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Old 29th March 2011, 10:57 AM  
Pano is offline Pano  United States
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Default What is Gain Structure?

Gain structure (AKA Gain Staging) is a concept that gets talked about a lot in pro audio, but most home audio folks have never heard of it. Understanding gain structure can help you get the cleanest signal possible out of your system and avoid some nasty things. Things like noise and clipping,...

Last edited by Variac; 1st April 2011 at 11:34 PM.
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30th January 2012
Antoinel
diyAudio Member
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.
31st January 2012
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adason
diyAudio Member
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.
31st January 2012
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Pano
diyAudio Moderator
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.
31st January 2012
Antoinel
diyAudio Member
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.
23rd February 2012
sgrossklass
diyAudio Member
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.
23rd February 2012
Antoinel
diyAudio Member
Thank you sgrossklass for the above writeup and also your article on noise. Got to balance noise and practical flexibility in design. Any possibility that you can publish it on DIYAudio? Best regards
14th March 2012
SafeandSound
diyAudio Member
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
2nd June 2012
audiolic
diyAudio Member
Could you please do another article on how to fix gain structure with passive attenuator and talk about passive attenuators... (like the new with SMD resitors ones)
4th June 2012
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Pano
diyAudio Moderator
Good idea. I'll see what I can do. Thanks for the idea.
4th June 2012
audiolic
diyAudio Member
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

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