Damping Factor - Interesting article

@kijanki  Interesting!

@kijanki  @roberttdid was referring to the idea of an amplifier that operates as a power source rather than a voltage source.

Otherwise how this particular speaker sounds when connected to that particular amp becomes rather unpredictable.

Regarding Bruno's comment from the link above, speakers designed to operate in the Power Paradigm usually have level controls for the midrange and tweeter, as the voltage response of the amplifier is an unknown. If you look on speakers made in the 1950s these controls are fairly common. They are not there to help set up the speaker in a given room!

I am not saying a huge improvement, but the low DF will in many cases be better.

That's an interesting idea- but I suspect one that has not seen a lot of study.

What is likely to product worse cone breakup, a high DF or a low DF?

To my understanding, cone breakup has nothing to do with the amplifier.

How does damping factor apply to a zero feedback amp like an ayre .

Generally speaking, the output impedance of a solid state amplifier can be so low with respect to most speakers that it can behave as a voltage source, even if it has no feedback. This is why there are zero feedback class D amps that also behave as a voltage source.


One thing I've noticed a lot while working in high end audio is the phenomena known as 'tight bass'. I regard it as a coloration, since in real life tight bass doesn't seem to exist. The head engineer of Electro-Voice wrote an interesting article that pointed to this many years ago but I've failed to find a link to his article. Essentially though, it points out that no speaker made needs a damping factor more than about 20:1 and many need a damping factor much lower, some as low as 1:10 and you read that right (certain kinds of OB speakers).


The sound difference you hear between systems that have much greater damping factors is often related to distortion rather than FR errors, due to how the brain interprets distortion (if often favors such tonality over actual FR errors or the lack thereof).

Every amplifier has some feedback. Even emitter resistor is a form of local feedback. The problem with global feedback is, that it corrects with a delay (phase shift from input to output). This delay produces overshoot in time domain (odd harmonics in frequency domain). 40dB feedback means, that amplifier has 100x higher gain without feedback. Since amplifier delays signal from input to output, signal fed back and summed at the input is late. It make very little difference for slow sinewaves, but for fast changing input signals amplifier, for a moment, has 100x higher gain and overshoots. Benchmark is trying to time correct it with separate error amplifier (two sets of output transistors).  This overshoot shows in some Stereophile reviews as square wave response.

+1

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.

@roberttdid   I've yet to see a tube output section where on its own without feedback, is able to behave as a voltage source. If you can point me to one I would be very interested. You might want to take a look at this image:https://www.radiomuseum.org/r/fisher_80_az80az.html
This is a Fisher 80-AZ, typical of a number of amps from this period of the mid-late 1950s, prior to when the voltage rules were adapted by the audio industry. It is equipped with a Damping Control, which is a variable voltage and current feedback system.


Note that at 12:Noon the control is marked 'Constant power'. At the extremes the control is allowing the amp to be a voltage source or a current source, as the control operates both forms of feedback. When the two feedbacks means are balanced against each other, that is about the same as zero feedback, hence 'constant power'. Now if you spend time with zero feedback tube amplifiers, and happen to have measured their power response with respect to load, you find that above a certain impedance (depending on the overall output impedance of the circuit) the power decreases quite slowly as impedance is increased, in fact doubling the impedance sees only a small percentage loss of power. Its not perfect of course, but 'constant power' is really not a stretch; a zero feedback tube amp will do pretty well with this as long as the load impedance is high enough. No amplifier is perfect of course and this includes all amps that behave as voltage sources as well.


So my description as not too simplistic. It was simply correct.

Their response w.r.t. voltage, is fairly flat from mids-highs, with usually a bit of a dip at high frequencies. An amplifier that doubles in power as the impedance is squared will keep the most consistent anechoic output.

Huh?? What kind of amp doubles power as impedance is squared? Even a constant current amp only doubles power as impedance is doubled. At any rate this statement is entirely false, as ESLs don't do that. Here's a rather famous ESL impedance curve, the Quad ESL57:
http://www.quadesl.com/quad_main.shtml
You can see that while it does flatten a bit in part of the midrange, its on the decrease all the way from the peak in the bass. We have a lot of customers with Quads and Sound Labs (80% of our MA-2s built over the last 30 years are running on Sound Labs); these speakers don't seem to behave around voltage rules nor should they, as their impedance curve is not that of a driver in a box with its attendant resonance. This is of course not the only example of a modern high end audio loudspeaker that doesn't use the voltage rules; keep in mind that most SETs are zero feedback and so tend to behave more as power sources than voltage sources, and yet there are speakers on which they do quite well as the designer of the loudspeaker intended that it be that way.

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!

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.