Andrew T, your note implied that you have a high-resolution hearing, and you know quite a bit about the factors which affect the integrity of the audio signal. I have 3 points: 1. DIYer Adason (above) and I ask whether you know of a CD player which has a volume-controlled headphone amp which "cuts the mustard"; commercial and high end calibers. 2. Please tell us more about the audio system which you are using, and how well does it jive with the teaching of DIYer Pano. 3. Given any caliber CD player at hand, the "fidelity" of the output signal emanating from the headphone amp can be readily compared with that appearing simultaneously at its RCA output by using a THD analyzer. The signal at the RCA port is the pristine reference in this comparison. This partly sheds light on the concern by Adason that OPA headphone amps are/ can be mediocre performers. The other meaningful comparison is between the signal from the headphone amp (already processed) and the signal from the RCA port after processing it in a preamplifier/attenuator etc. Both signals are thus compared side by for THD and noise, plus a listening A/B evaluation with the same power amp and loudspeakers. This is a fair and balanced multi-way signal evaluation to nail down relative performance; measurement and audible. Best regards.
Antoinel to adason:
Maybe you've seen the write-up on the Objective 2 (O2) Headphone Amp DIY. A lot of visiting traffic on it. The author used OPAs therein with a surprisingly (alleged) excellent performance. He basically externalized the internal headphone amp of a CD player; using its RCA output to feed it. One key difference between the internal CD headphone amp and its external counterpart is that it does not have or need an output current capability because it is driving a normally high input impedance of the power amp (>22 K). Essentially, the CD headphone amp is working in an almost STASIS (R) mode; which is an almost ideal performance. I believe that you are aware that STASIS(R) was invented by Nelson Pass.
Hi Antoinel,
Yes, I have seen that thread and I have built many OPA based headphone amplifiers myself. No, I do not like the sound of them neither as headphone amp nor as the output for amplifier. I find the sound of them to be sterile, antiseptic, uninvolving, tiring in comparison to good tube headphone amplifier.
The same applies if they are used only as buffers. I hope you are aware that most cd players are plagued by the pesky OPAs on the output, anywhere between two to six per side, yes, even six OPAs in a row in some. There is enormous improvement in signal quality if these OPAs are bypassed by single FET or tube stage. Google “lampizator” for more details. I have done it on three JVC cd players with great success. In some cases I kept the same original output and added new connectors for tube output, just that I can compare and show my friends (non-believers) the huge improvement. The headphone outputs on those JVC cd players (OPA based) just does not even come close in sound quality of good tube headphone amp.
Maybe you are aware there is effort to replace OPAs with “discrete” OPA, that is small board populated with few transistors and resistors with pins for OPA socket. Google “Burson” and you will see. Even Nelson Pass has an article about discrete OPA. I would never expect Nelson to use OPA, since I know his motto, simple circuits sound better…and that is true, six OPAs in cd player just does not sound good to me, since each of those OPA has two dozen transistors which generate enormous amount of odd order harmonics which have to be forced down by enormous amount of negative feedback…boy we discussed this ad nauseam already...resulting in sterile uninvolving sound. No thank you.
Maybe you've seen the write-up on the Objective 2 (O2) Headphone Amp DIY. A lot of visiting traffic on it. The author used OPAs therein with a surprisingly (alleged) excellent performance. He basically externalized the internal headphone amp of a CD player; using its RCA output to feed it. One key difference between the internal CD headphone amp and its external counterpart is that it does not have or need an output current capability because it is driving a normally high input impedance of the power amp (>22 K). Essentially, the CD headphone amp is working in an almost STASIS (R) mode; which is an almost ideal performance. I believe that you are aware that STASIS(R) was invented by Nelson Pass.
Hi Antoinel,
Yes, I have seen that thread and I have built many OPA based headphone amplifiers myself. No, I do not like the sound of them neither as headphone amp nor as the output for amplifier. I find the sound of them to be sterile, antiseptic, uninvolving, tiring in comparison to good tube headphone amplifier.
The same applies if they are used only as buffers. I hope you are aware that most cd players are plagued by the pesky OPAs on the output, anywhere between two to six per side, yes, even six OPAs in a row in some. There is enormous improvement in signal quality if these OPAs are bypassed by single FET or tube stage. Google “lampizator” for more details. I have done it on three JVC cd players with great success. In some cases I kept the same original output and added new connectors for tube output, just that I can compare and show my friends (non-believers) the huge improvement. The headphone outputs on those JVC cd players (OPA based) just does not even come close in sound quality of good tube headphone amp.
