Low noise headphone amplifier design - Page 1
Forum section: electronics / projects

author: Kawakneurder
author_rank: Contributor
timestamp: December 19, 2019, 04:27:13 am
content: Hello everyone! Nice to meet you all.I have been a long time lurker on these forums, but I have decided to make my first post a big one.As a short introduction: Currently I am 26 years old, a dutch native but temporarily living in Japan for an internship. I have been busy tinkering with electronics since 10 years old, having done plenty of personal projects and later competing in student teams in university. Currently I am finishing up a masters in EE with a specialization in robotics and mechatronics, but have done plenty of courses in other fields such as microwave electronics, biomechatronics, advanced programming, discrete circuit design etc etc.Recently I have decided to brush up on low noise and precision opamp circuits. Being a master student in EE, I have of course had experience with opamps, and I have used them in a fair amount of circuits, but never beyond relatively simple applications. Usually courses just hint at the existence of noise and offset voltages, but do not really delve deeply into it.For figuring this all out, I thought a great project would be a headphone amplifier design. I was thinking to buy a new set of higher impedance headphones, so a driver would be required for those anyways. Although offset voltages and currents are not as important, noise and distortion are in this kind of project. It seems that it would be fun to just design something, and itterate and improve on it as I go along.I have not really ran into any issues yet, so this isnt a ‘help required’ post, but more for documenting the project such that others can enjoy the process as well, and that people can chip in their feedback so that we can all learn something.So let’s dive straight into it!The specifications I put up for the project are:Frequency range of 10hz to 20khz (max 100mdB attenuation)Input impedance range of 32 to 600 ohmsMaximum output power of 100mW (with Beyerdynamics DT880s this would be 116dB spl!!)Distortion figure as low as possibleNoise figure as low as possible, (more than 120dB signal to noise ratio)Use as many jelly bean components as possibleAt least 10h battery lifeFrom this we can deduce the expected voltages and currents needed.Impedance RMS voltage P-P voltage RMS current P-P current 321.83.25680802.8435502505720286007.75111318Thus this calls for a peak to peak voltage of at least 11V, and a minimum maximum current of 100mA.There is fairly little information available about output signals of phones and computers, and number vary wildly. Numbers I read range from typical line outputs of 0.45V RMS to 2V RMS for computers, and 1V RMS for phones. Considering this, the following table shows the amplification required for a given input:Inputsrc / Impedance3280250600Line46.21117Phone1.21.863.37.75PC0.91.42.53.875As can be seen, these values seem to vary wildly as well. Reducing volume with a pot reduces the signal to noise ratio, so we want as high of a gain as possible without turning down the volume.. Currently I want to have a three way switchable gain to give three options. A ‘pass-through’ setting without gain, a ‘regular’ option with an amplification of around 4, and a ‘high’ setting with a gain of around 10. This would nicely be respectively 0, 6 and 12dB gain. For now, the amplifier is based on a gain of around 4.For supplying the power, I would like it to be portable. An added bonus of batteries is that we obviously don’t have to worry about added noise from mains or a DC-DC converter. I would like to avoid any switchmode converters for obvious reasons, so the easiest choice would be a pair of 9V batteries, giving plenty of voltage headroom and a nice split rail for the circuit. However, they have limited capacity and are relatively expensive.For now, I think I want to use a 6 cell long, series cascaded string of 18650 batteries. They are easy to find, cheap, have a reasonable power density, and are not too big. This would give a total voltage of at least 21V, or +-10.5V rails. At the maximum current draw of 56mA RMS max for a 32 ohm load, this would mean a total current draw in the range of maybe 100mA. Even at this extreme end it would equate to a 22 hour battery life at full volume, easily satisfying my battery life criteria.I am thinking of maybe implementing a series battery charging and balancing circuit later, so that the device can be plugged in and charged on the desk. However, I will first try to create the amplifier circuit and then worry about these additional challenges.For the system level overview, I have considered mainly two typologies. These can be seen below. Excuse the crudity of the model, didn’t have to build it to scale, or to paint it.Configuration 1Configuration 2In configuration 1, there is a voltage buffer buffering the signal that goes to the attenuator, then followed by a gain block. In configuration 2 the gain block and buffer have been switched around. Note that the buffer is needed such that we can reduce the impedance of the attenuator.The advantage of configuration 2 is, that all the noise after the gain block is attenuated by gain G when referred to the input. The main disadvantage is however, that if the input level is big, it can cause clipping and therefore extreme levels of distortion. Changing the volume would not solve this. You would have to change the volume in the PC or phone itself, which would cause an effective reduction in the number of bits of the internal DAC (and thus increasing noise).I.e. if well adjusted and not clipping, configuration 2 has a superior noise floor, making the input and gain stage the dominating factor in noise performance. Configuration 1 is more insensitive to large input signals.I opted to go for the more non-traditional configuration 2.I have already come to a basic circuit design:The choice of opamps falls onto bipolar types. This I chose mainly because the phone/computer/Line presents a reasonably low impedance source, thus a bipolar type can be used to minimize voltage noise. Secondly, the distortion of common, relatively cheap and easy to find bipolar opamps is exceptionally low.Naturally, the first choice is the NE5532/5534 because of their exceptional performance to price ratio.An obvious disadvantage of this chip is their relatively high current draw, but with the choice of batteries this probably will not be an issue. If each opamp draws about 8mA maximum quiescent per pair, this presents about 16mA of current total. With a regular 18650 cell being able to provide about 2200mAH, this would equate about 140h standby battery life. Plenty for me.Another disadvantage is of course the relatively large input offset voltage, however this should not be an issue because it is an AC coupled amplifier (although there is a gotcha later in the design!).Because of the low impedances I have chosen not to bother with calculating the input current noise of the opamps as this is probably negligible. I should verify this still.As for the input circuitry, C1 is obviously a DC blocking capacitor. It is chosen to ensure that the cutoff frequency (together with R4) ensures that there is at most 0.1dB attenuation through the audioband. (10hz to 20khz). R4, next to providing the low pass filtering, is necessary to present a ‘ground’ to C1 and a way for the input bias current of U2 to flow. Otherwise, the input bias current would charge up C1 and eventually saturating the opamp. Obviously a bad deal. I do not want to increase C1 much because it would mean slow rise times. R4 should be sufficiently low to reduce Johnson voltage noise, yet big enough to give a good response for the low pass filter. Bigger resistance values also create a higher input impedance (making the amp easier to drive). I have found 6.8k/22uF to seem a reasonable compromise, giving around 10nV/sqrt(Hz) noise. This is still about 2-3 times more noise than the input noise of the opamp, so maybe this should be reduced and the filter capacitor increased. I have also been toying with the idea of putting another opamp as a voltage buffer at the input, such that these resistors can be decreased without any issue, but I am afraid that an unpredictable large DC offset from a random source would saturate the buffer, and that a relatively large capacitance at the output would make the buffer unstable.R1 is an input protection resistor. Following the original specifications of protecting against +-60V input transients, it limits the current into the NE5532 to under 10mA. This is of course assuming a very rapid transient rising quicker than the opamp slew rate. Considering that U2 is a bipolar opamp, it is probably relatively sturdy against transients anyways, so I am not too worried about it. Maybe I should add some TVS diodes for protection extra though.U2 is of course a classic non-inverting amplifier configuration. The resistors chosen are such that I do not load U2 unnecessarily high (overloading an opamp significantly increases distortion) yet still present relatively low noise. I did the noise calculations on R2 and R3 and their contribution is negligible. The gain provided is about 6dB (4.3x voltage gain).R5/R6 make the voltage divider (potentiometer) for controlling the volume. I am slightly worried that mismatches in the dual pot would create an imbalance in the two channels. Maybe I should add some way to tweak this, if possible.Another thing that I am not sure of, is if the very low DC voltage over the pot would be a problem. Of course the input of U2 is AC coupled, however the input offset voltage of U2 amplified would still create a DC signal of about 15mV at the output max. Is anyone with more experience with audio equipment able to verify whether this enough to cause the ‘scratching’ sound? If so I would need to decouple the pot from U2 as well.C2 and R7 again form a low-pass, to remove the (amplified) DC offset of U2 to make the voltage over the output with no signal as close to 0V as possible to protect the headphones against DC voltage. As far as I can see the solution now seems the best possible, because I do not want to increase C2 beyond 22u for space/size reasons, yet a 10k resistor still gives 2.8nV/sqrt(hz) input referred noise, which is far below the noise of R4 anyway.U1 drives an AB class output stage, simulated here with BC547/556 transistors, but probably BD140/BD139 is a safer choice for power handling and maximum current. To reduce distortion, two diodes are used as bias spreaders and biased at 1/10th of the expected maximum output current. This is because the hfe of the BD pair is at least 100, so 1/10th of that seems a safe bet. Maximum peak to peak current of 79mA gives a base current of ~8mA. Currently they are biased around 6-7mA. However, this is now supply voltage sensitive, so I was thinking to replace this with a current source/sink pair. Without that, voltage regulators would not be necessary, right?On the topic of regulators, I would rather avoid them since even the venerable 78xx series seems rather noisy, the 317 being especially bad. Never mind that there is not a lot of voltage headroom anyways. Because of the low voltage headroom it would also be impossible to use for example a capacitance multiplier circuit to reduce the noise. An LDO could be used, but they are more noisy than regular regulators as well.Lastly the load. Usually it is decoupled with a fairly big decoupling capacitor, that is not present in this circuit. Properly working, there should be at most the maximum offset voltage of U1 present over the load, which would be 5mV. With the lowest impedance headphones, 32 ohms, this would be less than 1 uW of power. Am I correct in thinking that this is generally safe for headphones to handle? I do worry about possible failure conditions of the output transistors putting the full supply voltage over the headphones… From my experience BJTs tend to fail shorted.Simulation results:Initial simulation results seem quite promising, this is the output to a 2V p-p input signal:This is with an output load of 250 ohmsThis is with a worst case load of 32 ohmsThe first (gain) stage introduces most of the second order harmonics, which is comfortably down by about 117dB for the typical 250 ohm load. The output stage contributes most of the odd order harmonics, which are even further below. Considering the second harmonic dominates the distortion, It can be estimated at about 0.00015% THD. Not too shabby I think.For the heaviest, worst case load of 32 ohms, especially odd order harmonics increase somewhat as expected, but we still have about 115dB of SNR. Barely any higher than 250 ohms.However, I am curious about the rise in noise floor for lower frequencies. Is this an artifact in LTSpice or the way I produce the FFT plot? Or is it inherent in my design? If the latter, then this is obviously unacceptable.If I have to believe the higher frequency noise floor though, it seems to be at about -160dB, giving a signal to noise ratio of almost 170dB, which is far exceeding initial specifications and seems very promising. I do think noise is currently dominated by the input impedance of the circuit, as well as the opamp input noise. Am I correct in that? For the rest, distortion seems to be mostly dominated by the output stage. Are there any ideas to improve performance here, next to constant current sources?And the frequency response with a load of 32 ohms:The 1khz, -0.1dB and -3dB points marked. Seems all good!About the actual components, the resistors need to be metal film types for low noise. For the decoupling capacitors I am not sure if standard aluminium polarized ones would suffice. Or would I need film types?I am also considering some kind of relay output switching, but still not sure about it.Do you guys have any ideas, input, comments or feedback? It would be highly appreciated! Thanks!edit: Forgot to add the frequency response graph!

