C2L seems in reverse polarity to me. I'd like to avoid any cap bigger than 10uF to stay with small ceramic ones, and maybe modernize other things if you find flaws... Can you help me ? :)
Edit: after looking at different class-AB amplifiers on google, most of them don't have the caps equivalent to C2L, C3L & C4L. Maybe they are meant to optimize something ?
This is a B class amplifier to begin with.
Decoupling capacitor C1 placed wrong. Should be split against GROUND. This configuration of decoupling will not help much.
I planned initially to use a digital amplifier IC but since we want the ability to drive high impedance headphones (600 ohms) with studio-quality someone told me it would be better to use a classic approach with a ~1W class-AB amplifier, powered with +15V/-15V rails.
We found this schematic, from http://sound.whsites.net/project113.htm (http://sound.whsites.net/project113.htm) :
You say there is nothing studio quality about the simple amp, then you slap here a discrete one that will be likely even worse due to having rather lower open loop gain. And then I see C4: "C4 has been added to provide a greater internal gain." ... making the transfer function of the voltage amplifier stage inherently highly nonlinear. No real measurements of that amplifier provided.
I will not comment on the other one with a dozen of paralleled buffers, that is just one typical audiophool :bullshit: right there.
If you want fancy pancy, look for a diamond buffer, these work decent. Not all of them, but here you get an idea:
https://www.atlhifi.com/wp-content/uploads/2016/02/headamp-2-bp.png (https://www.atlhifi.com/wp-content/uploads/2016/02/headamp-2-bp.png)
Ooh I see ! The polarity of C2 confused me since it will see positive and negative voltages, but it's not the first time I see electrolytics used like that, it doesn't seems to matter when the current/voltage is limited enough with a resistor. I was planning to replace them all with 22uF ceramic caps.
b) Short circuit protection => you're right. Since the circuit is ready to drive already high-impedance headphones (600+ ohms), isn't the simplest solution simply to replace the 10 ohm resistors by a higher value ?
No real measurements of that amplifier provided.
Input impedance (without P1) 10kΩ Bandwidth 3.4 Hz - 2.4 MHz THD + Noise (1 kHz, 1mW/33Ω) 0.005% (B = 22 kHz) THD + Noise (20 Hz - 20 kHz, 1mW/33Ω) 0.01% (B = 80 kHz) Signal to noise ratio (ref. 1mW/33Ω) 89 dB (B = 22 kHz) 92 dBA Max. voltage (into 33Ω) 3.3 V (THD+N = 0.1%) Max. input voltage 0.57 V (with P1 set top maximum volume)
The 1 nF close to the OpAmp inputs can make a big difference on sound quality in a RF poisoned environment.
...
I did not propose to use ceramic capacitors in the audio path, but only for C3 and C4, that are uncritical in this respect (effects eliminated by negative feedback). The 10 uF i proposed for C2 can be film, maybe 2x 4u7 50V.
...
Concerning short circuit safety: It's fairly easy to test how long your final design survives a short cicuit at reasonable audio levels and how it dies. Certainly you don't want a fire.
You're right pointing that, I was forgetting that those voltages and current handling are absolute max in ideal cooling conditions (my developer background isn't helping here!). Since the sot-23 package probably can't dissipate even 0.1W without getting hot, we'll go for the SOT-223 with some pcb aera forming a heatsink.In the ear phone range there are quite some amplifier chips available. Depending on the power needed (too much is not good for the ears anyway), one could even consider 2 channels of the NE5532 in parallel. No real need to go for a discrete design at this power level. AFAIK it is not that uncommon to have some series resistor (100 Ohms range) at the output, so that a 600 Ohms and 32 Ohms speak could produce a similar sound level.
This is a B class amplifier to begin with.
I am not sure if two diodes are enough for any bias to flow through the output stage, are they? Such current will be mostly ill-defined.
