O2-based headphone amp - Page 1
Forum section: electronics / projects

author: microbug
author_rank: Frequent Contributor
timestamp: February 28, 2016, 12:09:34 am
content: After realising that I was only likely to use it at home, I abandoned my ideas for a portable headphone amp. Instead, I designed aclonedesign inspired byNwAvGuy's O2that made waves a few years back. I've named it Ozone (O2+1 '=' O3). Main changes are:- AC only (built in 230V/115V transformer)- Headphone relay for transient protection during power up / down.- Surface mount construction- Should fit insideHammond 1455K1201case (78*43*120mm)The features shared by this and the O2 are:- Up to 7V RMS output- Up to 166mA max current- Very low THD+N, crosstalk, output impedance; flat frequency response- Gain switch (set at 8dB / 16dB here)- Cheap compared to similarly-performing commercial productsNwAvGuy licensed his work under a No Derivatives license, but asothers(I haven't based my design on agdr's, despite some similarities in design goals) have pointed out, it is a grey area: NwAvGuy has not responded to contact for several years, and there is the question of how similar a design has to be in order to be considered a 'derivative'. Since I'm re-doing the PCB I feel that it's different enough to be acceptable.Provided that the PCB layout is good enough, it should perform near-identically to the O2, which hasexcellent measurements.I have attached the PDF of my schematic. Questions:- Will the transformer interfere with the audio circuitry, given its proximity to it? I could use a toroidal one (lower EMF) instead, for a little more money (and possibly a larger case).- Is anyone likely to want either an unpopulated PCB, kit or functional unit once the prototype has been built?- If the answer to the above is yes, does not having 1/4 inch sockets bother you? IMHO they are clunky and outdated, but they could be inluded.I will release the EDA files for this project as public domain.EDIT: Changed to a Triad Magnetics FP30-400 transformer as the other one wasn't available on Digikey (my aim is for all part to be available there).

author: microbug
author_rank: Frequent Contributor
timestamp: February 28, 2016, 07:34:31 pm
content: Bump

author: Audioguru
author_rank: Super Contributor
timestamp: February 28, 2016, 11:02:36 pm
content: Why use Japanese opamps?

author: microbug
author_rank: Frequent Contributor
timestamp: February 28, 2016, 11:05:49 pm
content: When I first saw the O2 design I was dubious too, but the JRC4556 especially is very well suited for the job. See here:http://nwavguy.blogspot.co.uk/2011/08/o2-details.html(there's a section on op amps)

author: NiHaoMike
author_rank: Super Contributor
timestamp: February 29, 2016, 02:57:03 am
content: You don't want to put the PSU (specifically the AC parts) right in the same case as a low noise design if you can help it. At the least, make sure to use good shielding.Beware that the offset voltage of some opamps is enough to make a very noticeable click when the relay engages or disengages. Here's how I implemented active offset correction in my headphone DAC/amp:https://github.com/NiHaoMike/OpenDAC-HD/blob/master/OpenDAC_sch_ana.png

author: microbug
author_rank: Frequent Contributor
timestamp: February 29, 2016, 06:50:09 pm
content: Quote from: NiHaoMike on February 29, 2016, 02:57:03 amOK, I'll use a full wave voltage inverter and an external transformer like the original O2. That will cut down the size too; the limiting factor in height and length was the transformer.I can see what's happening in your gain correction circuit, but I don't know how to calculate values for such a circuit. I attached a screencap with your solution bolted onto my buffer — would this work? If not, I might just go with the other solution attached (from TI'sAN31).

author: true
author_rank: Frequent Contributor
timestamp: March 01, 2016, 03:40:36 am
content: I have interest in trying this design.Do you have any way to test this design? I have limited audio test gear but could run it with what I have. Always had a dream to get an AP analyzer for some passive headphone EQ designs I've done, but have nothing to sell to pay for it

author: NiHaoMike
author_rank: Super Contributor
timestamp: March 01, 2016, 04:23:14 am
content: That's not gain correction, that's offset correction. You'll want to use an opamp specifically designed for low offset. For example, the LT1024 is designed to have less than 50uV offset. Compare that to the TPA6120A2 output driver that may have as much as 5mV - 100 times greater.For your design, the resistor values you show should be fine.

author: Zero999
author_rank: Super Contributor
timestamp: March 01, 2016, 08:54:59 am
content: Quote from: Audioguru on February 28, 2016, 11:02:36 pmBecause they're good value for money.