author: awallin
author_rank: Frequent Contributor
timestamp: December 19, 2019, 06:13:08 am
content: do you have the measurement-gear to verify the performance numbers? QA401 or similar?would it make sense to build an O2 first, learn to verify its performance and get reliable numbers, and then think about improvements?http://nwavguy.blogspot.com/2011/08/o2-details.htmlIt seems the more experienced EE designers you ask the less they rely on LTSpice - so beware of simulations...

author: mrjoda
author_rank: Regular Contributor
timestamp: December 19, 2019, 06:47:22 am
content: Check LME49610.

author: Kawakneurder
author_rank: Contributor
timestamp: December 19, 2019, 06:59:18 am
content: Quote from: awallin on December 19, 2019, 06:13:08 amNot yet, but I am thinking of purchasing one. First step though is getting a decent amplifier design on paper and that performs well on paper.That QA401 is definitely interesting though. I wonder what they put inside it.Quote from: awallin on December 19, 2019, 06:13:08 amI know about the O2, and it seems like an excellent design. However in that sense, it is already fairly optimized and I doubt that I can make significant improvements on it.More importantly though, the project is mostly a learning experience for more challenging opamp design and finesse. There are plenty of 1 chip solutions (like mrjoda mentioned) that solve the entire problem quite satisfactory for 99% of people. Then again, where's the fun in putting a chip on a board and seeing it work!On the same note as the O2, this 3 opamp circuit of Karl Adams also seems excellent, definitely an interesting design:Quote from: awallin on December 19, 2019, 06:13:08 amI know, but currently I am in Japan with no access to my personal lab, and limited access to simulation software. For now, it has to do. Row with the oars you have got and all that.

author: magic
author_rank: Super Contributor
timestamp: December 19, 2019, 10:13:08 am
content: SPICE simulations of Linear Tech opamp distortion are completely unrealistic. You can set them up for 60dB gain and still get ppm-level distortion. Look up Samuel Groner's opamp distortion measurements paper to see what happens to opamps in real life when they work at 60dB noise gain. I don't know what NE5532 model you are using, but be careful.It seems you ended up with an O2 anyway, plus a discrete output stage.NE5532 is not that great on noise, NE5534 or NJM2068 is quieter, AD797 and some LME49xxx and OPA16xx parts are better still.Erm, wait. Your input series resistor already gives 10nV/rtHz noise. Forget low noise, select for distortion only

