GL2400 schematic isn't on the product page like the GL3300 is:https://www.allen-heath.com/ahproducts/gl3300/https://www.allen-heath.com/ahproducts/gl2400/But I did eventually find it. A search for "mixer allen&heath gl2400" on this site:https://www.eserviceinfo.com/index.phphas this in the results:https://www.eserviceinfo.com/download_start.php?downloadurlid=127055The middle of page 2 in that PDF is the headphone amp. Looks like they've simplified it from the GL3300. Same opamp and a similar current boost, but:The GL2400 does not bias the boost transistors like the GL3300 does. So the GL3300 headphone amp is Class A-B, whereas the GL2400 version is purely Class B. So I would expect a small amount of distortion in the GL2400 version as the current waveform crosses zero and the opamp makes a near-instant jump of about 1V or so. (the sum of the two Vbe's)The GL2400 version is non-inverting, after an inverting stage that itself drives a line-level output (LOCAL_MON_L/R), while the GL3300 combines both the inversion and the power boost into the same opamp without that other output. I wouldn't call this one significant, but it *is* a difference that could throw you off at first glance.The GL3300 is surprisingly straightforward for a big-name product: lots of "textbook" modules, and some seemingly omitted finesse like frequency compensation in some opamps' feedback, or the rectifier before the "traffic light" channel meter, both of which are included in the GL2400. In general, it seems to me like the GL2400 is a more mature design, except for its clearly worse headphone amp.

author: magic
author_rank: Super Contributor
timestamp: February 11, 2022, 10:58:19 am
content: Quote from: stefan_o on February 11, 2022, 01:56:16 amYep, I checked them too. They are good except for current noise, which has tendency to increase with frequency for reasons that I don't quite understand yet and it's almost never documented because it affects almost nobody. Hopefully TI puts more effort into modeling current noise of their bipolar opampsQuote from: stefan_o on February 11, 2022, 01:56:16 amAre you talking 0.7nV/rtHz noise on the supply rail or in the headphone outputs?The former is harmless - see how much positive rail PSRR those opamps have. The latter would suggest that maybe virtual ground filtering is insufficient. Noise on supplies is unavoidable - even if you filter external sources, the supply has nonzero impedance and it will ripple due to AC currents drawn from it by all those chips, particularly the output drivers. The circuit needs to work despite noisy supply rails, unless you go audiophool crazy with separate linear PSU for every chip.Quote from: stefan_o on February 11, 2022, 01:56:16 amYou can make a resistor noise generator and buffer it withe- the ideal VCVS. (The E parameter is its gain, you need to set it).Alternatively, use AC simulation to see how much crap from the supply couples into various circuit nodes.Quote from: stefan_o on February 11, 2022, 01:56:16 amIt can run standalone. ASUS Xonar STX soundcard uses it. But if it oscillates with high source impedance, it could turn out that you would need a buffer between each pot and the TPAOr maybe you wouldn't and that guy screwed up something else.The TPA is a larger chip so that's a potential problem. And being a driver for properly impedance matched transmission lines, it goes crazy with capacitive loads. Most headphone applications put 10Ω in series with the output.editI just tried TI's TL072 noise model

author: stefan_o
author_rank: Regular Contributor
timestamp: February 11, 2022, 06:09:23 pm
content: Quote from: AaronD on February 11, 2022, 04:33:08 amI've also done that before, the only issue is, that you have to plug both in at pretty much the same time, otherwise weird things happen and even plugging in the other one doesn't get it working.Quote from: stefan_o on February 11, 2022, 01:56:16 am

author: AaronD
author_rank: Frequent Contributor
timestamp: February 11, 2022, 06:31:35 pm
content: Quote from: stefan_o on February 11, 2022, 06:09:23 pmPut them on a power strip or AC splitter, then switch that buss. But I'm sure I've plugged in one at a time too, and it figured itself out as soon as it got the correct supply. Are you sure you didn't do something weird with the circuit being powered?Quote from: stefan_o on February 11, 2022, 06:09:23 pmI wouldn't be surprised if it *was* a cost decision, even at that price point. Maybe someone thought the later design was "good enough"? The earlier versions of those modules are certainly more expensive, but not by much.Another interesting difference, though probably not significant either from a performance standpoint, is that the GL3300 is entirely through-hole while the GL2400 seems to be mostly surface-mount.

author: AaronD
author_rank: Frequent Contributor
timestamp: February 11, 2022, 06:43:48 pm
content: Something else you might do for the volume controls, is to use a linear pot with a load resistor between the wiper and ground. With a load resistor of about 20% of the pot's value (10k pot and 2k resistor, for example), the combination produces a better audio taper than a lot of cheap piece-wise linear "audio" pots. (and you can get far better variety in linear taper than any other)If the volume pot is feeding a virtual-earth, then the input resistor can do double-duty. No additional parts required; just make the VE input resistor about 20% of the linear pot that feeds it.

