Feedback

Ralph (@atmasphere), what do you think of Mr. Putzeys’ statement that ...

... the idea that feedback causes TIM is probably most noteworthy for being not just wrong, but also the exact opposite of the truth. TIM happens in the input stage. An increase in global feedback makes the input stage work less hard. That causes a disproportional reduction in TIM.

@almarg Bruno is spot on here. The problem is that certain elements of the amplifier design can lie outside the feedback loop, but nonetheless be affected by the operation of the feedback. When you're dealing with something like this you get distortion.


Feedback has mostly been used in the last 40 years to decrease output impedance so that the amp can behave as a voltage source. That is why in most cases there is brightness/harshness that isn't part of the original signal since the feedback needed to get the amp to behave this way is always going to be less than what is needed for the feedback to have the ability to correct for distortion it adds.


One issue that's not seen a lot of attention is the simple fact that a speaker does not need more than about 20:1 for damping and many need considerably less. If overdamped, you get a phenomena often called 'tight bass' which is pleasurable but a coloration. So right here you can see that adding the feedback that is really needed for the amp to do its job right is overkill for the needs of the loudspeakers. 


I've seen nothing to address this; audio is still a developing field.

Nobody with a solid working knowledge of feedback theory chooses to avoid it. “I choose to avoid feedback” is an all too transparent code for “I’ve yet to figure it out”.

The above is from the article. Its not exactly correct, although the gist of the article is.


We don't use feedback in our designs simply because so far we prefer to use tubes. Its almost impossible to add enough feedback in a tube amplifier due to all the frequency poles which limit phase margin and decrease Gain Bandwidth Product. And since not enough feedback is actually worse than none at all, being pragmatic, that's the direction we went. But if we could actually add enough (35dB being an absolute minimum to any design) then we would. 

So actually what's been happening is most designs using feedback have not used enough, rather than 'too much'. When the feedback is insufficient, it is unable to cause the amplifier to compensate for the distortion added by the application of feedback itself. The distortion caused by the application of feedback (from Norman Crowhurst, so this has been known since the early 1960s) adds a host of higher ordered harmonics as well as intermodulations at the feedback node while suppressing some of the amplifier's innate distortion. This results in the noise floor of the amplifier being harmonic and inharmonic information rather than just a gentle hiss. The addition of the higher ordered harmonics causes the amplifier to sound bright and harder since the ear assigns a tonality to all forms of distortion and is keenly sensitive to higher orders since it uses them to discern sound pressure. I've seen a lot of people have problems with this fact over the last 40 years, it helps to understand the ear's sensitivity in that it has a 125-130dB range.


That is why nearly all solid state amps until very recently have been bright; certainly **all** solid state designs from Japan in the 1970s to possibly about 5 years ago. So in a way, the people criticizing the author's Pioneer for having too much feedback might have been right; if you can't add enough in your circuit you might be better off with none at al!


But if you can add enough feedback, and by that I mean in excess of 35dB then the circuit has the chance of being able to compensate for the distortion added by feedback: now you have a circuit that is actually really low in distortion. It has to be really low because the higher ordered harmonics are so easily heard by the human ear- 0.005%THD is easily heard if its all higher ordered harmonics!


While a lot of solid state advocates talk about neutrality, its hardly 'neutral' if its bright. That's a more heinous distortion than the 2nd or 3rd order (to which the ear is not that sensitive) by far!


Now some designers have taken a different approach, sculpting the distortion signature to include the 2nd and 3rd harmonics. Its been shown that if these lower orders are present (both treated by the ear as 'richness', 'bloom', etc.) in great enough degree, they can somewhat mask the presence of the higher orders. Nelson Pass takes this approach in his designs, which are some of the best sounding solid state amps out there. I've found certain early solid state designs that have this property as well although in the case of the latter it does not appear to be with intention.


Self-oscillating Class D amps appear to have the ability to run the level of feedback required. In fact such amps use the high amount of feedback as the means to set the switching speed. Its tricky- you have a lot of variables but in a nutshell the feedback loop is set with enough feedback that the amp exceeds its own phase margins and goes into oscillation. The loop then has timing constants within it that cause the oscillation to settle on a constant 'solution'; a constant switching speed. Since its easy to make a lot of gain in a class D amp, you can easily have over 40dB of gain and still have 25-30dB left over for actual amplification.