author: Kawakneurder
author_rank: Contributor
timestamp: December 19, 2019, 11:02:15 am
content: Quote from: magic on December 19, 2019, 10:13:08 amThis opamp is not from LT themselves, but from TI I believe. I indeed do not know how realistic it is, but frankly I don't have any better model.Samuel Groner's page does seem mighty interesting, I will read through it tomorrow.Quote from: magic on December 19, 2019, 10:13:08 amI didnt have a proper look at the O2 schematics before, and thought they were decently more complicated, but having a look now it does seem that it is not very different. Just that he seems to use some higher power opamps to directly drive the output.Quote from: magic on December 19, 2019, 10:13:08 amI have considered the 5534, however, one opamp is in a buffer configuration which would mean it needs additional compensation capacitors, and I think these decreasing the gain would virtually make them identical to the 5532 (AFAIK the 5532 is just a unity gain stable internally compensated 5534, with two in one package). Also, the two opamps in a single package is nice for space savings.Quote from: magic on December 19, 2019, 10:13:08 amRight, so I am wondering how this problem can be solved. I need decent input protection, mandating a high input impedance, low roll-off frequency, mandating a high input impedance, low loading of the source, which means a high input impedance, yet low noise, which would mean a low input impedance.To be honest it feels kind of like I am at a standstill and I cannot figure out on how to improve this at the moment (without increasing the capacitor to more ridiculous levels). Do you have any ideas how to improve this dilemma? Of course I could just omit the protection resistors all together, but that doesn't feel very sound from an engineering perspective.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 19, 2019, 02:26:13 pm
content: If you can get a copy of Doug Self's small signal book and his power amp book, they have some interesting solutions including parallel opamp designs.http://douglas-self.com/

author: SiliconWizard
author_rank: Super Contributor
timestamp: December 19, 2019, 02:36:18 pm
content: Just a couple things:* There are many available opamps these days that will give you better noise/distortion figures with less power draw (LME49720 is an example);* You could do without the first amplification stage IMO and include the voltage amplification as part of the feedback loop in the final stage.

author: David Hess
author_rank: Super Contributor
timestamp: December 19, 2019, 05:20:46 pm
content: Quote from: Kawakneurder on December 19, 2019, 04:27:13 amThat is flicker (1/f) noise. It would not be so high except that the source resistance is too high for the input current noise from the NE5532. Low frequency noise tends to be inaudible so can often be ignored. Midband noise is much more of a problem.For lowest noise, the input stage needs moderate gain to overcome the noise of the following stage. On the other hand, latter stages can be low impedance making them lower noise anyway so this is not a demanding requirement.I question whether the lowest possible noise will be an advantage in a headphone amplifier when the source will likely be noisy anyway. I suspect a low noise non-inverting JFET input with low to moderate gain will be good enough.

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: December 19, 2019, 05:23:22 pm
content: As a general design rule, remember that noise is set in the first stage. Nothing that comes after can improve it.

author: Kawakneurder
author_rank: Contributor
timestamp: December 20, 2019, 12:39:09 am
content: Quote from: Conrad Hoffman on December 19, 2019, 02:26:13 pmActually I have his 'small signal audio design' book, definitely and excellent read and one of my original motivators to start this project.I was not aware that he had a power amplifier book though. I will check that out! So far, for power amplifiers I liked Bob Cordell's "Designing audio power amplifiers".Quote from: Conrad Hoffman on December 19, 2019, 05:23:22 pmThat is what I suspected. For now it seems most noise improvement can be made in the input circuitry, whilst it seems that most distortion happens at the output stage (Especially odd ordered distortion).Quote from: SiliconWizard on December 19, 2019, 02:36:18 pmI am fully aware that there are better opamps available, however they are also higher price.More importantly, I am not terribly interested in end performance perse, but in how to achieve optimum performance out of a circuit, pushing the existing components to their limits.The LME49720 seems to be an almost direct drop in replacement for the 5532 though (no doubt aimed at audiophiles), so it would be interesting to see how they perform with some minor tweaks to surrounding circuitry.EDIT: I come back to this! In terms of noise, the LME49720 would perform worse in this circuit, mainly because of the first stage. The voltage noise is about half, but the current noise about double. As discussed in this same post, the current noise through R1 is the dominant contributor in noise I believe.However, the LME does have substantially lower distortion which arguably is more of an issue in the actual audio fidelity of this amp.Quote from: SiliconWizard on December 19, 2019, 02:36:18 pmI have thought of that, but my thought was that the volume control would need to be higher impedance, creating significantly more noise. That was the idea of putting a buffer before the volume control, after which I had the thought of switching around the gain and buffer stage.Quote from: David Hess on December 19, 2019, 05:20:46 pmYou are right. I did the input current noise calculation and it seems that R1 has a worst case noise contribution of almost 20nV/sqrt(hz), definitely being the primary contributor of noise.I was not aware that the low frequency noise tended to not be audible, so that is definitely a relieve, because this noise contribution drops to about 5nV/sqrt(hz) for 1khz. Thanks for the explanation though, it makes a lot of sense. Some quick ballpark calculations show that the noise of a 5532 in this circuit is very similar to the noise of a jellybean TL072 FET input amp. I suppose the input protection resistor really is the limiting factor here. Is there any way to improve this inherent noise whilst still retaining input protection?On another note, it just occurred to me that the noise contribution by R4 is useless anyways. This because for an AC signal it is shorted through the source anyways, at most contributing its inherent noise voltage multiplied by Rs / (Rs + R4) making it not important. Am I correct in thinking that? In this case it would be nice to increase this resistor, and with it decreasing the input coupling capacitor.Quote from: David Hess on December 19, 2019, 05:20:46 pmThat is a fair point, if the noise floor really is below 120dB, at least a 20 bit DAC would be needed to have the appropriate SNR with just quantization noise. That would mean only a half decent 24-bit DAC would be good enough to not be the primary contributor of noise, and even then the software should not be controlling the volume such that the DAC can use its full output range.Do you have any suggestions on how to minimize output distortion without just whacking in some more fancy pancy parts, especially for the second stage?

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2019, 04:23:10 am
content: Quote from: Kawakneurder on December 20, 2019, 12:39:09 amThe common thing to do is to bypass R1 with enough capacitance to short out its noise contribution but this relies on a low source impedance. This decreases protection for high frequency AC but has no effect at low frequencies.The easiest thing to do is use a low noise high input impedance input like with a JFET amplifier so that the value of R1 can be increased without increasing its noise contribution too much. And now the bypass capacitor can be smaller for a given noise reduction and the source impedance is less important.If I was going to design a no holds barred high impedance integrated input stage, I would start with a large gate low noise JFET operational amplifier. (1) The problem with a large gate is that it means high input capacitance which varies with common mode voltage contributing distortion. Further, JFETs have lower common mode rejection and lower open loop gain contributing to higher distortion in the non-inverting configuration. So I would bootstrap the operational amplifier from its *inverting* input node. Now the power supply voltages follow the input signal and common mode effects are reduced including input capacitance.The bootstrapping can also drive the input protection network composed of low leakage shunt diodes relaxing the series resistance requirements somewhat.This would be overkill for any general purpose audio application that I am aware of. Even better performance could be achieved with a discrete or composite implementation of the same circuit.(1) This could also mean multiple JFET inputs in parallel which lowers noise but also increases input capacitance.QuoteThat is correct; the high frequency noise from R4 is bypassed into the source through the input coupling capacitor assuming that the source is lower impedance than R4.R4 needs to have a relatively low value in this case because of the high input bias current of the NE5532. With a lower input bias current, the value of R4 could be considerably higher making bypassing its noise more effective.QuoteThe first thing is to use inverting amplifier configurations where possible because doing so removes common mode distortion. This is generally feasible on every stage except the first stage.The next thing I would do is unload the output of every amplifier which is driving a low impedance load. This means adding buffers within the feedback loops which might be as simple as class-a emitter followers but voltage followers or buffers could also be used. The reason is that thermal feedback from the output transistors to the input differential pair in an integrated part limits open loop gain and causes low frequency distortion.