author: magic
author_rank: Super Contributor
timestamp: February 11, 2022, 07:09:24 pm
content: I think those 100nF in parallel with elcos are a bit excessive. Opamp inputs don't need extremely low source impedance. In particular, don't put some microphonic ceramic junk on your vitrtual groundsSome chips may be unhappy without their own 100nF in close proximity, so you may need more than one on the supply rail. Ceramic is OK here.It seems common to have a small RC lowpass on the input, like 100Ω/100pF or similar to kill some RF picked up by the cable. See Objective2 for example. I don't know how much difference it makes in practice. NP0/C0G ceramic caps are available in such sizes and can be used. Some people also include a bleeder resistor to prevent charge accumulation on the external side of the input cap and subsequent insertion pops, although it's not clear how charge could get there in the first place. Maybe after use with some pathological device with high output DC offset.Not sure what's the deal with R11. If anything, 100kΩ to ground could go there to prevent funky business if the pot goes open.You could reduce the output bleeders to 4k7 and remove one BOM lineRegarding simulation, try it in pieces. See what the TI model thinks about using OPA1688 with capacitive load up to a few nF at low closed loop gain. See if maybe a few Ω in series with the output could make a significant difference. AC simulation of the virtual ground generator alone is certainly feasible too.

author: AaronD
author_rank: Frequent Contributor
timestamp: February 11, 2022, 07:40:46 pm
content: Quote from: magic on February 11, 2022, 07:09:24 pmIt's common to do that if the input current is significant. The idea is to present an equal impedance to both inputs, but most of the time it's not really needed. (the latest schematic here is not a good example of it anyway, because the pot changes output impedance based on the setting) A DC-blocking filter at the output of the problematic stage is more than enough, and is often there anyway for other reasons.Instead of 100k to ground there, you could do 1k to ground with a 5k linear pot, which would do what I mentioned in my previous post in addition to preventing the funky business if the pot goes open.Also, the OPA1688's datasheet from TI specifies a short-circuit current of +/-75mA TYP, with no MAX or MIN. Since you're using it as a power amp to drive headphones directly, is that enough to avoid a self-protection shutdown before it becomes useful?

author: magic
author_rank: Super Contributor
timestamp: February 11, 2022, 08:32:58 pm
content: I don't think it has any protection shutdown, just current limiting. It will clip if you try to pull more, but that's quite a lot already and the original board wasn't any better.The limit decreases with die temperature, hitting 45mA typical at 125°C. It seems to have no thermal shutdown either, which is not great, but what can you expect from a generic CMOS opamp with "Burr-Brown Sound Plus" badge slapped on itIn the absolute worst case of full scale square wave into a dead short on both stereo channels at high frequency with barely any reactance in the coupling cap, you are looking at 2·45mA·6V=0.54W burned in the chip. A bit hot, but perhaps survivable for SO8. They are rated to withstand continuous short circuit on one channel at any supply voltage.Here's a commercial product using a bunch of 1688 in parallel for moar powah.https://www.schiit.com/products/magni-1It looks like they run them at unity gain with dead shorts for feedback and 1Ω series resistors and then straight to the protection relay and output jack. So I guess they are stable that way, although I would like to see somebody test that amp with 1nF or 10nF.And unity gain with full output swing is a bit dodgy. I was wrong, the input common mode range does extend all the way to the rails, but look at figure 5 in the datasheet. 'Nuff said.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 11, 2022, 09:47:06 pm
content: Regarding R11: In the TI datasheet there is mentioned that in certain situations the protection diodes could kick in and the input current should be limited to 10mA (page 22f). It also states that noise is increased with those resistors (doesn't seem to included in the spice model), so I decided for a compromise of 470Ω to limit the current (could be changed later). If not needed I'll remove it (gives me some board space)Quote from: AaronD on February 11, 2022, 07:40:46 pmIn fact a linear pot is even more difficult to get in the specific form factor that I need (Alps RK12L series fits: PCB mount, dual gang, 30mm height, 6mm D type axis).Quote from: magic on February 11, 2022, 07:09:24 pmThat's a good pointQuote from: magic on February 11, 2022, 07:09:24 pmI'll hope I can fit them in: I noticed that 0805 parts are to big for the second stage feedback network (my preferred size), 0603 is still ok, but for me 0402 gets really annoying with hand-soldering.Quote from: magic on February 11, 2022, 07:09:24 pmAdded, hope they will fit on the board.Quote from: magic on February 11, 2022, 07:09:24 pmYes, even 10nF causes oscillation, 10Ω in series fixes that, even with 100nF (4.7Ω doesn't seem to be enough). Should also slightly improve low-frequency response of low-impedance headphones (at the expense of volume). Other simulations are fine.

author: magic
author_rank: Super Contributor
timestamp: February 11, 2022, 10:33:12 pm
content: Alright, I understand. You are talking about the last paragraph of 9.4.2.Current through those IN+ to IN- diodes can be limited by resistance on either side. In this case there are resistors already on IN-. I imagine output short circuit with rail to rail input signal and volume at 100% (which is beyond clipping level due to 2nd stage gain - no one listens like that) could be one bad situation when this can matter and you are still at 9mA in such case.Feedback network resistance could be increased further if you are worried about even more abnormal situations, like +12V input signal right at the amp's turn-on when the virtual ground and output capacitor aren't charged yet. Effects on noise would be the same as adding a new IN+ resistor because it's the sum of resistance on both sides that matters for current limiting and for noise too.So I think R11 can be removed to gain board area.Quote from: stefan_o on February 11, 2022, 09:47:06 pmI guess opinions differ on whether series resistance is an improvement or notBesides, the effect is dependent on individual headphone, so it's hard to say anything with certainty.You could try tweaking that 47pF capacitor too.