The best IC for a headphone amp I've tried is the LM7171 , however it is not an easy opamp to deal with, requires a good layout and P/S decoupling, plus some careful filtering on the input and the output.That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble. :palm:
Cheers
Alex
If the NE5532 is used, as suggested, the polarity of C2 is wrong, it will be biased by the opamp input current (about -0.3uA for NE5532) x 22K feedback resistor or about -6.6mV on pins 2(6). It would be better to reverse the C2 polarity with NE5532.Not a big deal given that elcos can safely take up to 500mV of reverse voltage (IIRC) but I am actually autistic enough that I would do the same ;)
AFAIK it is not that uncommon to have some series resistor (100 Ohms range) at the output, so that a 600 Ohms and 32 Ohms speak could produce a similar sound level.Certainly not common on modern low voltage portable devices driving modern low impedance headphones as it would unacceptably limit power output. And people say that headphones these days are designed to be driven from zero impedance, so there is that.
The problem is increased distortion, even if the capacitor isn't harmed, although it's probably negligible in this case.If the NE5532 is used, as suggested, the polarity of C2 is wrong, it will be biased by the opamp input current (about -0.3uA for NE5532) x 22K feedback resistor or about -6.6mV on pins 2(6). It would be better to reverse the C2 polarity with NE5532.Not a big deal given that elcos can safely take up to 500mV of reverse voltage (IIRC) but I am actually autistic enough that I would do the same ;)
That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble. :palm:
Speed is good, but having a lot of it just makes life harder, with no benefit.
It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.
Yes, I got the decimal point wrong when looking at the current noise graph. I must've been looking at the scale for the voltage noise!
That's a video/RF op-amp. Using it for a headphone amplifier just seems like asking for trouble. :palm:
Speed is good, but having a lot of it just makes life harder, with no benefit.
It will also be very noisy, especially at bass frequencies around 50nV/√Hz and 60pA/√Hz, at 30Hz! The NE5532 with a suitable BJT follower on the output will be much less noisy and less prone to oscillation.
The LM7171 is the best integrated solution I've heard. On the noise side - it is not a problem in a line level amp, it is not a phono stage! The current noise is about 7pA/√Hz at 30Hz, not 60, and 2pA/√Hz at 1kHz, so a 10K resistor on the input will only increase the noise by about 3dB on top of the voltage noise, which is fine. I've designed a respected headphone amp using this chip about 20 years ago, nobody ever complained about the noise level ;) .
Cheers
Alex
No doubt it can be done, but why bother? There are far more suitable solutions which are far more stable and are better suited to the application.
Too bad if there's a risk of RFI, especially with long cables. :palm:No doubt it can be done, but why bother? There are far more suitable solutions which are far more stable and are better suited to the application.
The reason is very simple - the sound quality when used with various real world headphones.
Cheers
Alex
When i read claims that an amplifier sounds different, my first thought is: Maybe it got a problem. Any amplifier with low output impedance, a reasonably flat frequence plot and less than 0.1 % THD up to 10 KHz at somewhat elevated output levels sound the same even for a trained listener. That amplifier is easy to build using modern parts.I mostly agree but beware of THD figures which don't always tell the full story. Some types of distortion are noticeable at low levels of THD, such as crossover distortion, whilst others such as slew rate limit distortion aren't noticeable until much higher THD numbers. In this case, my suspicion is the LM7171 could be ringing at certain frequencies which could be seen as sounding brighter, boosting treble response.
Most tube amplifiers sound "interesting" since they don't fulfil these criteria (higher output impedance and THD more like 1 or 2 %). Higher THD simulates higher sound level..
Regards, Dieter
When i read claims that an amplifier sounds different, my first thought is: Maybe it got a problem. Any amplifier with low output impedance, a reasonably flat frequence plot and less than 0.1 % THD up to 10 KHz at somewhat elevated output levels sound the same even for a trained listener. That amplifier is easy to build using modern parts.
Most tube amplifiers sound "interesting" since they don't fulfil these criteria (higher output impedance and THD more like 1 or 2 %). Higher THD simulates higher sound level..
Regards, Dieter
I think the Rod Elliott circuit shown in the beginning is a little more rational. Depending on the OpAmp it will certainly outperform all those diamond circuits.