author: Someone
author_rank: Super Contributor
timestamp: December 20, 2019, 06:03:45 am
content: Quote from: David Hess on December 20, 2019, 04:23:10 amWalt Jung presented a nice starting point in Analog Devices Application Note 232:https://www.analog.com/media/en/technical-documentation/application-notes/742022599AN232.pdfWith some more modern devices adding gain in U2 could also be interesting.

author: magic
author_rank: Super Contributor
timestamp: December 20, 2019, 07:48:21 am
content: To fully bypass R1 noise you need to set the RC corner frequency below audio band, at which point you might equally well rely on the R4C1 highpass for protectionI think the common solution is not to bother. Use low enough series resistance to avoid unacceptable noise and rely on ESD diodes to handle transients and on C1 to block DC which could overheat the diodes in the long term. Sometimes a sufficiently strong transient pulse may blow something up, but it's rare. We did however have a thread like that in the repair forum a few days ago.Attached is line input schematic from Realtek ALC899 soundcard datasheet. If you look at O2, I think it isn't even AC coupled and uses only 250Ω series resistance. You can look at some other amps or soundcards, it's probably similar story. Don't even look at expensive audiophool gear and certainly don't post schematics here without trigger warning.By the way, notice the capacitors to ground. Oftentimes they even go behind the input series resistor. The purpose is to shunt RF picked by the cables into ground and prevent it from demodulating into AF junk in the input stage. So that you don't hear that beeping and buzzing noise when a cellphone is near.And if you want to have that series resistor, kindly put it somewhere where input bias current doesn't flow through it

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2019, 08:53:30 pm
content: In his book Small Signal Audio Design, Douglas Self does not do anything special to reduce distortion and mostly relies on the NE5322 operating with an input impedance of about 5k.

author: magic
author_rank: Super Contributor
timestamp: December 21, 2019, 10:41:53 am
content: Probably related to Self's other belief that there is no practical difference between 100pppm and 10ppm distortion.These days some $100 headphone amplifiers are advertised as single digit ppm and there is even a relatively influential audio reviewer who measures a lot of them and performs informal sighted listening test where the good measuring ones invariably sound good too.Audio is a branch of marketing, not engineering

author: David Hess
author_rank: Super Contributor
timestamp: December 21, 2019, 11:34:32 am
content: I suspect it is more about what is easily and economically achievable. Better performance than a NE5322 operating with an impedance of 5 kilohms requires significantly more complexity for an improvement of 12 dB at most which most sources are not going to support anyway.

author: Kawakneurder
author_rank: Contributor
timestamp: December 23, 2019, 01:32:02 am
content: Quote from: David Hess on December 20, 2019, 04:23:10 amBut this in effect reduces the input protection as well. But I see that there is no other way to achieve protection AND lower noise.Quote from: David Hess on December 20, 2019, 04:23:10 amI was afraid so, and already expected this. It cannot be helped then.Quote from: David Hess on December 20, 2019, 04:23:10 amI have seen this bootstrapping idea a few times in low noise designs now. It is definitely quite interesting but I need to read more about it before implementing it. Do you have any particular articles/books as a recommendation for reading into opamp bootstrapping?Quote from: David Hess on December 20, 2019, 04:23:10 amThis is very helpful advice. I suppose especially the last point is not something you easily find in book etc for design. Thank you very much for the advice.Quote from: Someone on December 20, 2019, 06:03:45 amI indeed had the same idea about adding more gain in the second stage, which is most simple with a more modern opamp. Having a quick look for example the AD797 seems to be excellent with ~10x the unity gain bandwidth and similar or better noise figures, although it is a bit pricey.The application note is a nice read though. I quickly brushed over it now, and will read it later today.Quote from: David Hess on December 20, 2019, 08:53:30 pmYes I noticed, that is why I came to the forum to see if there are other ideas on reducing distortion and noise. Again nothing practical if only to learn more about it.I think though that the biggest challenge in reducing noise and crosstalk will be in the PCB design.Quote from: magic on December 21, 2019, 10:41:53 amI fully agree, and now that you remind me, I was in a big electronics store yesterday and saw all kinds of silly audiophoolery going on. From 800$ cables to 15k$ single vacuum tube amps. Whatever floats someone's boat I suppose.Quote from: magic on December 20, 2019, 07:48:21 amAlright, I think I will change the design then to lower valued resistors since it does not seem to need a lot of protection. And add a few ESD diodes as well then instead of relying on the internal 5532 diodes.Quote from: magic on December 20, 2019, 07:48:21 amI just pulled up the O2 schematics and it seems you are right. It isnt AC couples and just has a 100-250series resistor. But I have heard horror stories of some audio devices outputting a DC voltage of a few volts, so I prefer not to risk that.Quote from: magic on December 20, 2019, 07:48:21 amI was already wondering why these designs included that capacitor! I assumed it was to have a slight roll-off but to reduce the noise bandwidth, but that didn't make a lot of sense to me since the DAC already has an anti-aliasing filter. This makes a lot of sense though. Definitely will include this into the design. I guess the ferrite beads in the schematic also help with this.Also I do think a harder challenge on this design would be to have a battery charging circuit that does not couple noise into the supply rails. I know that the op-amps have reasonably good PSRR, but if a boost converter is injecting noise into the rails that is certainly audible I think. Is it usual to use linear battery chargers in portable headphone amps?At a later stage I thought it would also be fun to design a DAC as well, so maybe the DAC and charger could be combined into a single design, and such that when the amplifier is connected to the DAC, it is also charging.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2019, 03:41:33 am
content: Quote from: Kawakneurder on December 23, 2019, 01:32:02 amThere may be other ways depending on what constitutes a failure. The input series resistor could be replaced by back to back depletion mode MOSFETs to limit fault current while presenting a low resistance. Or a small incandescent bulb can be used as a combined PTC thermistor and fuse but of course if it blows as a fuse, that is at least a soft failure.A current driven diode bridge can be used for overload protection on inputs and outputs; it can only pass up to a specific current. However it may add too much noise and distortion and it not suitable for high impedances. I have not looked into it for audio purposes.QuoteRaising the input impedance is the easiest thing to do and it makes input protection easier.There may be a place for using a low noise dual JFET as a differential preamplifier before a suitable operational amplifier but this raises the parts cost and complexity considerably. Essentially the input stage of the operational amplifier is replaced by a discrete JFET stage. This is common where the lowest possible input noise is required while maintaining a low input bias current.But JFET input operational amplifiers are pretty good these days. A composite design could improve the noise by a factor of 4 or 12 dB from 4nV/SqrtHz (ADA4625, LT1792, LT1113, OPA1656) to 1nV/SqrtHz which is almost as good as the best bipolar input operational amplifiers.Figure 6-180 on page 600 of Walter Jung's Op Amp Applications Handbook shows a modern example of this. His book is available for free online now:http://www.kelm.ftn.uns.ac.rs/literatura/mpi/pdf/Op%20Amp%20Applications%20Handbook.pdfhttp://www.miedema.dyndns.org/co/2018/Op_Amp_Applications_Handbook-Walt-Jung_2005.pdfhttps://www.eit.lth.se/fileadmin/eit/courses/etef15/Kursmaterial/Op_Amp_Applications_Handbook-Walt-Jung_2005.pdfQuoteThe previously linked application note from Walter Jung pretty much covers it. Before JFETs became avaialble, it was commonly done in discrete bipolar designs only to raise input impedance but it has other benefits with both devices.QuoteThis issue with integrated circuits is mentioned here and there but in most designs it is not the most significant distortion mechanism so can be ignored. It contributes to extended settling time in precision parts. It becomes very important however in audio test instrumentation and precision circuits in general when other distortion mechanisms have been accounted for. Discrete designs are immune although there are other thermal distortion mechanisms which affect both discrete and integrated designs.QuoteThings are easier for the following stages so lessor performing parts are more acceptable. They can be made low impedance and inverting and their input noise and distortion is less important or irrelevant. Total noise and distortion is likely limited by the first stage because of its high impedance.QuoteLayout and especially handing return currents properly can be critical. I think Self's book on audio power amplifiers discusses these things and if not, Bob Cordell's book does. Something to beware of is that a solid ground plane is not a panacea for all ground loops.QuoteRF can be rectified by the input circuits causing offset changes. In the case of AM, this results in demodulation. It is a big problem with modern cell phones which use time domain multiplexing which results in "snickety" or low frequency buzzing sounds.QuoteIt is something to be aware of but most of the noise will be above the audio band so both easy to filter and largely unimportant.