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 12, 2022, 05:47:00 pm
content: Quote from: magic on February 11, 2022, 10:33:12 pmIt's a good workaround for equipment with a high noise floor. You'll obviously lose maximum volume, but with criticism that portable devices tend to be capable of playing way too loud rather than not loud enough, you'll end up with more usable dynamic range. Also, with high impedance headphones, which need more voltage, the volume reduction would be a lot less. (Indeed, properly sized series resistors are the only reasonable way to have a headphone output be able to work well with both high impedance and low impedance headphones automatically.)

author: magic
author_rank: Super Contributor
timestamp: February 12, 2022, 06:34:09 pm
content: Other approach is to have low noise floor and high voltage swing at the same timeAnd then be mindful not to kill small headphones with excessive volumeHigh source impedance is not without effect on frequency response. Most drivers have variable impedance over frequency, which forms variable voltage divider with the source with variable attenuation. Common outcome is somewhat louder and bloated bass.It supposedly gets much worse in multiple driver headphones with internal crossover networks, which get screwed by impedance added in series. I dunno, never owned such stuff.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 13, 2022, 12:56:21 am
content: Quote from: magic on February 11, 2022, 10:33:12 pmI was able to fit it, so I can see in practice if R11 causes audible noise and decrease it if necessary.Quote from: magic on February 11, 2022, 10:33:12 pmIn the simulation 47pF seems to be pretty much the best choice, it gets worse no matter if I in- or decrease it.Quote from: NiHaoMike on February 12, 2022, 05:47:00 pmI don't think volume is going to be a problem.Quote from: magic on February 12, 2022, 06:34:09 pmI just measured my Sennheiser HD 380 pro, for the range of 30Hz to 13kHz it measures between 60Ω and 70Ω (mostly 60Ω, but with a increase between 200Hz and 300Hz), above 13kHz it increases to 90Ω at 20kHz. 10Ω series resistance would make a difference of about 0.09dB, even with infinite source resistance it's just 0.67dB (if didn't made mistake in my calculation). I don't know if that's common behavior or if these headphones just have a very linear impedance.

author: magic
author_rank: Super Contributor
timestamp: February 13, 2022, 09:27:30 am
content: Seems normal for HD380, though they are supposed to have a narrow peak at 2kHz too.It's equally common to have much larger variation; see HD600 for example.Or AKG K550 for something flatter than yours.https://www.stereophile.com/content/innerfidelity-headphone-measurementsI suspect those differences are responsible for half of the folk wisdom about how different headphones "synergize" with different amplifiers. And half of other half may have no basis in reality whatsoeverQuote from: stefan_o on February 13, 2022, 12:56:21 amWith 100dB/V headphones you will need to put many kiloohms in there to stand any chance of hearing it, but of course I would try to minimize noise anyway.Quote from: stefan_o on February 13, 2022, 12:56:21 am110dB if you can deliver 3Vrms, that's fairly loudSurely more than the original board with its 47Ω resistors.BTW, there are some tricks for stabilizing opamps with clever feedback networks, see for example figure 3 on page 9 in OPA604 datasheet. I have seen some of those circuits in some AD app note too so it appears to be legit. Some of them can be used with zero or low output series resistance. Never tried any of that myself.Regarding output resistors, I hope it goes without saying they need to be sized for power dissipation

author: stefan_o
author_rank: Regular Contributor
timestamp: February 13, 2022, 08:40:37 pm
content: Quote from: magic on February 13, 2022, 09:27:30 amLow noise is great, but for me ear safety is more important. As this is a device for playback/listening and not in a recording chain, noise that you cannot hear doesn't matter (in a recording chain that's different, you may amplify/equalize in post and noise could become audible).Quote from: magic on February 13, 2022, 09:27:30 amFigure 51/table 4 show the maximum output power (for +/- 5V) without distortion: 32mW for 16Ω load, so even for a ultra-low-impedance headphones a 0603 (100mW) is more than enough. Even if I set maximum possible volume, output a square wave, with a 50Ω headphone it's just 100mW at the resistor (for a sine wave 34Ω headphone)I cleaned up some names in the schematic, swapped the 470µF for the virtual ground with 47µF tantalums (couldn't fit the 470µF anywhere) and made a first PCB design. The only point I realized after almost having it finished: I'm not sure how the pots operate, if maximum volume is now all the way to the left or the right and I couldn't figure it out by the datasheet. Guess I'll first have to buy the parts and then measure it before I'll order the PCB.BTW, in case somebody is following this thread with the intention to also build this headphone amp, I'll post the Eagle files (I would love to use KiCad, but I'm so used to Eagle and I haven't found the time yet to get started with KiCad), the gerbers and the BOM after I build and tested it.Edit: The output coupling caps are not directly mounted to the PCB, but bent over the output jacks. One capacitor doesn't fit completely on the board, but it will still fit in the case