Maybe you are aware there is effort to replace OPAs with “discrete” OPA, that is small board populated with few transistors and resistors with pins for OPA socket. Google “Burson” and you will see. Even Nelson Pass has an article about discrete OPA. I would never expect Nelson to use OPA, since I know his motto, simple circuits sound better…and that is true, six OPAs in cd player just does not sound good to me, since each of those OPA has two dozen transistors which generate enormous amount of odd order harmonics which have to be forced down by enormous amount of negative feedback…boy we discussed this ad nauseam already...resulting in sterile uninvolving sound. No thank you.
Feeding the signal out of the headphone jack on the CD might be very good gain structure, depending on the input sensitivity of the power amp. It's certainly a very simple solution! There are even a few high end CD players with a remote controlled variable line level outputs that sound quite nice (E.G.,the Njoe Tjoeb)
As to whether or not you like the sound of the headphone amp and internal DAC is an entirely different matter.
As to whether or not you like the sound of the headphone amp and internal DAC is an entirely different matter.
Thank you Adason and Pano for your valuable replies. Our perception of sound is so diverse that no device satisfies all. I vaguely recall that Mr Nelson Pass had supplied one or more high end CD players at some point. If so, he may have incorporated discretes like FET using STASIS technology. A headphone amp may have been included.
Gain distribution is an interesting subject. Behind it there's a fundamental tradeoff between noise on the bottom and nonlinearity (up to clipping) on the top end of the level scale. Power consumption also plays in.
What makes the subject so tricky in practice is the issue of flexibility and practicality. Your average 45 dB of voltage gain found in typical integrated amps came about because people wanted to interface phono preamps (2.5..5 mV input, 40 dB gain @ 1 kHz) with the kind of output levels needed for, say, 80..100 wpc, with 6..10 dB of gain to spare. Your average MP3 player is in the same ballpark or a bit louder.
If you have all of that gain behind the (only) volume pot, the input will be extremely insensitive to overload, which makes the amp very foolproof.
At the same time, however, the volume pot defines your input impedance and as such must be sufficiently high in value.
That in turn increases source impedance and thus voltage noise, or for an equivalent view, available signal power for a given setting drops.
Now SNR always is the ratio of signal power to noise power, so whichever way you see it, it is obvious that you may get a noise problem. (Not to mention related input nonlinearity.) Many amps built as outlined tend to be quiet enough for medium-sensitivity speakers, but definitely won't be dead silent once we get into 95..100 dB SPL / 2.83 V / 1 m territory. The problem is even more readily apparent with the large range of sensitivity found in headphones.
In this situation, adding an input buffer may already benefit output noise levels considerably. A buffer does, in fact, not have "no gain". In fact, it's got plenty - current gain, that is. You can easily drive a 10k pot with it (or a value even lower, limited only by nonlinearity or allowable power dissipation) if you had a 50k one before. If the following amp has low voltage noise, that'll reduce noise levels by a ratio of up to sqrt(5) (7 dB). (You can't drop voltage noise levels indefinitely though, it gets very hard once you get into 1-2 nV/sqrt(Hz) territory.)
Of course you have already given up some input overload capacity at this point (supply voltages are obviously finite), though it wouldn't be an issue in practice. As you shift gain from after to before the volume pot, output noise levels drop fairly proportionally, but input overload becomes more and more critical, as well as general nonlinearity. Given usual input circuitry supplies and CD player output levels, roughly 10 dB is about it.
That's why top-notch preamps and integrated amps have tended to use a 4-gang pot, i.e. two coupled pots with the second one at the very front. That potentially aggravates channel imbalance, but can yield very good results in terms of noise. It doesn't have to be a second pot, that merely was found the most convenient to use.
What you may end up with could be: Input buffer - pot 1 / switched attenuator - +16 dB gain - pot 2 - +29 dB gain. Which in fact isn't terribly dissimilar to the setup with CD player headphone out connected to power amp input as suggested above.
All this analysis is based on looking at signal levels, noise levels and limits imposed by nonlinearity for each stage. Voltage noise densities are determined, integrated (20 kHz = factor of 141) and compared to signal levels (ratio expressed in dB) after multiplying/dividing by gain if necessary. At least that's what I do.
As it happens, I wrote an article on the subject of noise in amplifiers myself last year, not being aware of this one. A bit of redundancy certainly can't hurt.
What makes the subject so tricky in practice is the issue of flexibility and practicality. Your average 45 dB of voltage gain found in typical integrated amps came about because people wanted to interface phono preamps (2.5..5 mV input, 40 dB gain @ 1 kHz) with the kind of output levels needed for, say, 80..100 wpc, with 6..10 dB of gain to spare. Your average MP3 player is in the same ballpark or a bit louder.