author: Kawakneurder
author_rank: Contributor
timestamp: December 25, 2019, 03:33:21 am
content: First of all, thank you for the elaborate reply. I do appreciate it a lot.Quote from: David Hess on December 23, 2019, 03:41:33 amRight, makes sense. Then again as you mentioned before it is probably nothing to worry about. It would definitely be something interesting to look into.Quote from: David Hess on December 23, 2019, 03:41:33 amI have seen this setup used for different very low noise instruments yes, but as you mention cost and complexity go up a lot.Quote from: David Hess on December 23, 2019, 03:41:33 amIt keeps amazing me what these chip designers manage to squeeze out of some silicon. Something that would be unheard of 20-30 years ago can now be purchased for less than a bag of crisps.Thank you for the link of the book! I think I should not just look at that example, but read through the whole book. It seems mighty interesting and definitely a lot of food for thought for me.Quote from: David Hess on December 23, 2019, 03:41:33 amYes I have noticed that. And this has had me thinking, would it be better to use the dual opamp per channel, or 'share' the input amplifier opamps with both channels, and the output opamp with both channels? I.e. one opamp that does the amplification and the other that does the output buffering.I have read before that in general you should avoid routing left and right input channels to the same opamp, to reduce crosstalk between channels. But I can imagine that if the buffer stage runs warmer, it might induce more of this thermocoupled distortion in the amplifier stage, whilst otherwise the amplifier stage would reduce the (input referred) noise of the buffer amp.Quote from: David Hess on December 23, 2019, 03:41:33 amIt seems most people do not even recommend a solid groundplane, but instead always rely on a star ground. Seehttps://www.diyaudio.com/forums/power-supplies/138482-lt-dual-tracking-3a-regulated-power-supply-1-25v-20v.html?pagenumber=2&pagenumber=2https://www.diyaudio.com/forums/chip-amps/142712-star-ground-vs-ground-plane.htmlHowever, this does seem more relevant with higher power amplifiers more so than headphone amplifiers.I think I will try to do a star ground, even if it is just for trying to design a system with star ground once.Quote from: David Hess on December 23, 2019, 03:41:33 amOkay, that is noted then. I suppose this is one of those things just to try out and see if it matters. I was afraid that it somehow might modulate or resonate to create audible signals.On that note, since my last post I have decided to first build a mains powered amplifier, and later build a portable one. For now I have picked 12V rails followed by a capacitance multiplier to get rid of some of the linear reg noise. See the schematic below.As for the circuit, I have integrated the changes to the input circuitry, including the cap for getting rid of RF noises.I have also calculated the proper gains required, and changed the biasing of the output transistors with current sink/sources.Also, some of the harder to source components with long lead time have been ordered such as the case, RCA/phono jacks, opamps (both NE5532 and LME49720, just for fun to see if the difference is actually audible at all) and expensive audio log pots.Now I will get started with the PCB design.Schematic so far:EDIT: Fixed some dumb mistakes in the schematic

author: Calvin
author_rank: Regular Contributor
timestamp: December 25, 2019, 05:45:20 am
content: Hi,just looked at the latest schematics and spotted several issues.input circuitry:I'd start with a 100k to gnd, then the series dc-blocking lytic, followed by a smaller R (100R-1k) and a 100k to gnd, paralleled with a film cap (220p-1n). The first 100k drain any possible charge build-up of the lytic and forms together with the second 100k the 50k ac input impedance. The 100R-1k resistor and the small film cap form the lowpass protecting the amplifier from hf entering. Putting a 100n cap first doesn't form a effective filter, but just places a serious burden on the preamplifier at the other end of the cable.-amplifier gain:switching gains by switching the feedback resistors results in loud, possibly destructive pops, since at switch over the OPAmp might loose feedback, running with openloop gain. Also a small cap of a few pF is typically connected in parallel to the feedback R (for bandwidth limiting and stability). Switching caps is a true source for loud pops. Instead keep a single R plus parallel cap only and switch the gain by changing the resistor from feedback to gnd. That'd temporarily reduce the gain to 1 at switchover and remains almost inaudible.-buffer:I'd suggest to use 3-4 biasing diodes and emitter resistors.-buffer ccs:Using a red led in place of the two diodes and couple led and transitor thermally will give You a more stable ccs.-supply rail decoupling:just as a tip. Besides the small 100nF caps at the pins of the OPAmps also place at least 10uF up to 100uF lytics as close as possible.regardsCalvin

author: Kawakneurder
author_rank: Contributor
timestamp: December 25, 2019, 06:39:44 am
content: Hey Calvin, thanks for your inputQuote from: Calvin on December 25, 2019, 05:45:20 amThe capacitor was wrongly dimensioned in the circuit I see. It should have been 100p which would have made a negligible contribution. The setup you propose makes a lot more sense though.I was afraid that that 100k after the input cap would give a large DC offset being amplified by the opamp, but after calculating it seems it would not exceed 500mV at the highest amplification setting.Thank you for the suggestion, I will change it in the circuit.Quote from: Calvin on December 25, 2019, 05:45:20 amThis makes an awful lot of sense when I think about it, and it is strange to me as to why I missed this when I designed the gain switching circuit in the first place. Then again, it would have not been a big deal, since I have thought of it such that you set the gain for the headphone impedance you intend to drive and then leave it there. It is of course better if it can be switching without pop, so thanks for the notice!Quote from: Calvin on December 25, 2019, 05:45:20 amMight I ask what the advantage of this may be? The emitter resistors is of course for emitter degeneration, but what about the multiple biasing diodes?Quote from: Calvin on December 25, 2019, 05:45:20 amOkay, I will replace it with a 3mm led.Quote from: Calvin on December 25, 2019, 05:45:20 amI left them out since I thought I would be tight on space, however, I have found there is plenty of space on the board, so I will probably put some 22uF caps in there as well (since they are already on the BOM).For those reading this thread at a later date, I have updated the schematic in my original post to reflect these changes.