author: microbug
author_rank: Frequent Contributor
timestamp: March 01, 2016, 04:50:27 pm
content: Quote from: true on March 01, 2016, 03:40:36 amI'm glad that you're interested. I don't have good enough equpiment to make meaningful measurements past checking that it works, but I might be able to use a friend's. No promises; I need to find out whether he has the right equipment. As long as the PCB layout is fine, the design is close enough to the original O2 that it should measure similarly (i.e., very well).Quote from: NiHaoMike on March 01, 2016, 04:23:14 amThat was a typo — I'm more used to talking about gain correction with op-amps. I chose the OPA2188 for this: 25µV of offset should do! Good to know that it should work. I used a single op-amp for each NJM4556 as the two drivers are both on the same chip.Quote from: Hero999 on March 01, 2016, 08:54:59 amDefinitely; the NJM4556 is $1 in single quantities from Digikey!I have attached an updated schematic. I modified the decoupling to be in line with the O2's design, and took the electrolytic decoupling caps down to 220µF since apparently that's optimum:Quote from: NwAvGuyEDIT: Accidentally had the black and white PDF export mode on. Now in colour!

author: bson
author_rank: Unknown
timestamp: March 01, 2016, 06:39:46 pm
content: Quote from: NiHaoMike on February 29, 2016, 02:57:03 amOr use a small toroidal. Also, an EMI blocking inlet filter.

author: bson
author_rank: Unknown
timestamp: March 01, 2016, 07:06:59 pm
content: I've built a bunch of headphone amps and started by measuring the load. The normal listening levels ranged from 10mVrms to 100mV for the headphones I looked at: Audeze LCD-3, Beyerdynamic T1, Grado RS2i, Audio-Technica (AT55 I believe), Sennheiser HD650, AKG K702. I seriously doubt you'll find any headphone other than perhaps from Hifiman that needs more than 100mV. Almost all ended up around 60mV rms. This means a headphone "amplifier" has a ~175-350mV rms (some sources will be hotter than others) inputs and 1/3 to 1/10 the output. It's a net attenuator. You can see power levels will also be measured in uW. Any op amp can drive even two headphones without breaking a sweat, even to deafening levels. Forget about measuring at 1Vrms; you will never ever listen that loud.You absolutely don't need a unity buffer on the output.Remove the output AC coupler and move it to the input. Use 1uF polypropylene here; this part is outside the feedback loop and any nonlinearity will faithfully reproduced. In fact, swapping this part and trying a bunch of different ones might be worthwhile, it definitely affects the sound. PP caps are fairly big and cost a bit more.With the AC coupler on the input pick a low-noise low offset op amp. (I like the LME49710.) Use one per channel since even 0.5mV noise will be audible on a 50mV output. (It's 1% distortion into a headphone that might measure 0.05% at 1V, and even better at listening levels.)For a headphone, post-attenuation is the way to go rather than attenuating the input. The op amp could be operated at unity gain, but this will give a weird volume response. Gain steepens the volume control and makes it more sensitive. I like 3X (10k,5k) but for some of the headphones this requires dialing it up to 2 o'clock but this is something most people get used to quickly. I use a volume knob without a marking to avoid the notion that it should be in some particular position - less attenuation is better, so the more it's dialed up at listening levels, the better. This is the opposite of consumer electronic expectations... people tend to view volume as something that adds rather than removes.Use a good audio taper pot. I like ALPS Blue Velvet and the taper works well with low gain. Works very well with IEMs, too.Use proper audio jacks - RCA or XLR. None of that TRS toy stuff for line level.I'd strongly suggest a 1/4" TRS jack for the putput... all quality headphones come with 1/4" TRS plugs and microphone cable.Just go PTH, no SMD for audio - you'll want the op amps socketed which means DIP so people can try different ones (the differences are very minor and relate to noise modulation, but at 175-350mV input 1V out it doesn't really matter). With DIP socketed op amps, 1uF polypropylene caps, transformers, SO-220 regulators, etc... no point using SMD for the few remaining passives.Oh, and the AES nominal crest factor for audio is 17dB. I've measured 20dB on uncompressed audio from a high end DAC though (Auralic Vega). You need better than 17dB headroom for transients if you don't want to clip them. Clipping transients tips looses that sense of clarity and makes audio sound subtly muffled and textureless, as if you played an instrument inside a cardboard box, but without the cardboard box resonance. (Unlike clipping closer to the mean where it starts sounding raspy and then badly distorted.) I'd use 15V rails and 10V signal peaks. Op-amps perform better at closer to max rail voltage, so using 15V instead of 12V should be a no-brainer. Just drop in 7815/7915 instead.Good luck!

author: bson
author_rank: Unknown
timestamp: March 01, 2016, 07:25:23 pm
content: Oh, and move the fuses to immediately following the secondary taps. This prevents blowing up your regulators and/or regulator caps if you drop a screw driver in there.