I've looked at the OPA2134 and the fact the datasheet mentions 600ohm load and +/-18V VCC is very interesting.No. the datasheet says the voltage loss when driving a 600 ohm load is the negative supply plus 2.2V to the positive supply minus 2.5V. So if the supply is +/-15V the output swing into 600 ohms is 25.3V p-p which is 9V RMS which is 135mW which would be very loud. Without the positive swing clipping a little more than the negative swing then the output power into 600 ohms with extremely low distortion is 130mW.
However it mentions the voltage output to be max +/-2.5V at 600 ohm, which is I=U/R = 4mA. 4mA @ 2.5V is about 10mW, which seems low for those purposes.
Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection. Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.
Any measurements should be performed with the speaker or headphones connected to the amplifier, via a suitable length of cable. I'd say this is even more important when you're using an IC out of specification, as would be the case for the LM7171 driving headphones, rather than the purely resistive load it was designed for.
Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection. Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.
The LM7171 in a DIP8 package can output about 0.7W RMS into a 100 Ohm load continuously with a +/-15V supply, do you even need that much for a headphone amplifier? On the other hand, driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life (or put it differently, why measurements don't tell a complete story) and why so many different amplifier designs exist.
Cheers
Alex
Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection. Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.Yes, the LM7171 isn't suitable for use as a headphone amplifier. There are much better alternatives. Just because it can be done, it doesn't mean it's the best way. :horse:
Another solution which avoids using discrete parts is use several audio operational amplifiers in parallel to get the needed power dissipation. If rail-to-rail output parts are used, then increased output power for a given supply voltage will be available also. There are several ways to configure operational amplifiers in parallel.Yes, it also results in lower noise and distortion.
driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real lifeTrue that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.
Any measurements should be performed with the speaker or headphones connected to the amplifier, via a suitable length of cable. I'd say this is even more important when you're using an IC out of specification, as would be the case for the LM7171 driving headphones, rather than the purely resistive load it was designed for.
Yes, different amplifier designs exist for different purposes, which is why using a video amplifier for audio is daft, even if some people like the colourful ringing and distortion it might add to the sound.Distortion and noise will be higher on the LM7171 because of lower open loop gain and lower common mode and power supply rejection. Perhaps more importantly, the output power is limited by limited output voltage range and limited power dissipation from a single 8 pin package.Yes, the LM7171 isn't suitable for use as a headphone amplifier. There are much better alternatives. Just because it can be done, it doesn't mean it's the best way. :horse:
driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real lifeTrue that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.
I'm not aware of any instances of such demonstration actually being done. I suppose it would be all over the Internet if it happened, given the amount of controversy.
:palm:I don't doubt that it's possible to build a headphone amplifier with the LM7171, just that it'll make life harder, with no benefits, over ICs specifically designed for it.
I personally find this amusing. I have shared some information which I consider valuable, as it is the result of many hours of work at the time I did that design. I am not trying to sell anything. I have a practical experience using that particular chip for that particular application in a production headphone amplifier which was produced for many years and in many thousands of units. And I get theoretical arguments from people who most likely never tried to work with that particular IC and not even read the data sheet carefully (as it clearly states the LM7171 can be used for driving ADSL lines and transformers for instance). ADSL drivers in general are not a bad choice for headphone amplification (low distortion, high output current and ability to work on difficult loads) and there are designs which use them*.
Cheers
Alex
P.S. Worth noting that TI actually sells the THS6012 Dual Differential Line Driver as a "High Fidelity Headphone Amplifier" chip under a different name (TPA6120A2) ;) .
I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real lifeTrue that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.
I'm not aware of any instances of such demonstration actually being done. I suppose it would be all over the Internet if it happened, given the amount of controversy.
It is getting completely off-topic, however I will only ask a simple question: can you provide a link to an example of a statistically valid controlled DB listening test with a positive result?
Cheers
Alex
I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
I still don't know what you're going on about. Please provide a more detailed explanation of what you're asking.I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
So, you can not provide a link to a statistically valid DBT with a positive result (meaning that there is a difference in the sound confirmed in the test) ? So, you have a "gold standard" with negative results only, don't you think it is funny?