author: Zero999
author_rank: Super Contributor
timestamp: December 25, 2019, 02:38:03 pm
content: You can increase the output voltage range by AC coupling the bases of the transistors to the output of the op-amp.

author: David Hess
author_rank: Super Contributor
timestamp: December 26, 2019, 05:31:19 am
content: Quote from: Kawakneurder on December 25, 2019, 03:33:21 amOne way increases crosstalk and the other way increases thermal effects. Buffer requirements do not match input amplifier requirements so using a single dual for both functions is not ideal for performance.QuoteCrosstalk is more significant than low frequency thermally induced distortion.Since the buffer is within the feedback loop, its distortion and noise is reduced by the excess gain. The buffer might be as simple as a single transistor class-a emitter follower. The high speed and high output current capability of a current feedback amplifier makes it a good candidate for a buffer.Note that delay through the buffer directly reduces phase margin so it needs to be faster than the operational amplifier and frequency compensation may need to be added to preserve stability.Quote from: Zero999 on December 25, 2019, 02:38:03 pmOr use a common emitter output stage like that shown below from Jung's book. It is not really suitable for a dual part like the NE5532 unless both operational amplifiers are run in parallel but maybe that would be worth it given the low cost of the NE5532.

author: Calvin
author_rank: Regular Contributor
timestamp: December 26, 2019, 08:39:57 am
content: Hi,here´s a design relying on only one OPAmp per channel plus bipolar Booster. Nothing fancy nothing new, but provenOne of the largest noise contributors would be the series input resistor R2 (~12nV/sqrt(Hz)) and the feedback-to-gnd resistors, even if their overall noise contribution to circuit noise remains small.The OPAmp noise dominates. So I wouldn´t worry too much about lowest R-values, especially if they´d become so low that they may create other problems.If You wanna stick to Your cicuit then following small traps still exist:The feedback resistors (R13,R14) and the potis are so low in value that the NE5532 might be loaded down too much (600R capability iIrc).Also there´s no cap across R13 and R14. Might be needed for stability and is limiting the bandwidth to a sensible value. Something between 22pF to 100pF should suffice.I´d also connect the largest feedback-to-gnd (100k here) permanently and parallel the resistors for higher gains.Choose straight dB gain steps (preferrably 3dB or 6dB steps). 3dB steps are already small regarding audibility.Switch positions of the potis and C27, C28 and increase R21, R22 to 100k. The output offsets of U3A, U4a will very probabely be alot larger than the input offset of U3B, U4B. the potis form a bias current path to gnd, so R21, R22 then only function for safety in case the poti wiper looses contact.You already added emitter resistors. Besides reducing tolerance sensitivity of the output transistors they allow for precise setting of the idle current and make measurement of idle current easy.Oh, a final tip: add testpads to Your PCB -or those special test-pins where you can hook up a Osci-pobe to.regardsCalvin

author: David Hess
author_rank: Super Contributor
timestamp: December 27, 2019, 02:11:25 am
content: Another thing I have seen in some practical designs is instead of using a larger and more expensive BD139/BD140, several smaller but faster parts like 2N4401/2N4403 transistors (1) were used in parallel with the emitter resistance made up of the ballasting resistors which enforce good current sharing. This would be more important with surface mount parts where power dissipation per part is more limited.In the past there were lots of good and fast medium power transistors like the BD139/BD140 but they are not as common now.(1) If your budget is unlimited, use those perforated or ring emitter TO-92 sized parts which are commonly available but more expensive now. Unfortunately they never became common in the TO-225 package.

author: Kawakneurder
author_rank: Contributor
timestamp: January 06, 2020, 07:46:56 am
content: First of all my apologies to everyone for the late reply. I was traveling during the holidays, and had no access to proper internet.In any case, a belated happy new year!Quote from: Zero999 on December 25, 2019, 02:38:03 pmCurrently I do not need more output voltage swing, but I am toying with the idea of making a lower voltage battery powered version that could very well benefit from this. Nice little trick, thanks!Quote from: David Hess on December 26, 2019, 05:31:19 amThats as I expected as well then. I already figured that crosstalk was likely going to be a bigger issue.Quote from: David Hess on December 26, 2019, 05:31:19 amI am a bit confused what you imagine with the buffer stage. Do you have any reference or circuit diagram?Quote from: David Hess on December 27, 2019, 02:11:25 amI picked the BD139/140 pair purely because I still have a lot of them in stock myself (over 100 for each type) so cost is not an issue. It is my 'standard' go-to medium power transistor. As for now I am not planning to produce these in high volume at all, maybe would give a try later but like you suggest I would step over to full SMD assembly for that. I have thought of the paralleling transistor idea before, except with common bc547/847/557/857 parts because the current requirement of the device is quite low. Power dissipation for a transistor is under 400mW so putting a few of those in parallel should be sufficient too. If the previously mentioned idea of yours with the buffer also uses one of these transistors, it would be even more attractive for mass production by reducing the BOM.Quote from: Calvin on December 26, 2019, 08:39:57 amHey Calvin, thanks for your input!Of course there are tons of proven designs out there, I especially like the O2 design as well, but again, this is mostly an exercise and designing something from scrap for fun.Quote from: Calvin on December 26, 2019, 08:39:57 amI have been thinking about this too, however the problem solved itself since I could find some nice quality audio pots, but only in 50k versions. Will stick with those for now.Quote from: Calvin on December 26, 2019, 08:39:57 amYes I thought so as well. The 5532 should be unity gain stable so I will try the design out without, and if it oscillates we already have a reasonable suspicion of where it comes from.Quote from: Calvin on December 26, 2019, 08:39:57 amWhat might be the advantage of this? Less influence on the circuit when changing values of the switch?Quote from: Calvin on December 26, 2019, 08:39:57 amThis is a good idea. I am still not settled on the exact values, because I do not know yet how much gain is needed exactly. I was thinking 0 6 12 20 dB (1x 2x 4x 10x voltage gain).Quote from: Calvin on December 26, 2019, 08:39:57 amThat makes a lot of sense. I will definitely change this. Tomorrow I will come with an updated schematic.Quote from: Calvin on December 26, 2019, 08:39:57 amOh yeah, I have learned this the hard way already a long time ago. Never fun measuring a chip leg when your probe slips off, accidentally connecting a high input voltage of an (of course expensive) dc dc chip to another pin that cannot take a high voltage...

