Please explain amplifier output impedance

Negative feedback in high levels is quite audible and it doesn't sound good.

Agreed. A badly designed circuit will sound terrible. Most manufacturers try to avoid building unstable circuitry. However, in the pursuit of ridiculously high damping factor specifications (for marketing purposes), it is certainly possible to build a dangerously unstable circuit. Extremely high amplifier gain will lead to instability and oscillations. This occurs when feedback times open loop gain approaches negative one. In this case, the closed loop gain will approach infinity...which is of course not possible and everything becomes oscillatory, distorted and clipped.

In general, typical SS amplifier circuitry (with negative feedback loops), although very linear when operated within tolerance, are not at all forgiving when they are over-driven; typically when over-driven they sound harsh and then damage speakers fairly quickly. Certainly, high levels of negative feedback are likely to lead more quickly to catastrophic behavior.

Since music is very dynamic, it is relatively easy to over-drive equipment. Some SS gear has built in protection circuitry that is designed to detect and protect equipment from damage.

Arthur: Those are very good points and well worth clarifying. Thanks for taking the time to point them out AND explain them.

I have often said that it is the sum of manufacturer spec's that count more than any individual spec by itself. Even then, most manufacturers don't provide the quantity of spec's that one needs to make such information truly useful.

As to your comment about most amps having a linear output impedance up to appr 50 KHz, that is kind of generous in my experience. Many amps exhibit a noticeable increase in output impedance at or slightly above 10 KHz. How severe this is will depend on the design of the amp. By 50 KHz - 80 KHz or so, performance is starting to suffer quite noticeably. This is why many amps round the leading edge of a 10 KHz square wave. That is, the higher output impedance is part of a bigger problem i.e. limited bandwidth due to the amp being too slow to properly respond. Combine the limited bandwidth / lack of speed with the rising output impedance and you end up with that slightly rounded square wave that you see so often in Stereophile test measurements.

If you think that this sounds "bad", there are REALLY slow / limited bandwidth / higher output impedance amps fail the 1 KHz square wave test. When this type of amp encounters a very fast high energy high frequency transient, most of the attack, definition and duration is lost. This translates into a soft sounding blur, which some people like. This is probably more true with digital recordings and playback, which tends to sound hard, bright and glaring in many systems.

Other than that, this thread could go on and on contrasting various designs and goals. Suffice it to say that there are a LOT of variations that come into play with any / every design. When all is said and done though, the end result is a summary of what the designer / engineer thought was most important. Whether or not you like that product will depend on your own personal preferences and how well that specific component blends with the other gear in your system. As far as i know, there are no spec's to quantify personal preference. Sean
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To bring this discussion back around to my original question, when a reviewer states that a particular loudspeaker would likely work best with a low output impedance amplifier, should one assume (based on comments made earlier in this thread), that a low output impedance measurement is .1 ohm? At what measurement (expressed in ohms) does the threshold cross from low output impedance to marginally high output impedance...1 ohm?

There is no hard and fast threshold but IMO, anything over 1 ohm is definately high output impedance. Many SS amps will be about 10x lower and tube amps are up to 10x higher so 1 ohm threshold seems to a decent rule of thumb.

Sean - My comment about amp impedance rising above 50kHz was based on at least a 3dB rise so it would visibly start to deviate around 20kHz. And you are definately correct in saying that many amps can't even do that well. I read all the Stereophile measurements several times each, no matter the piece of equipment. I am happy they are still around to do the good old fashioned measurements, even if I realize they aren't very applicable to musical reality. Nonetheless, I feel there is a lot to be said for an amplifer with an elegant set of curves.

Arthur

High damping factor is allways desirable. Since speakers are a reactive load they respond to the amplifier signal with a counter eletromotiv force that the amp must deal with, and with a low damping factor the amp will have more intermodulation distortion because of this fact. Low damping factor will cause the amp to have more problems in controlling the speakers drivers properlly.