author: AaronD
author_rank: Frequent Contributor
timestamp: February 14, 2022, 03:23:43 am
content: Quote from: stefan_o on February 13, 2022, 08:40:37 pmI did the same thing. Started with Eagle when it was still its own company with a decent free version, kept that version despite it being way out of date, and then switched to KiCad. I put off switching for way too long because I was waiting for a clean break. I eventually got that break, when all of the old projects finally settled down and I wasn't doing revisions on them anymore, and then I fumbled around a new project in KiCad.It's a lot different from Eagle - which is probably true regardless of what you switch from or to because this type of program became popular before UI's were standardized, and people really like their favorite workflow - but I'm actually starting to like it better. What I REALLY like, is that it's a mature open-source thing, which means all kinds of cool toys, all for free! (once you figure out that they're there and how to use them)

author: magic
author_rank: Super Contributor
timestamp: February 14, 2022, 08:38:06 am
content: I noticed that there is one thing missing from the original version, and that is any sort of reverse power supply protection. The sad truth is, if anyone grabs your amp and connects it to some AC wall wart or one of those rare DC wall warts with negative center pin, virtually everything will blow up. T1 BC junction is a diode just waiting to conduct reverse current, T1 BE junction is a 6V zener which will break down under reverse bias. And T1 is the only thing between the input and the VCC rail - all those chips and elcos will get -5V on them (from a -12V PSU).Schottky is one option but it has some voltage drop, another option is this little MOSFET circuit.https://components101.com/articles/design-guide-pmos-mosfet-for-reverse-voltage-polarity-protectionGiven the low current consumption of <0.5A max, perhaps it would be possible to find even a SOT23 MOSFET - power dissipation in the FET is very low with good RDS(on).A few thoughts about grounding and power delivery. As I see it, there are two distinct modes of operation:Positive output: current is taken from VCC by 1688, charges the coupling cap, passes through the HP, flows from HP ground back to the PSU.Negative output: current is discharged from the coupling cap through the HP, flows back from opamp ground pin to HP ground.The first case results in horizontal current flow which may lead to different voltage at grounds of different headphones. I suppose nothing can be done about it save for throwing as much copper there as possible.The second case results in vertical current flow and different voltage at HP grounds and 1688 grounds. Since opamps have some PSRR and headphones have zero (change the signal-ground offset by 1mV and you have 1mV signal seen by the HP), it seems reasonable that input ground, signals and virtual grounds should accurately track headphone grounds. This means, good copper and no large AC currents between HP grounds and the input jack and 1644. So I wonder if it wouldn't be beneficial to cut the bottom right corner link between 1688 ground and connectors ground (the EMI filter stays on the connector ground, of course). You can always leave some footprint to join them with a solder blob and see if it makes a difference.Did you calculate resistance of the supply trace? I wouldn't be surprised if voltage drop there ends up being even a millivolt at load and the 1644 has only 60dB PSRR at 10kHz (OPA1688 are better, though). Perhaps 1644 could have its own supply at the upper edge of the board, freeing up space for different routing of that virtual ground, which frees up space for more ground copper between C2x4.Also, C2x1 and other capacitors there seem a little crowed. Can't things be shifted down a little to open space between C2x1 for more ground?Another thing, when ground current flows horizontaly, the floating 12V PSU pulls current from headphone grounds and develops negative (with respect to the input jack and signal) voltage at its negative terminal. And your cap multiplier very accurately decreases VCC (with respect to input ground) by the same amount. Obviously things wouldn't be any better without it, but I'm not sure if it's really achieving much useful function.

author: magic
author_rank: Super Contributor
timestamp: February 14, 2022, 12:10:26 pm
content: Here's my attempt at star-grounding and Kelvin-sensing this layout a little.I have separated the connectors ground from the PSU so that no significant headphone current flows horizontally there.Headphone current flows through the vertical ground links in parallel with headphone output lines. Importantly, it does so regardless of output polarity - during sourcing and sinking alike. The effect is that ground of the opamps bounces proportionally to the signal. If any of that gets injected into the signal, we will see a tiny increase/decrease in gain and a little left/right crosstalk (crosstalk is still bottlenecked by headphone jacks anyway). OTOH, injecting only positive or negative output pulses into the signal or headphone ground connectors would be pure distortion - and this is avoided here.Halfwave-rectified (only positive pulses) current flows through the ground trace of opamps back to the PSU. The result is that pulsed ripple again appears at PSU ground and it's somewhat worse than before because connector ground no longer participates in conducting that current.The capacitor multplier is now referenced to the connector ground to remove that ripple from Vcc. A downside is that β times lower pulsed current is still taken by Q1 base and dumped through Cmult into sensitive ground. Perhaps the whole thing could be replaced with an LDO like LD1117 or LD1085. These LDOs have very high current gain (almost zero ADJ current variation with load current) and also need little capacitance to filter their divider.EMI filters and virtual ground dividers are referenced to the precision ground. OPA1644 is connected to OPA1688 ground because it dumps a few mA worst case of negative-only pulses from all those pots and feedback networks to its ground. Grounding OPA1644 to the PSU would take that ripple away from OPA1688 but expose OPA1644 to much greater PSU ground pulses. Probably OPA1644 comfort is more important, because its output is followed by more gain.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 14, 2022, 03:24:26 pm
content: Quote from: AaronD on February 14, 2022, 03:23:43 amI'm also still using the last free Eagle version. I installed KiCad and I tried to start the project there, but it's also very slow for me (zooming in out/moving parts, in Eagle that's instant, in KiCad it takes about half a second). I tried different setting, but I assmume KiCad is using some 3D acceleration that's also used for the 3D view and my graphics card (GT 1030) is just very outdated for my 4K monitor (I've got similar issues with some other programs as well). I'm looking for a new card since I got that monitor that's reasonable priced and has reasonable power consumption (I'm not into gaming), seems I have to wait another 3 month and hope that Intel can deliver something.Quote from: magic on February 14, 2022, 08:38:06 amTo avoid that voltage drop I didn't installed that diode, and I thought a MOSFET solution takes up to much board space, but I was able to fit it in with a SOT23 MOSFET. The biggest evil are these universal DC wall warts that have that connector you can fit both ways to swap polarity.Board space: It may seem that there is a lot of space left because I didn't bother drawing the entire pot, but the top line shows where it ends and there is also very little space in between the pots, so the only place for the 470µF caps is above the jacks.Power supply/ground: I separated the OPA1644 supply on the top edge as you suggested, but is ground really an issue? I've got ground planes on both sides and a lot of vias to connect them (btw, the vias for the opamps are directly at the pin without soldermask, so the solder will flow through the via and connect them almost directly to the top plane. The resistance between the opamps and the headphone connectors should be really low.What I could easily do is to split the ground plane along C2L1 / C2R1, so that the op amps are closer to DC ground than the headphones (and the headphones closer to the op amps than to DC in ground).capacitor multplier: My idea was to remove high frequency noise that's introduced by bad power supplies next to the amp (cheap phone chargers, bad Led lamps). I noticed some noise from devices like that in the Behringer design.