If you have all of that gain behind the (only) volume pot, the input will be extremely insensitive to overload, which makes the amp very foolproof.
At the same time, however, the volume pot defines your input impedance and as such must be sufficiently high in value.
That in turn increases source impedance and thus voltage noise, or for an equivalent view, available signal power for a given setting drops.
Now SNR always is the ratio of signal power to noise power, so whichever way you see it, it is obvious that you may get a noise problem. (Not to mention related input nonlinearity.) Many amps built as outlined tend to be quiet enough for medium-sensitivity speakers, but definitely won't be dead silent once we get into 95..100 dB SPL / 2.83 V / 1 m territory. The problem is even more readily apparent with the large range of sensitivity found in headphones.
In this situation, adding an input buffer may already benefit output noise levels considerably. A buffer does, in fact, not have "no gain". In fact, it's got plenty - current gain, that is. You can easily drive a 10k pot with it (or a value even lower, limited only by nonlinearity or allowable power dissipation) if you had a 50k one before. If the following amp has low voltage noise, that'll reduce noise levels by a ratio of up to sqrt(5) (7 dB). (You can't drop voltage noise levels indefinitely though, it gets very hard once you get into 1-2 nV/sqrt(Hz) territory.)
Of course you have already given up some input overload capacity at this point (supply voltages are obviously finite), though it wouldn't be an issue in practice. As you shift gain from after to before the volume pot, output noise levels drop fairly proportionally, but input overload becomes more and more critical, as well as general nonlinearity. Given usual input circuitry supplies and CD player output levels, roughly 10 dB is about it.
That's why top-notch preamps and integrated amps have tended to use a 4-gang pot, i.e. two coupled pots with the second one at the very front. That potentially aggravates channel imbalance, but can yield very good results in terms of noise. It doesn't have to be a second pot, that merely was found the most convenient to use.
What you may end up with could be: Input buffer - pot 1 / switched attenuator - +16 dB gain - pot 2 - +29 dB gain. Which in fact isn't terribly dissimilar to the setup with CD player headphone out connected to power amp input as suggested above.
All this analysis is based on looking at signal levels, noise levels and limits imposed by nonlinearity for each stage. Voltage noise densities are determined, integrated (20 kHz = factor of 141) and compared to signal levels (ratio expressed in dB) after multiplying/dividing by gain if necessary. At least that's what I do.
As it happens, I wrote an article on the subject of noise in amplifiers myself last year, not being aware of this one. A bit of redundancy certainly can't hurt.
Excellent thread, this is largely aimed at people who are starting out as a fool proof way of practically reducing distortion and mishaps during recording/mixing.
Gain structure
cheers
Gain structure
cheers
thx a lot.. i'm actually interesting in the gain structure of a 3way 3 amp speaker with digital crossover.. (similar to minidsp).. my 3 amp have volumes but not passive. I tought of bypassing those volume and have a passive smd volume control on the inputs and i have digital volume on my dsp but not sure if i would loose bitdeph using it.. anyway thx for your articles.. will help me find the gain structure sweet spot
Hi,
Mr.Stephan Großklaß (sgrossklass) and Mr.Michael Mardis (Pano),
Thank you very-very much for both of your contributions. Topics about "Gain Structure" and "Noise in Amplifiers", really are invaluable and encouraging articles for a novice at diyaudio. Specially, for those who are seeking for a good and reliable source of information to build better audio projects, and amplifiers or even just to improve the skills.
After reading your articles, I am now a lot more in a better position to design, execute and share my audio amplifier project, at diyaudio.
Also, i found the answers to lot of my questions which were floating inside the no gravity space region of my mind since my teenage or even my childhood.
We are keen and interested to wait for your future articles on Audio amplifiers, high current buffers, noise, speakers, crossover designs, filters, equalization techniques, op-amps, surround sound techniques and other such related articles mostly contributing towards the better sound quality or achieving a perfect diy audio systems. We would also like to see circuits in action (if possible and applicable) along with the theory part.
Thanks a lot to both of you.
Mr.Stephan Großklaß (sgrossklass) and Mr.Michael Mardis (Pano),
Thank you very-very much for both of your contributions. Topics about "Gain Structure" and "Noise in Amplifiers", really are invaluable and encouraging articles for a novice at diyaudio. Specially, for those who are seeking for a good and reliable source of information to build better audio projects, and amplifiers or even just to improve the skills.