Cheers
Alex
I still don't know what you're going on about. Please provide a more detailed explanation of what you're asking.I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
So, you can not provide a link to a statistically valid DBT with a positive result (meaning that there is a difference in the sound confirmed in the test) ? So, you have a "gold standard" with negative results only, don't you think it is funny?
Cheers
Alex
I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
Yes, a DBT is a tool. If the results are that no one can differentiate between two amplifiers, any better than chance, then the two amplifiers both sound the same. This is a scientific fact. If no one can tell the difference between the two amplifiers, then saying the they are sonically different is lying. The same is true of a quack claiming their new spiritual healing treatment cures cancer: without a DBT it's total lies.I still don't know what you're going on about. Please provide a more detailed explanation of what you're asking.I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
So, you can not provide a link to a statistically valid DBT with a positive result (meaning that there is a difference in the sound confirmed in the test) ? So, you have a "gold standard" with negative results only, don't you think it is funny?
Cheers
Alex
OK, let's go back to your earlier post:I'm not sure what you're asking? Double blind tests are a the gold standard for determining whether anything is detectable or effective. If your LM7171 amplifier is audibly transparent i.e. doesn't distort the sound, the difference between it and any other amplifier which is also audibly transparent, should be undetectable when subject to a double blind test.
The DBT is a tool. To use any tool you should understand its capabilities and limitations, otherwise you can not trust the results. My question is very simple - could you provide an example of a statistically valid DBT with a positive result, i.e. where the difference between the sound from two pieces of equipment was registered. Clearly, if you have 50% distortion on one of the sources it should give a good positive result, but no one will bother with this kind of test. However unless we have examples of positive results it is not clear where the test sensitivity border line is. Saying that it is "industry standard", "gold standard" and such doesn't prove anything, it is only a convenient smoke screen. DBTs in audio are notoriously insensitive, that is why their use is essentially a biggest can of a snake oil in this industry, IMHO. Well, again, could you provide a link to an example of a statistically valid audio DBT with a positive result?
Cheers
Alex
Yes, a DBT is a tool. If the results are that no one can differentiate between two amplifiers, any better than chance, then the two amplifiers both sound the same. This is a scientific fact. If no one can tell the difference between the two amplifiers, then saying the they are sonically different is lying. The same is true of a quack claiming their new spiritual healing treatment cures cancer: without a DBT it's total lies.
No doubt two amplifiers can sound different and will give a positive result when subject to a DBT. This is likely to be the case if the amplifiers are over-driven or genuinely do produce distortion in normal operation. Some people might like certain types of distortion, so keeping THD figures down isn't always the goal.
The idea people can hear things which can't be measured is snake oil. It's true the human ear is more sensitive to some types of distortion than others, so THD figures alone aren't always that useful, but everything which is audible is measurable and is the whole point in having expensive test equipment.
No doubt tests have been done to determine what level of distortion is audible, tolerable, and immediately noticeable, in order to develop efficient lossy audio compression algorithms and determine the bare minimum standard for an audible transparent audio amplifier.
To answer the question: no, I've not seen an example statistically valid DBT with a positive result, comparing two amplifiers, but that doesn't mean to say that DBTs aren't valid. It most likely means that all decent amplifiers quality sound the same, as long as they're operated within their specifications.
Any audio-related discussion inevitably ends like that ;DYes it does get on my nerves how audio threads invariably turn to audiophoolery and trolling. I apologise for my part in it and am no longer responding to it.
I know your suggestions here may be using higher quality parts, but what about this one :That's a good idea. I'll add that to my list of decent op-amps.
http://www.ti.com/lit/ds/symlink/rc4580.pdf (http://www.ti.com/lit/ds/symlink/rc4580.pdf)
It's cheap, has some oomph to be used as a headphones amplifier, has low enough THD/noise... and the implementation seems dead easy.
I'm a musician but I like when choices are made with measurable, scientific proofs, since audio as it is perceived by our ears is way too much subjective. Some opamps with thousand of V/uS slew rate and hundred of Mhz bandwidth may sound very good but I feel it's just unneeded complications (unneeded costs too), like you can buy a race car to drive at 30mph in a city...but WHY ? ;D
driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life
True that, but after all those years of hearing about it I would like to finally see a demonstration of two amplifiers which both show no perceptible noise, 0.1dB gain flatness from 10Hz to 50kHz and some low THD figure, all measured into the actual demo speaker, and yet sound different enough that somebody (bonus: everybody) can tell them apart.
