Solid state design options...

To me the design parameters are important to me in the foll. order:

* Zero global feedback - reduces distortion & improves 3D imaging
* Class A output - for best linearity. We can have a class-A amp w/ lots of feedback but its worth very little!
* Instantaneous Current Delivery - this implies a very well designed power supply. Clayton Audio amps seem to rank the highest here. They are rated at 200A continuous! One could weld with them.
* High Damping Factor - this speaks about the amp output impedance. Since power amps are voltage sources, the best voltage source has zero output impedance. Of course, this is not physically possible so the next best is as low as possible over the entire audio band. Such amps will have killer bass response such as Symphonic Line & Wolcott tube amps. There are others, of course, but these come to mind right now.
* Balance In/Out - according to me this feature is the most hyped & most marketed! It has some advantages but not as much as what is hyped. I have no personal experience w/ BAT but this co. seems to make fully differential designs from input to output rather than create a single-ended design & then create an out-of-phase signal & call it balanced! Fully balanced designs seem to make sense if common-mode noise is an issue. I have heard single-ended designs that sound awesome so I'm not fully convinced that a balanced design brings a significant advantage to the table. Of course, JMO.

Karls,

Interesting feedback comments!

Likewise I was reading the GamuT Audio website (http://www.gamutaudio.com
On the right side click "FAQ" & then click "what is damping factor") & they claim that damping factor of just 25 is sufficient.

When I think of damping factor I mostly think "amplifier output impedance". The lower it is, the better off you are (& the more naturally you get bass control). I'm not aware of the various techniques of obtaining high DF but your comments have me pondering....

The more the speaker impedance changes, the more the variation in DF, which is not so good i.e. load dependent DF. I have inferred that those amps that have a constant high DF over the audio band have low output impedance over that same freq. range. Might be a simplistic view but it is one criteria I use to short-list an amp. Listening to it provides the final verdict.

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!

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.

Nrenter,

Understand why you posed the question in the 1st place. I agree that it's very hard to discern an amp's sound quality by just looking at the specs & knowing what sort of technology was used to design it. I also agree that the best is to listen to it.

You know, we might be surprised when we open up the M-100 & DNA-500 & place them side-side! It is quite possible that the 2 independent designers used techniques more similar than dis-similar to design their resp. amps! Neither designer provides much info about the techniques used & we have to rely on the IAR journal to ferret these out for us. I suppose that you are referring to that IAR80 article, which I also read with a lot of attention, that put both these amps in the same class.

We just don't know what these designers are putting into their amps esp. if they as tight-lipped as Wilson Shen & Steve McCormack. I read a review on the M-100 amps, which was a reprint of a Bound For Sound review, where I was informed that I should not use high impedance 10-16 Ohm speakers nor should I use a tube pre with the M-100 if I really wanted the amp to sing. J. Peter Moncrieff never mentioned this in his article! So, should I/we append this tit-bit of knowledge on the M-100 to the knowledge gained from the IAR80 article? Don't know......