Solid state design options...

The way you get really high DF (ie, really low output impedance) is with negative feedback in the output stage. This isn't the only factor, just the easiest way to reduce the output impedance to really low numbers. And with negative feedback comes the issues I mentioned above. Visit vac-amps.com; there are numerous thought-provoking essays on this and other topics. I'm not saying they're right, or that there aren't other issues, but they do offer some interesting points to think about.

I would like to make a comment here:

The post 'Karls' made on 7-30-03 seems to suggest that ALL NEGATIVE FEEDBACK is bad!! This statement is not correct!

As an EE, I know that we rarely design an amplifier (with large gain) WITHOUT feedback! It will clip to the + or - supply rail the moment you feed it an input signal esp. with music transient signals that are rampant in classical, Jazz & blues music.

The key to negative feedback is that the designer must be intelligent to use it properly. I will agree with Karls that it if global feedback (from output of amp to input of amp as was done a lot in the 1970s) is used then the sonic signature of the power amp is botched up pretty damn well nicely! However, if local feedback is used most/all of the negative feedback issues can be overcome. Local feedback is defined as feedback that exists local to 1 device or a few devices or an amplifier stage.

I cite 2 prominent examples that use negative feedback. Let me point out here that I simply use these 2 amps as examples - I'm not bashing nor am I praising. Pass Labs X series amps claim "no negative feedback" & many would believe this. However, read his white paper on his website that clearly states that he (Pass) considers "no feedback" to synonymous with "local feedback". He even challenges the reader to submit to him a "true no feedback" circuit so that he (Pass) can point out the hidden feedback! Per his literature, it is true to say that the X & XA series amps operate heavily into class-A hence use no feedback in the output stage (but there is local feedback in the 1st gain stage). That is because, for a MOS device, output impedance is inversely proportional to drain current. If one uses a "healthy dose" of bias drain current in the output stage, output impedance will drop to practically nothing. This will result in a NATURALLY high damping factor.

The second example is Spectron Musican II. The designer clearly states the advantages of negative feedback on his website. The Musician II seems to use local negative feedback & those of you who own it, can testify to its great sound. I will agree with the Musician II designer that the amp has to be high bandwidth (i.e. fast) for the feedback to work. Most of the time feedback makes the amp sound sloppy 'cuz the amp stage(s) to which feedback are applied are not high enough in bandwidth & by the time the amp reacts, that particular music passage is done & long gone. In this case, the issues with feedback that Karls pointed out are very valid. This does not mean that feedback is bad! It just means that the amp designer is incompetent!

However, not every amp on the market is class-A operation. In fact, most are class-AB & some of those are class-AB with emphasis on class-A upto a certain wattage. The better sounding amps in these 2 classes use LOCAL negative feedback. They HAVE to! No way the above cited amps can have the much sought after attributes of unconditionally stable into any speaker load, low distortion, low output impedance, low noise, etc, etc. w/o some sort of feedback.

Anyway, for power amp design, negative feedback is a fact of life. Less in being today as compared to ss designs from the 1970s but it is STILL being used! With intelligent use of negative feedback, the issues associated with this technique can be overcome with good/great results. My experience is with solid-state designs only. Thus, I will not comment on negative feedback or lack of as used in tube designs. Some of the concepts of tube design can be ported over directly to the ss domain but there are a number of pecularities of tube design that cannot. These are exploited positively by creative designers to produce some great sounding amps that not always measure A1 in the lab. It is very possible that what Karls read on the VAC site is particular to tubes only & that technique cannot be applied to ss designs to reduce output impedance. BTW, I have yet to read that VAC site info.

Just how high the D.F. has to be to get good/great bass - I don't know for a fact. The Gamut site says 25 is enough. The Pass site says a large number is not needed but doesn't state how large is large (1? 10? 100? 1000?). You can be quite sure that, with the X & XA series amps producing killer bass, their D.F. is on the high side rather than the low side. Providing a low impedance to the woofer VC thru the amp output impedance is important to start/stop it quickly. That's what gives us "fast" & "tight" bass. Also, do not forget that D.F. IS frequency dependent 'cuz the loudspeaker impedance is frequency dependent. Each time the loudspeaker impedance dips, the D.F. reduces. If the amp. has to provide "fast" & "tight" bass regardless of this impedance fluctuation then the amp output impedance BETTER be real low! Local negative feedback can help us here unless you have a high power dissipation pure class-A power amp. Many of us cannot afford such monsters but that doesn't mean we cannot have a good/great sounding amp!

Bombaywalla, great post. Please do not assume I have bought into the "all negative feedback is bad" camp, as that is not the case. I merely mentioned VAC's site as interesting food for thought, and much of what they have to say is probably applicable primarily to tube amps due to the presence of the output transformer. I will say, however, that two of my favorite amp designs presently are Pass X and Ayre V_x, both of which are touted as "no feedback" designs, and both of which sound purer than most anything else out there. And I would venture a guess that both of them have quite high DF to boot.

Karls,

Thanks!
OK! Sorry to club you with the "all negative feedback is bad" group. Your post seemed to indicate that but it seems that I misinterpreted it.

Very interesting and well put, Bombay! (The EE in you manifested himself, thankfully:))

Engineering a constant current device is a bit tricky, (albeit nice for speakers) especially when we go over the sacrosant 2,83V level. Astonishingly, it seems that some SS amps have been designed with the full throttle in mind, linearity falling as we lower the volume.

For that matter, how many of us nowadays listen at more than ~4V at the amp output -- with a 89-90db efficiency speaker that's enough to tire the ear pretty fast!