I'm not aware of any instances of such demonstration actually being done. I suppose it would be all over the Internet if it happened, given the amount of controversy.
driving a speaker coil is not equivalent of driving a resistive load, that is one of reasons why amplifiers with similar traditional measurements could sound different in real life
The reactive part of the load changes the phase relationship between the current and voltage which can be seen in a class-b design where the crossover distortion will move away from zero volts. But in a well designed amplifier, it makes no practical difference assuming that the reactive load characteristics were designed for.
A more important effect is that the reactive load shifts the current to voltage across the output devices which can result in violation of their breakdown rating. The protection circuits and output device derating need to take this into account.
The main problem is not that loudspeaker is a reactive load (at least that is a reasonably measurable and easy to deal with effect) , but that it is a very non-linear load, and not only load but a source as it acts as a microphone picking up vibrations and sounds. All this rubbish gets back to the amplifier and enters the feedback loop, as well as other drivers in a multi-way speaker.
Yes, speakers and room acoustics are the weakest links in the chain as far as distortion is concerned and dominate over even mediocre amplifiers. The audio amplifier is a problem which has been solved a long time ago. The only improvements made recently involve reductions in sized and increased power efficiency.The main problem is not that loudspeaker is a reactive load (at least that is a reasonably measurable and easy to deal with effect) , but that it is a very non-linear load, and not only load but a source as it acts as a microphone picking up vibrations and sounds. All this rubbish gets back to the amplifier and enters the feedback loop, as well as other drivers in a multi-way speaker.
I have difficulty imaging that being a problem with the low dampening factor at the feedback point of a solid state amplifier combined with the cable and speaker coil resistance.
Speakers are specifically designed to be linear because otherwise excessive intermodulation distortion is produced by the speaker itself. This takes the form of using a narrow voice coil within a large field of magnetic flux or a wide voice coil with a narrow field of magnetic flux. Woofers present the most difficulty because they must support a large throw to produce good loudness at low frequencies and base-reflex enclosures help considerably in this respect by lowering the impedance seen by the driver.
Interaction between drivers should be prevented with active or passive crossovers.
But I would agree that a lot of speakers intended for the consumer market are trash. But hey, the customer knows what sound they want.
Yes, i agree there is a huge difference between somebody ignorant using some sound system and a technical person using a sound system designed or combined by himself. Sound causes feelings! I know lots of well educated technical people who go crazy about horn speakers and the selection of the right foil capacitor.Sound causes feelings, so many of the perceived differences in amplifiers are often down to the placebo effect, rather than anything real. There is nothing wrong with this, until people start to make extraordinary claims, which cannot be proved.
But human hearing is extremely flexible.
Think about the differential analog transmission still used in professional audio. The input stage can be made such as to yield -6 dB when one of the two contacts in the connection fails. Or it can be made to yield -60 dB in that case. Now you may wonder how many of those connections in the real world are jumping between 0 dB and -6 dB without anybody taking notice.
Regards, Dieter
It is getting completely off-topic, however I will only ask a simple question: can you provide a link to an example of a statistically valid controlled DB listening test with a positive result?I can't because I don't care, but of course such tests must have been done if there are people talking about "scientifically established" thresholds of audibility of various defects.
Say that I swapped a few paralleled NE5532 into one of those LM7171 amplifiers of yours, would you tell it apart?
I will add an LM7171 amplifying its own noise and mix it with the input signal, just for you :P
With a gain of 23? Possibly if you have some very sensitive earbuds.Say that I swapped a few paralleled NE5532 into one of those LM7171 amplifiers of yours, would you tell it apart?
I could; the LM7171 will produce noticeably more hiss because of its greater input noise. This might not be noticed in a noisy environment however.
317 is quite low noise by itself. Doubt it will have any major contribution to the amp output noise.