author: magic
author_rank: Super Contributor
timestamp: February 14, 2022, 06:12:03 pm
content: Ground is an issue. Many milliamps through a few milliohms is many microvolts.I turned my diagram into a working simulation which can be experimented with. Load resistance is two 50Ω channels per socket and I replaced OPA1688 with some LT opamp because the TI model simulated very slowly. It runs at 1V RMS and I suppose results will just scale proportionally at other levels. 1V is a convenient number and this LT chip can't do more output current anyway.Resistances are estimated based on supposed 0.5mΩ per square sheet resistance of 35µm copper. Particularly the "north" and "south" resistors, the central vertical links are a wild guess due to weird shapes.It shows halfwave rectified ground bounce on headphone grounds and the further to the left, the worse it gets. Actually, my idea was not as effective as I hoped, because nothing forces U4 current to flow horizontally - it can go to the north, meander between headphone jacks, come back to the south. Increasing resistance of the central links reduces headphone ground bounce, at the cost of worse opamp ground bounce (still, opamps are supposed to have over 100x more PSRR than headphones over full audio band).Ground bounce at opamp grounds is higher than at headphone grounds, but distortion content is actually similar: FFT shows second harmonic at -90dB on op1g and -100dB on hp1g. It's the fundamental which makes most of the difference in amplitude. You may say that -100dB harmonic distortion is not a huge deal, but it's not the only distortion source. And there is the problem that loud outputs may generate loud distortion which will sound even louder in quiet outputs.Note that I don't model the virtual ground, just perfectly level-shift the signal by 6V. It's equivalent to assuming that virtual ground is noiseless, which likely requires separation of virtual ground dividers from the noisy ground of output driver opamps, in the bottom right corner near EMI filters. If that means that OPA1644 stays on the "quiet" ground and pollutes it with its ground current due to layout constraints, so be it. It's still an improvement to get at least the output driver ground current out of there.I'm not sure if there is much current flowing through ground pours between VCC and pot mounting holes, some maybe it would be better to increase VCC track width instead? (At least up to C7). I don't know its exact dimensions, but I would guess it's some 10~20mΩ.Have fun with the simBTW, there is no such thing as DC ground - the 12V supply is floating. The input jack is ground - everything else is noise and distortion

author: stefan_o
author_rank: Regular Contributor
timestamp: February 14, 2022, 08:58:52 pm
content: Thanks, I played with the simulation: I added a resistor between -12V and Rn to simulate the board between the DC input (that's what I meant by DC ground) and the left output jack. That resistor made it worse, so splitting the ground planes there should be an improvement. Obviously reducing {rn} improves the the output grounds. In practice this could be done quite easy: Create a strip of 3mm without soldermask on the bottom layer between all 5 jacks and solder some used solderwick along. Resistance should decrease a lot.I couldn't find a way to calculate the resistance inside Eagle and so I got an old (unpopulated) board out with a ground plane and a massive amount of traces all over and measured the ground resistance across about 9cm (by feeding 1A and measure the voltage drop directly at the board): 3mΩ. On the headphone board it's just about 3cm distance from the op amp to the output jack and there are less traces in between, so 1mΩ should be the maximum. Rs is about 1.5cm, so .5mΩ. Rn with the solderwick should a lot less, maybe .1mΩ.With this values the simulation gives me 11.5µV maximum amplitude of ground at the headphones and 120µV at the op amps. FFT shows first harmonic at -111dB at the worst output and -99dB at the worst op amp ground.I think that's good enough. It's "just" an headphone amplifier, I'm not trying to measure gravity waves. It should be a lot better than the original Behringer in every aspect now (the pcb traces look horrible on it, either someone engineered it for perfect matching trace impedance and therefore created useless vias and extra long traces or it's the result of a bad autorouter (by looking at the price I assume the later)).

