Damping Factor - Interesting article

Let's assume for a moment that wire is perfect.

The only problem with that is wire isn't, so the math can't be realized.

The example of the constant power amplifier as a tube-amplifier with transformer taps, is in my mind no different from the voltage amplifier paradigm presented, the only difference is the taps on the transformer impedance match the output to what is still essentially a voltage amplifier. The output power of those amplifiers will still change as the load impedance changes, perhaps not as much as if there was more feedback to compensate for the low output impedance of the amplifier, but it will still change as it is inherently a voltage amplifier.

@roberttdid

One thing you are not getting has to do with the application of feedback. What I have said in that paper is true if the amp has none- what you say above is true if the amp has enough feedback to allow it to behave as a voltage source.

Now Duke touched on something of high importance, that relates to @douglas_schroeder 's comments quoted from his review. Loop negative feedback is not a trivial matter in any amplifier, and the amount used can have profound consequence on the sound that derives from the amplifier. I am immediately asking- in an amplifier which has variable damping the easiest way to set that up is by the use of variable feedback- so what is the minimum and what is the maximum feedback?

This is a bigger deal that it would seem to appear; if the amplifier has too little feedback (less than about 35dB) the consequence is that the feedback itself will introduce distortion, mostly composed of higher ordered harmonics (and some IM). Somewhere in the area of 35dB and north the amp finally has enough feedback such that is can actually compensate for the distortion introduced by the feedback itself.

Now the ear converts all forms of distortion into tonality and can favor that tonality over actual FR errors. The ear is particularly sensitive to the higher ordered harmonics and IMD; the former are used by the ear to calculate sound pressure. If they show up, the amplifier will sound brighter and harsher and louder than real life, even if in 'tiny' amounts that we are used to seeing on spec sheets.


This simple fact is at the root of the tubes vs solid state debate! Tubes don't make the higher ordered harmonics in the same way as solid state and so sound 'smoother' as real music does not have these harmonics enhanced either.


So its understandable that an amplifier with variable feedback would sound quite different, and not because of damping factor, even though that is being varied. When an amplifier has insufficient feedback it will have colorations and those colorations will overshadow frequency response errors on account of how the brain perceives distortion. This is why two amps can measure flat on the bench but one can sound bright and the other doesn't!

Now some of you may have noticed something- that most amplifiers made in the last 70 years don't have enough feedback. This is why solid state amps have been bright and harsh all this time- its only been recently that newer semiconductors have been available to allow amplifiers to be made with enough loop gain. But it appears that you can count those amps on one hand at this point.

So the alternative is to simply use no feedback at all- and thus avoid the highly audible distortion caused by the feedback itself. This results in an amplifier with a high output impedance and thus low damping, but many speakers don't need much damping to sound quite realistic. This is why things like SETs exist- put them on the right speaker and the result is excellent.


The bottom line is this is all about Gain Bandwidth Product and the resulting loop gain- both of which have been insufficient in the prior art. The Benchmark amplifier is one of the very few non-class D designs that actually gets the feedback into the ballpark. So if you want really natural sound, you either go with an amp like that or go with an amp that uses no feedback at all- and deal with the simple fact that it won't work on all speakers, which is also true of an amplifier that is a perfect voltage source! So you'll have to audition the speaker and amp combination in any event.

My understanding is that low damping factor generally calls for high amounts of global negative feedbac

Doesn’t have to be global.  Local feedback can also be used to lower distortion and increase DF.  Also, a high output impedance (low DF) can be offset by having more output devices.

Still, yes, it is easy and cheap to achieve low damping factors with high amounts of global feedback.

Another dimension of DF not often discussed is having high DF, and high current through the treble.  While conventional speakers tend to have their low points in the mid-bass, where most amps have the highest DF, ESL's are essentially capacitors, and have their lowest impedance in the peak frequency.

Having an amp that can do high current and low output impedance at 20 kHz can restore the treble.  This is a reason why a lot of tube amps can sound dull with ESLs.

Of course, no one cares more about this than Roger Sanders, and his Coda based amps are optimized for low DF and high current across the spectrum.

https://www.sanderssoundsystems.com/

Another dimension of DF not often discussed is having high DF, and high current through the treble. While conventional speakers tend to have their low points in the mid-bass, where most amps have the highest DF, ESL's are essentially capacitors, and have their lowest impedance in the peak frequency.

If an amp has 'high current' (which is a bit of a myth; current can't exist without voltage) then it will at all frequencies.


The problem with ESLs is that they typically vary by about 9 or 10:1 in impedance from bass to treble, but their efficiency doesn't vary in lockstep as it is supposed to like you see with box speakers. So an amp that doubles power as impedance is halved is typically way too bright on most ESLs. Martin Logan got around this (sort of) by making their ESLs very low impedance in the bass (4 ohms) so they are only 0.5ohms at 20Khz. Even most solid state amps have troubles into that impedance, thus reducing the brightness that would otherwise manifest.


Generally speaking most ESLs don't follow the voltage rules; IOW their impedance curve does not match their sensitivity through their frequency range!

This is too simplistic a view, and I was thinking specifically just related to the basic output stage which does typically behave much like a voltage source, and is usually configured as a voltage follower, and with a light load (lighter than a speaker), behaves as a voltage source, and with load, as a voltage source with an element of constant and variable impedance.

One thing you are not getting has to do with the application of feedback. What I have said in that paper is true if the amp has none- what you say above is true if the amp has enough feedback to allow it to behave as a voltage source.

Again, I find this is too simplistic of a view. Simply saying 35db is too little feedback without taking into account the frequency response of the feed-forward and feedback paths, not to mention what the inherent feedback is in the output stage if you are considering that separately makes any hard number in the sand questionable. The statement makes assumptions about the linearity of the feedback network as well. Ditto for Gain-Bandwidth, which is one number, but gain at frequency is far more relevant. Instrumentation op-amps may have very high gain-bandwidth, but are useless at 20KHz.

This is a bigger deal that it would seem to appear; if the amplifier has too little feedback (less than about 35dB) the consequence is that the feedback itself will introduce distortion, mostly composed of higher ordered harmonics (and some IM). Somewhere in the area of 35dB and north the amp finally has enough feedback such that is can actually compensate for the distortion introduced by the feedback itself.

The bottom line is this is all about Gain Bandwidth Product and the resulting loop gain- both of which have been insufficient in the prior art. The Benchmark amplifier is one of the very few non-class D designs that actually gets the feedback into the ballpark. So if you want really natural sound, you either go with an amp like that or go with an amp that uses no feedback at all- and deal with the simple fact that it won't work on all speakers, which is also true of an amplifier that is a perfect voltage source! So you'll have to audition the speaker and amp combination in any event.