author: microbug
author_rank: Frequent Contributor
timestamp: March 01, 2016, 07:27:45 pm
content: QuotePerhaps I'm missing something, but while 1Vrms is loud I do get there when listening to my DT-770s (80 ohm) on my phone in a loud area. 80 ohms is low as headphone resistance goes -- surely you need more than 100mV to drive 600 ohm headphones?!

author: Monkeh
author_rank: Super Contributor
timestamp: March 01, 2016, 07:38:11 pm
content: Quote from: microbug on March 01, 2016, 07:27:45 pmUhm, no, it's not, by a long shot. I have some going down as low as around 20 ohms. My every day pair are 54, and my folding pair 32..

author: microbug
author_rank: Frequent Contributor
timestamp: March 01, 2016, 07:44:10 pm
content: Fair enough; I'm comparing to the 600 ohm headphones that bson listed. The point remains.

author: dom0
author_rank: Super Contributor
timestamp: March 01, 2016, 08:27:58 pm
content: Quote from: microbug on March 01, 2016, 07:27:45 pm1 V into the DT-770 Pro 80 ? means about 110 db SPL. That's quite some average volume...

author: microbug
author_rank: Frequent Contributor
timestamp: March 01, 2016, 08:29:28 pm
content: Yes - I have rarely used it at that level but it seems counterintuitive to have an amplifier that actually has less max output than my phone

author: dom0
author_rank: Super Contributor
timestamp: March 01, 2016, 08:34:41 pm
content: This isn't about maximum output, the point bson (I assume) was making is that the normal listening condition has low levels and thus THD+N tends to be limited by N, not actual THD. Maximum output must be much higher than that anyway, to account for the dynamic range and crest faktor of audio. I'd say for 250+ ? phones you want at the very least 10 Vpp voltage swing. Actual operation should never touch the voltage swing limits.

author: bson
author_rank: Unknown
timestamp: March 02, 2016, 12:22:55 am
content: Quote from: microbug on March 01, 2016, 07:27:45 pmIt also depends on the sensitivity, which is typically high for a high-impedance driver.

author: microbug
author_rank: Frequent Contributor
timestamp: March 03, 2016, 08:31:56 pm
content: OK, time for a megapost.Quote from: bson on March 01, 2016, 06:39:46 pmToroids are slightly more expensive than an external power pack. Given the added size that they would inevitably bring to the enclosure, I'm inclined to go with an AC-AC wall transformer solution, with a voltage doubler. Do I need an EMI filter for that, given how much capacitance is present?Quote from: bson on March 01, 2016, 07:06:59 pmDesigning an amp from scratch is not a road I want to go down. Much as the O2 (on which this design is heavily based) could be improved, it does an excellent job of driving nearly any headphone on the market (see attachment screencap orthis pagefor details), and past a certain point (which IMO the O2 easily achieves) there is no audible difference when an amplifier is improved.Quote from: bson on March 01, 2016, 07:06:59 pmALPS Blue Velvet does look better. I'll go with that, thanks.Quote from: bson on March 01, 2016, 07:06:59 pmOK, will do.Quote from: bson on March 01, 2016, 07:06:59 pmI'll mostly use this with my phone, which has a TRS output. TBH I can't think of an occasion on which I might need a 1/4 inch input (read: yes, I know that others may need one), but since I'm already getting a 1/4 inch output jack I might as go with that for input too. Adapters are cheap enough.Quote from: bson on March 01, 2016, 07:06:59 pmYou said it yourself — the differences between op-amps are very minor; they're likely inaudible past the placebo effect. While I'm not interested in testing different op-amps, others likely are and the power dissipation of DIP-8 is around twice that of the alternative DMP-8 (especially relevant with the increase to a 15V power supply). The transformer won't be inside the enclosure though, and regulators are available in D2PAK / TO-252 (they're probably better for power dissipation in a surface mount package with a large ground plane than freestanding TO-220). Polypropylene caps are admittedly large but I only need four: I'll stick with surface mount for the majority of components. Call it pointless but a small form factor is appealing to me and makes any future conversion to a portable amp easier. Even if I have to use the same enclosure for SMD vs PTH, the PCB will be smaller and therefore cheaper.Quote from: bson on March 01, 2016, 07:06:59 pmOK, I've changed it. The original author of the O2 writes that the max dissipation with 15V power supplies is 1.1W per channel (i.e., per IC), which is very high for a DIP-8: I'll put a small heatsink over the output buffer chips. Good PCB layout should help there too (vias to the back ground plane).Good idea.Quote from: bson on March 01, 2016, 07:25:23 pmGood idea, I will.Quote from: bson on March 01, 2016, 07:06:59 pmThanks, I appreciate it.Thanks for all your help. New schematic attached.