author: magic
author_rank: Super Contributor
timestamp: February 14, 2022, 09:24:24 pm
content: Yes, I know what you meant by DC ground. And what I meant is that DC ground is not a very useful ground if it bounces a few mV with respect to the source signalResistance depends on copper thickness and is measured in ohms per square. A square of standard 35µm ("1 oz") copper is said to be approximately 0.5mΩ between its opposite sides. Two such squares in series are 1mΩ and two such pairs in parallel are 0.5mΩ again because they are a square. Every square is 0.5mΩ and this makes life easy.Just divide trace length by its width and multiply by 0.5mΩ.

author: Bassman59
author_rank: Super Contributor
timestamp: February 14, 2022, 11:15:14 pm
content: Quote from: AaronD on February 11, 2022, 04:33:08 amI used to participate in a forum dedicated to live audio production, and the head engineer at A&H participated regularly, too. I mixed countless shows on the GL3000, the GL3300, the GL2400 and the GL2800. The 2400 and 2800 were in every small club after the Mackie 8-bus consoles they had finally died. The 2800 was a solid performer.The aux outs on the GL mixersarebalanced.The PCBs were designed so that in the default configuration, the aux outs used the circuit called "impedance balancing" which we know is redundant (because balanced circuits are impedance balanced) which has the hot driven through a low-value resistor and the cold connected to audio ground through a same-value resistor. The boards had the DIP footprint for a DRV134 or THAT1646 or SSM2142 if the users thought that they needed differential drive. A&H sold a kit that included enough of the driver chips to outfit the console.Since the internal buses ran at -2 dBu, the standard configuration drove the output at -2 dBu. If the differential drivers were installed, the outputs ran at the "standard" +4 dBu. In practical use, the aux outs saw either an effects processor or an EQ/processor for monitors, so the difference in level was not noticeable, and anyway mix out levels depend on what you feed to it. The A&H engineer said that very few customers bothered with the "upgrade."I think that the direct outs on each channel had the same idea.And of course it was a cost decision.I don't recall any issues with the headphone amp in any of them driving the usual 7506 headphones.QuoteThe 3300 was released a few years before the 2400/2800.

author: Bassman59
author_rank: Super Contributor
timestamp: February 14, 2022, 11:25:28 pm
content: Quote from: stefan_o on February 09, 2022, 06:11:08 pmMy suggestion was to use something like this. The power jack connects to an wall wart of at least 16 VAC.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 15, 2022, 02:02:38 am
content: Quote from: Bassman59 on February 14, 2022, 11:15:14 pmYes, I but there is an important downside of impedance balancing that I experienced: Using a long and not well shielded multicore, the aux outs at high level, and a pair of a mic with a very low signal probably next to it for 50 meters, there can be significant crosstalk. Therefore I populated the balanced line drivers (I used THAT 1646 bought from Mouser, not from A&H support), crosstalk wasn't an issue anymore.Quote from: Bassman59 on February 14, 2022, 11:25:28 pmThat's a nice solution, but AC wall warts have become pretty rare: The EU has some restrictions, that a device that is powered off may not consume more than 0.x Watts (not sure of the value), a normal transformer can achieve that only at higher cost (more material inside), therefore almost everything uses switching power supplies or has the transformer inside the device, so the power switch will disconnect mains of the transformer. I have savaged several AC and non-switching DC wall warts, but I just looked in a few online stores for electronics, they only got switching supplies. Of course you still buy a transformer and a wall-wart case to build your own (but then you could just use a transformer with +/- 15 V).

author: magic
author_rank: Super Contributor
timestamp: February 15, 2022, 11:47:47 am
content: Quote from: magic on February 14, 2022, 06:12:03 pmTriple-mega-facepalmI cannot believe I wrote this. I knew that the origin of those halfwave rectified pulses flowing back to the PSU is not the 1688 ground pins (they are the origin of pulses flowing back to HP ground) but the HP ground pins, when the HPs and their coupling caps are fed positive current pulses from the DC supply. I even wrote about it earlier.There is nothing forcing that current to flow neatly through each headphone's corresponding Rc and then through Rs to the PSU. Of course it will spread all over the place through RnThe solder wick trick should help a lot and it may be good enough. That being said, I apologize for suggesting that my yesterday diagram had much to do with star grounding, and here's what a true star ground looks like. Every Rh is only a short extension of the HP's ground cable, every Ro is an extension of the opamp's ground pin. Still ripple on every component connected to ground and it will be somewhat larger (ground has been cut up into fragments), but only the opamps see nonlinear ripple. For HPs, it's just attenuation (and very little L-R crosstalk).That's the theory. In practice, I suppose it may be impossible to route so many grounds separately, perhaps the opamps could share their local "plane" as currently. One trace could go from the middle of that plane up and separate into four headphone ground returns. At the separation point the input and OPA1644 could be connected. And the Vbe multiplier. And the PSU would have to go somewhere.But it's a fairly radical redesign, so I guess I won't be very surprised if you leave it as is.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 15, 2022, 01:35:10 pm
content: I just simulated the star ground, the simulation doesn't turn out better, because in practice: a) The resistance values will be a lot larger (traces instead of planes) and b) The resistance values to the ground star will also differ (unless you very precisely match width/length of all traces, but manufacturing will probably not be that precise).Therefore I think I'll stuck with the two improvements I made reagarding to ground: Split the ground planes between the DC jack and the output on the left and remove soldermask along the ground pins of all 5 jacks to solder a solder wick there to reduce resistance