author: David Hess
author_rank: Super Contributor
timestamp: January 06, 2020, 05:44:35 pm
content: Quote from: Kawakneurder on January 06, 2020, 07:46:56 amZero999 is suggesting the use of AC coupling to the bases of a class-AB emitter follower output stage to increase the maximum output voltage. I suggested a rail-to-rail common emitter output stage as shown below. (1) My way is more susceptible to distortion.Check outLinear Technology application note 18for more examples of how to buffer the output of an operational amplifier.QuoteThe BD139/BD140 are also on my short list of medium power audio transistors as well as the 2SB1143/2SD1683.(1) Update: Also shown below is the same idea from the 1973 Fairchild ua741 datasheet.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: January 07, 2020, 11:20:46 pm
content: 2 Alternative paths:A few years ago, some electronics Magazine published an article with the title "opamplifier".It used a whole lot (32?) of NE5532 opamps in parallel to make an audio amplifier.As far as I know the NE5532 works well downto an output impedance of 600 Ohm, so with 32 parallel you should not go below 18 Ohm, and it seems you would need 150 of them parallel for a 4 Ohm speaker...But still, I find the idea intriguing.NE5532 is a pretty cheap opamp, so putting a bunch of them parallel can be done with a modest budget, and putting them in parallel lowers the noisefloor.In the Elektor article about this amplifier there was also a section which listed some relevant differences of NE5532 opamps from different manufacturers, and some differences (noise?) were quite significant.Search to get you started:https://duckduckgo.com/html?q=%22opamplifier%22Different, but similar project:https://ggianluca.wixsite.com/opamplifierAnother design is published in one of the application notes of the LME49720.It uses (of course) the LME49720, combined with a class A buffer in chip form (I think it was BUF600)This search:https://duckduckgo.com/html?q=lme49720+headphone+amplifier+angets me to:https://www.ti.com/lit/ug/snaa052a/snaa052a.pdfIt uses the LME49600 which is probably an updated version of the BUF600. (BUF634?)It also shows links tohttps://www.diyaudio.com/] https://www.diyaudio.com/ which probably is the best forum around for DIY'ing with electronics on Audio related stuff.

author: IDEngineer
author_rank: Super Contributor
timestamp: January 08, 2020, 05:31:00 pm
content: QuoteSorry for the late response to this comment.Audio is what got me into electronics long, long ago. One of the more interesting results from my long slog in that discipline was that noise is FAR more important than distortion, as long as the latter is not ridiculously extreme.A key element of this realization is that regardless of how awesome your straight-wire-with-gain signal path may be, the transducers on either end of it distort by at least integer percentages. Input devices such as microphones distort by several percent. Speakers can approach 10%, and even the very best, cost-is-no-option speakers are still well over one percent. And that's BEFORE you consider intermodulation distortion due to the fact that you're asking a small number of transducers to reproduce the full frequency spectrum. Headphones are no exception to this rule.There are certain near-massless drivers, such as flame and plasma speakers, that might yield very low distortion but 1) I've never had access to them so I haven't been able to confirm this theory, and 2) the microphone(s) used to evaluate them would themselves distort their output, making interpretation difficult. One possible answer to the latter might be to measure their behavior optically... I'd love to play with that.A more subtle aspect is that distortion is at least related to the original material, and (while I have no hard evidence for this) I believe human hearing is more forgiving of related foreign content than it is of (psuedo-)random foreign content that has no relationship to the original signal. Put bluntly: Even "untrained", non-golden ears can identify very small levels of noise yet may be unable to discern small levels of distortion. Noise is just obviously not supposed to be there.For these reasons, I long ago stopped worrying much about distortion levels below one percent. When dealing with analog (and especially audio), my focus is on noise prevention.

author: magic
author_rank: Super Contributor
timestamp: January 08, 2020, 06:57:55 pm
content: The standard audiophile answer to that is that transducer distortion is low order but electronics distortion can easily be high order and therefore different. Case in point, class AB.1% is a bit crude. Typical measurement puritans like Self put it around 0.01%These guys claim to have easily passed ABX at, if I got my math right, 2% THD.https://benchmarkmedia.com/blogs/application_notes/power-amplifiers-the-importance-of-the-first-watt

author: IDEngineer
author_rank: Super Contributor
timestamp: January 09, 2020, 06:31:06 pm
content: Quote from: magic on January 08, 2020, 06:57:55 pmGranted it's easy to design audio amps with well below 1% THD. My point is that it doesn't matter much if the transducers in the signal path are one or more orders of magnitude worse. Many amplifiers spec "THD + N" but if you look at the numbers it's the N(oise) that dominates at most listening volumes, hence my focus on the dominant problem.WRT the article about distortion at low power levels: I completely agree and am surprised that detail hasn't gotten more attention. It surfaced as a point of discussion back in the 70's but I haven't heard much about it since.

author: magic
author_rank: Super Contributor
timestamp: January 09, 2020, 11:00:44 pm
content: You can hide behind transducers if your distortion is 2nd or 3rd order and presumably "compressive" (peaks are soft-clipped rather than expanded). Otherwise this argument doesn't really work.But the 0.01% argument sounds believable. And those who say otherwise have a peculiar tendency to complain that existing tests are unrealistically difficult to pass but at the same time make no effort to suggest better tests, more resembling of their preferred listening conditions and presumably easier to pass, but still fully blindMany people intuitively assume that low power must be "easier" than high power, and they would even be right if not for crossover. That being said, serious designers and fanatic hobbyists do talk about it and THD vs output level should be a standard test. You may just not hear it on your standard issue Internet forum full of noobs reposting wisdoms they have read from older noobs a week ago without even giving it much thought.

author: IDEngineer
author_rank: Super Contributor
timestamp: January 09, 2020, 11:51:14 pm
content: Quote from: magic on January 09, 2020, 11:00:44 pmClipping isn't the only, or even dominant, mode of transducer distortion (though I'd certainly say it's the most objectionable!). The primary problem with transducers is that they aren't massless. You can roughly equate that mass to stray capacitance - the driver cannot respond instantaneously. Thus (bad) there is always lag regardless of how high a damping factor exists between the amp and speaker, and (worse) that lag/distortion is frequency dependent. Then add the wholly separate effects of IM distortion, and it's easy to see why transducers are the weak link in the signal path.Microphones have the same problem, again due to mass. Even without clipping. Indeed, ALL mechanical transducers suffer from this effect. We demonstrated it in turntable cartridges, both moving magnet and moving coil. Cutting heads would have it too, though we didn't test that. It's lovely that the purely electronic signal path can have such low THD but unless the source is purely electronic (synthesizer, theremin, etc.) and until we tie the output to a cochlear implant to eliminate the need for an electro-mechanical ear interface, the math says that low electronic distortion is mostly an academic exercise.QuoteI probably don't hear it because I tired of the whole audiophoolery mindset decades ago, so I don't hang out on audio Internet forums. Audio topics do surface occasionally on real EE forums like this one, and once in a while I let my guard down and comment on an audio topic. I generally regret doing so, but I don't seem to have learned the lesson yet.

author: magic
author_rank: Super Contributor
timestamp: January 10, 2020, 05:57:55 pm
content: As far as I understand mass is not the problem but nonlinear suspension complicity and nonuniform magnetic field is. And of course lack of stiffness, cone breakup, etc.Mass simply causes 12dB/octave rolloff of excursion under constant voltage drive. And reportedly you want it, because SPL is proportional to cone acceleration, which rises 12dB/octave under constant excursion.This is assuming that mass is constant, which it may not be when the amount of mass that is subject to movement varies with excursion.