After reading your articles, I am now a lot more in a better position to design, execute and share my audio amplifier project, at diyaudio.
Also, i found the answers to lot of my questions which were floating inside the no gravity space region of my mind since my teenage or even my childhood.
We are keen and interested to wait for your future articles on Audio amplifiers, high current buffers, noise, speakers, crossover designs, filters, equalization techniques, op-amps, surround sound techniques and other such related articles mostly contributing towards the better sound quality or achieving a perfect diy audio systems. We would also like to see circuits in action (if possible and applicable) along with the theory part.
Thanks a lot to both of you.
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Great article 
I'm already of the opinion that a preamp is not needed with an all-digital setup with 2V+ line output.
I have a low gain amp (the F3) and more traditional ones, and I find about 10x gain to be all needed for home reproduction.
Now the issue is where to attenuate. Doing it at the power amp imput is NOT the ideal solution, the best option would be to adjust the gain of the amp itself (like Ayre and ASR do).
The ideal gain structure would be
2-2,5V from digital source -> input buffer -> adjustable VAS from like 0,1x to 20x (to provide for less efficient speakers) -> unity current gain.
The main difficulty to DIY this is from remote controlling the "volume" and from lack of published schematics.
If we loose a bit of signal purity (and SNR), a buffer type preamp with a pot in the middle can work almost as well and there are cheap and easy schematics availalbe, but it is still a waste of 3 stages and a pair of cables that all degrade the signal.
I'm already of the opinion that a preamp is not needed with an all-digital setup with 2V+ line output.
I have a low gain amp (the F3) and more traditional ones, and I find about 10x gain to be all needed for home reproduction.
Now the issue is where to attenuate. Doing it at the power amp imput is NOT the ideal solution, the best option would be to adjust the gain of the amp itself (like Ayre and ASR do).
The ideal gain structure would be
2-2,5V from digital source -> input buffer -> adjustable VAS from like 0,1x to 20x (to provide for less efficient speakers) -> unity current gain.
The main difficulty to DIY this is from remote controlling the "volume" and from lack of published schematics.
If we loose a bit of signal purity (and SNR), a buffer type preamp with a pot in the middle can work almost as well and there are cheap and easy schematics availalbe, but it is still a waste of 3 stages and a pair of cables that all degrade the signal.
Glad you liked the article.
Yes, an adjustable VAS in the power amp might be the ideal approach, sonically. But as you point out, maybe not so practical or easy to find. For me, finding an amp that has a little more gain than I need, then attenuating it some at the input, is the most practical solution. With modern 2V sources it's gotten much easier.
Yes, an adjustable VAS in the power amp might be the ideal approach, sonically. But as you point out, maybe not so practical or easy to find. For me, finding an amp that has a little more gain than I need, then attenuating it some at the input, is the most practical solution. With modern 2V sources it's gotten much easier.
If the gain would be controllable with a single variable resistor (read linear pot), it would be much easier to diy... I want to do some research, with the many talented people roaming around, something may come out.
Things gets a lot more difficult with active multiamplified systems, where one needs to match amp and speaker gain, plus a desire to remotely control all the system at once (after level-matched). In a 2 way I was able to level-match with ears adjusting the pot of an integrated amp i have.
I'm mostly rumbling out loud.
Were you aware of a translation of your article on the italian webzine ReMusic?
Maximizing SNR is a very sensible topic for me, I'm reading the link posted here about an article on noise.
Things gets a lot more difficult with active multiamplified systems, where one needs to match amp and speaker gain, plus a desire to remotely control all the system at once (after level-matched). In a 2 way I was able to level-match with ears adjusting the pot of an integrated amp i have.
I'm mostly rumbling out loud.
Were you aware of a translation of your article on the italian webzine ReMusic?
Maximizing SNR is a very sensible topic for me, I'm reading the link posted here about an article on noise.
This is an issue I'm about to have to deal with because I'm going tri-amp, and since I can't afford those tube-crossovers that Marchand(?) make I'm going to have to use DBX or Behringer pro model. Something like the ART CLEANBoxPro Stereo Balanced/Unbalanced Converter to adjust the unbalanced lower consumer level output of my preamp to better match the input, but my power amps are consumer w/o gain controls. Suggestions?
Replace the output amps of the Behringer with balanced RC filters (Jan Didden wrote a very clear article on how to do that). Pano prefers using output transformers to do the same job. Either way, the performance of the Behringer is improved and the levels are more suitable for consumer gear. As a bonus, you can take an spdif output directly from your digital source, run it into the Behringer's spdif/AES-EBU input, and bypass the ADCs completely.