author: magic
author_rank: Super Contributor
timestamp: February 15, 2022, 02:23:06 pm
content: For the record, Rh is not a major problem. Another few mΩ added to the ground cable of the HP. They don't need to be matched between sockets at all. Nothing really needs to be matched, but it would be nice if Ro were low to reduce ground bounce experienced by the chips. In the current implementation, different chips also experience different amount of ground bounce - so much for matchingBTW, is it possible to also reinforce the OPA1688 ground strip under those volume pots? Probably not...OTOH, obsession with optimizing every damn last detail is one reason why I often get less stuff done than others so I'm not going to argue.I would cut the ground in the bottom right corner. Separate OPA1688 from OPA1644 and the input section. It doesn't seem right for load currents to flow between OPA1688 and HPs through the input ground. An option can be left to connect them together with solder blobs or copper wick or whatever to see what / if any difference is made.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 21, 2022, 03:26:26 pm
content: Quote from: magic on February 15, 2022, 02:23:06 pmNo, what I could do is ground the pots (currently the case is floating)Quote from: magic on February 15, 2022, 02:23:06 pmI know what you mean, I was told several times "A 95% solution ready in time is better than a 100% solution that is never completed".Quote from: magic on February 15, 2022, 02:23:06 pmI got the first delivery today (there was a time where you would search for the cheapest supplier, now you hope you find at least one with a suitable part in stock...) with the pots and as expected, there are the opposite, so I had to fix the connections and rearrange some resistors. I also cut ground in the bottom right by doing that I think it's a improvement in general, looks a bit cleaner.Unless there are any problems left I would order the PCB in the next days (the OPA1688 will not arrive before end of march, so no need to hurry)

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 07:06:19 pm
content: I know I sound like a broken record but I would cut that ground in the bottom right cornerAnd it seems you have reduced ground connectivity between IC4 and its headphone socket by moving those signal traces. I suppose they can't go back due to new resistor positions, but maybe they could go further to the right, one (if not both) of them behind the pot mounting hole.It seems there is some free space on the bottom layer near 1688 VCC pins, maybe their decoupling caps (with individual ground vias) could go there to put them closer to the chips.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 21, 2022, 07:42:57 pm
content: Quote from: magic on February 21, 2022, 07:06:19 pmHuh? I repositioned the traces in a way that the ground is cut there? The OPA1688 is connected to ground to the left with lower resistance now and farer away from the OPA1644. I thought that was your proposal? There are two other possibilities to cut ground (the yellow lines or the cyan), are you suggesting the cyan one (the yellow one would connect the input filter closer to the OPA1688 ground and far away from the OPA1644 ground, should but worse)?Quote from: magic on February 21, 2022, 07:06:19 pmGround connection should be much better now.

author: magic
author_rank: Super Contributor
timestamp: February 21, 2022, 07:58:32 pm
content: This looks nicer than I expectedI would cut cyan - remove the blue plane connecting the two red planes. The input filters etc need to stay on the input side.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 21, 2022, 08:41:03 pm
content: Quote from: magic on February 21, 2022, 07:58:32 pmDone, I'll have a look at it again tomorrow and if don't see anything wrong I'll order the PCB.

author: magic
author_rank: Super Contributor
timestamp: February 22, 2022, 09:40:12 am
content: I recall a discussion about grounding C1 closer to the I/O jacks to reduce VCC noise, but I guess it's not going to happen on this already cramped board.And you left an unused via near C1. Time for v9If we are at that, the ground trace north of IC3 could be made wider at absolutely zero cost.

author: stefan_o
author_rank: Regular Contributor
timestamp: February 22, 2022, 09:56:13 am
content: Quote from: magic on February 22, 2022, 09:40:12 amI could reconnect ground at the cyan mark, that would reduce resistance between C1 and the jacksEdit: Just spotted another misaligned via below C2R4

author: stefan_o
author_rank: Regular Contributor
timestamp: February 22, 2022, 10:41:44 am
content: Quote from: magic on February 22, 2022, 09:40:12 amAlso to a lesser degree at IC2, had already done that yesterday, so that change is already in v9

author: magic
author_rank: Super Contributor
timestamp: February 22, 2022, 11:38:33 am
content: Quote from: stefan_o on February 22, 2022, 09:56:13 amI don't know, maybe it's harmless. At any rate, if we accept the input jack as "true" ground, then the PSU is the noisiest area on this ground plane and the most sure improvement would be a dedicated ground trace for C1. But you are also using C1 as a via to stich ground planes in the PSU and there really isn't any space left to route more traces there, so things are staying as they are, I suppose.It wonder if some tools exist for simulation of ground currents in such PCB designs. SPICE is fun but it can only be made to approximate very simple patternsI'm starting to wonder again if ground connection between C2x1 would be beneficialBut IIRC, the SPICE sim suggested it would increase crosstalk...