author: IDEngineer
author_rank: Super Contributor
timestamp: January 10, 2020, 10:53:17 pm
content: Quote from: magic on January 10, 2020, 05:57:55 pmYes, there are a lot of other variables. I skipped them because otherwise this thread will start to resemble a lot of the audiophoolery forums out there. {grin}It's not just mass that creates frequency-dependent distortion. As you mentioned, stiffness/rigidity can play a role too. There can actually be standing waves on the cone itself, adding their own layer of distortion/mismodulation to the fundamental(s). Suspension... the degree of approximation of an "infinite baffle" behind the driver (vs. compression/rarefication, which would further affect the driver's ability to track the signal)... it's endless.I won't pay extra for audio distortion levels below one percent. That's just throwing money away, at least until electro-mechanical transducers get a LOT better. But I will pay for a lower noise floor.

author: David Hess
author_rank: Super Contributor
timestamp: January 12, 2020, 03:12:04 am
content: Microphones operate much more closely to small signal conditions so can have much lower levels of distortion than speakers. If a non-linear transfer function is operated over a small enough range, then distortion can be reduced to negligible levels.Speakers are a completely different matter and low frequency speakers which require large cone excursion are especially problematical. This neatly explains why base-reflex enclosures which reduce cone movement at the lowest frequencies for the same output offer such an advantage in low frequency distortion. The mechanical impedance transformation that a horn provides returns the same benefit by reducing driver excursion.Multi-way speakers with appropriate crossovers reduce intermodulation distortion from a single speaker cone which comes from lower frequencies moving the cone and Doppler (FM) shifting the higher frequencies.

author: Käsebrot
author_rank: Newbie
timestamp: January 13, 2020, 03:04:08 am
content: Regarding potentiometers: You could take a look at the different versions of the ALPS RK97 Series if you haven't already. They are commonly used in higher grade audio equipment and still reasonably cheap. Then there is the larger and more expensive RK27 "Blue Velvet" (Japanese audiophoolery...) series. Both of them come from 10k to 100k I think. Some of them are available at competitive prices on LCSC, but I suggest to use the alps or some other vendors website to figure out which one you'd like, as their naming scheme isn't that intuitive and LCSCs search is as crappy as ever.Also LME49720 = LM4562, at least thats what most people on the interwebz as well as TI and the datasheets agree on. The LME naming scheme is a relict from National Semiconductor, in this case the TI portfolio overlapped with the NS one and they decided to keep both parts. Unless you believe the conspiracy theories some people are spreading, just buy the LM4562 instead. There are also a few newer interesting OpAmps that can drive headphone loads pretty well, for example the OPA1688. They are like 1USD a piece on LCSC, even in a quantity of just 1.http://www.ti.com/lit/ds/symlink/opa1688.pdfhttp://www.ti.com/lit/ds/symlink/opa1622.pdf

author: Kawakneurder
author_rank: Contributor
timestamp: January 20, 2020, 07:23:44 pm
content: Hello everyone, thanks for all the messages.My input in the distortion debate would be, I tend to agree with the mindset that you want an 'as ideal as possible' sound chain, such that the headphones can be appreciated for what they are. Sure the headphones will have incredibly bad distortion and frequency response compared to the amplifiers, but this is what gives them their character. Kind of how some people prefer vacuum tubes even though they (generally) objectively are a lot worse in performance than solid state amps.In general, I think with modern opamps it is not very hard to design a sufficiently low noise, low distortion amplifier that is such good performance to exceed the limitations of human hearing. And again as I mentioned in the OP, this project is mostly about learning and improving opamp design skills.Quote from: Käsebrot on January 13, 2020, 03:04:08 amI actually found a 10k RK27 pot for not too much money, so I am gonna put that one in. I am still not 100% sure if its a genuine one or not, but only one way to find out. I can always exchange it for something else if its not satisfactory.Quote from: Käsebrot on January 13, 2020, 03:04:08 amI was not aware that they are the same chip, but I already got some LME49720s now anyways. I doubt it will make an audible difference with the 5532's anyways, but will be fun to find out.Quote from: Doctorandus_P on January 07, 2020, 11:20:46 pmThis is definitely a very cool idea, out of the box thinking. Maybe I will indeed build this as a second headphone amp.As for the circuit, find the current schematic attached.I already had a PCB design for the previous schematic, but I have started over with the new design. It has been ordered, and hopefully it can be processed and shipped before the start of chinese new year in 2 days...For the rest, most of the components have already been ordered, so when the PCBs are ordered only the assembly still needs to be done (and testing).

author: Kawakneurder
author_rank: Contributor
timestamp: March 10, 2021, 08:26:53 pm
content: A (late) update! After a busy busy year, I have finally found some time to work on the amplifier again. Although lots of progress had been made! I don't like leaving these kinds of forum topics dead in the water without a nice conclusion, so thought it would be fun to update.At the time of my last post I think I ordered the PCB, some other components and was still in Japan. In the meantime I had received them and gotten back to my home country. I soldered them and everything worked perfectly......Well of course it didn't, but isn't that part of the fun of tinkering with electronics? Slightly disheartened and busy with many other things (such as a masters thesis) I put the thing on the backlog together with many other projects. Until today!Went on quite the wild goose chase, as to why the amplifier went into clipping, until I discovered that I had put in NE5534's I had lying around, instead of 5532'sThe capacitance multiplier also went into oscillation (although strangely enough only for the negative rail, the positive one behaved fine).After fixing these two mistakes the headphone amplifier did work, but shows some distortion, I think it might be one of the following two issues:I seem to remember that I accidentally ordered a transformer with lower output voltage than necessary, it might be fine on low signals but with higher current draw dip low enough to make the voltage regulators drop outMaybe it is the biasing in the output stage, I will have a look at this as wellI am happy to say though, that there's no 'pop' when turning the amp on, no crackle changing volume, no crackle or pop changing gain and no strange noises plugging in the headphone. In general the amplifier is behaving wellexcept for the distortion (which is worse at higher volumes).As for some other general improvements that I want to do for the design:I read that using 100n caps from Vp to gnd and Vm to gnd with the NE5532 is not a good idea. Using just a single 100n from Vp to Vm seems to be recommended.Add heatsinks to the voltage regs and output transistors. They don't get ridiculously hot (about 45C quiescence state), but just to be sure.Add some larger supply capsAdd some snubber caps to the bridge rectifierAdd an indicator LED connectorAnything that might come up in the meantimeFor now, I will work through these, and probably post an update if these are resolved. If the kinks are ironed out I will post the schematic somewhere as open source (which it kind of already is, but whatever).And finally, of course I can't leave without some quick pictures!(Don't mind the lazy wire lacing)Edit:The distortion was caused due to a shoddy soldered resistor for the current source in the output stage. Re-soldering it solved the issue. The amp sounds pretty good, indistinguishable from another headphone amp I have, but I still do think linearising the output stage is useful (if only as a fun exercise) and I will do that for the final design.