author: stefan_o
author_rank: Regular Contributor
timestamp: February 22, 2022, 12:07:14 pm
content: Quote from: magic on February 22, 2022, 11:38:33 amFirst step could be an option to calculate trace resistance (including planes) in the PCB editor. Second step then would be an export option to export it to SPICE (including the trace resistors). I would use the parts placement from the schematic, remove all wires (or draw them as annotation) and replace them with nets and add all trace resistors at the bottom in, as there will be a lot (assume 10 parts connected to ground, just for ground for ground you'll get 45 resistors (10*9/2 or nCr(10,9)). As you have to add/adjust the SPICE file an option to only update the values when modifying the PCB would also be very helpful.Maybe that functionality could be introduced into KiCad...Quote from: magic on February 22, 2022, 11:38:33 amI think I'll keep in it's current state. In case I'll experience noise I can still solder a botch wire to bridge grounds at that place.Edit: PCB is ordered, expected to arrive by end next week/beginning of the week thereafter. I spotted another mispositioned via near C1L/C8, that was fixed. After building & testing I'll post all Eagle/Gerber files and the BOM with the values used.

author: stefan_o
author_rank: Regular Contributor
timestamp: March 06, 2022, 10:34:03 pm
content: Thank you all for your input (even though I have not used it in this project, there are some ideas I can use in future projects) and a special thanks magic for constantly helping me out!The OPA1688 arrived earlier than expected and also the PCBs have been in my post box on saturday, so I already built it: Works perfect, no noise even at maximum gain and open input, no ground issues anymore, bass response is a lot better than before, also better stereo separation and to my ears it sounds cleaner, more detailed (that's highly subjective and I could be completely wrong, I do not have the equipment to measure it and I also didn't do a blind listening test (=rating the sound while not knowing what device I'm listing to)). Tested with Sennheiser HD 380 pro (56 Ohms) and AKG K812 (36 Ohms).The maximum amplification is less compared to the original one, but for me that means more useable range at the pot. You can adjust the resistor values to increase maximum gain.I attached a few pictures of the assembled PCB, it also fits perfectly in the original case (note: The jacks are not as loose as the original ones, you cannot move the housing a lot after soldering them in, but there is a little tolerance positioning them before you solder them in. If you like to rebuild it and put it in that case, I would suggest solder the 4 output jacks before soldering the input and DC jack while having the jacks/board upside down the case to ensure correct positioning).I also attached a zip file with the EAGLE (version 7.6) schematic and PCB, PDFs of the schematic, the PCB and the BOM (including links to the Mouser/Digi-Key parts I bought, no link means I still had those parts, but they are nothing special) and a ZIP with the Gerber-files. I got the PCB done at Aisler, but I used the same job-file for creating the gerbers at OSHPark and JLCPCB before, so they should work there as well.

author: magic
author_rank: Super Contributor
timestamp: March 07, 2022, 07:45:59 am
content: Noise with open input is mainly the Johnson noise of your 100kΩ input biasing resistor. It's similar to LM358 noise level so I'm sure more sensitive HPs will pick it up, but it goes away when the input is connected. If somebody is bothered by it, input impedance of the amp could be reduced to 10kΩ.A computer audio interface and RMAA software can be used as a poor man's audio analyzer to some extent.

author: Dope_x101
author_rank: Newbie
timestamp: September 16, 2025, 03:26:56 pm
content: Hey Stefan!Im looking for reverse engineering a 4 channel headphone amplifier like FIFINE N6 and i came across your design for HA400this one:https://www.amazon.com/FIFINE-Headphone-Amplifier-Adapter-4x-Headphones/dp/B07QDN6JXG.I was searching through internet about similar design and came across your design which seems really interesting to me. I need to ask you few things related to your design:1. Could you tell me all the specs of your deisgn like output, input and their impedences2. Also if i were to alter your design to make it similar to FIFINE N6 which components i should replace to make them similarThanks in advance

author: stefan_o
author_rank: Regular Contributor
timestamp: September 16, 2025, 03:54:07 pm
content: Hey,the input impedance should be around 100k (as suggested in the previous post could also be lowered to 10k), the output impedance is 10 Ohms. Output coupling handles low frequencies for low-impedance headphones just fine.I don't know the FIFINE N6, but considering it's low price point I would assume it is very similar to the HA400. My goal was not be similar to those, but significantly better at still reasonable cost (without esoteric magic snake oil components). The HA400 has a few major weaknesses I addressed:Noise coupling into audio from DC inNo common ground, AC coupling is done at the ground side to reduce number of caps neededCoupling cap very small, resulting in a very noticeable low-cut (I think calculated -3dB point at 80 Hz) for my headphonesPots not very good and inconsistent, sometimes large volume jumpsRather than trying to mod the existing circuit/PCB to address these issues and I designed a new circuit and PCB. And as I used higher quality components and no class-2 ceramics in the signal path it should also have lower harmonics and distortion.If you have a FIFINE N6 that you want to upgrade, you would have to check dimension/distance of pots and connectors on the PCB and move them if needed to make the PCB fit into the case.

author: Dope_x101
author_rank: Newbie
timestamp: September 18, 2025, 06:30:59 pm
content: Right! Thanks man basically i want the same circuit like yours just want to replace those board mounted 1/4" TRS Jacks with something like solder terminals or screw terminals but i dont think so as these will take so much space so im considering using plated through holes instead so that i can solder them to my panel mounted jack in my pcb enclosure and wanted same specs like fifine n6 but yours are very similar to it infact much better it seemand any further suggestions you want to add if i were to alter some changes like replacing those TRS jacks im thinking of keeping everything else same like yours?