Please explain amplifier output impedance

Sean & Dave,

Thanks for talking this discussion even further.

There are quite different design philosophies behind the conflicting views on output impedance;

One philosophy is that the amplifier and cables should have a minimum affect on the speaker response - i.e. the speaker is designed for SS amps of many types and the speaker impedance variation with frequency is NOT intended to affect system frequency response. (provided the amp has enough power to drive the load this philosophy leads to consistent results => the flat speaker response will be maintained provided it is coupled with a low output impedance SS amp.)

The other philosophy is that amplifier and cables should be chosen in order to augment/adjust speaker response by behaving as a kind of "equalizer" or a tuned system; in this case, system response will vary as a function of speaker impedance. (amp ouput impedance - cable - speaker load act together as a filter/equalizer on the sound => this leads to a system which can be tuned to taste, according to the preferences of the user and the selected high output impedance amp)

It is the differing philosophy that leads to different views on amplifier output impedance. There are advantages to both. One is consistent and accurate; but you get what you get. The other approach offers greater flexibility for tweaking to taste.

This is why I qualified my earlier comments in this thread by saying they were generalizations for SS amps an not for tubes. Tubes seem to be desirable to those that like a certain flavor to the sound...a certain warmth or punch to the bass. (and there is nothing wrong with that. Doug Sax, one of the industry leading professional Mastering Engineers, uses tubes for mastering for this very reason. Indeed, most people seem to prefer an equalization curve with a boosted bass and a rolled off treble.

Apparently my point was missed about the subject of speaker damping. The point that I was making concerned the subject of optimum damping in response to a transient waveform. Music is made up mostly of transients whereby many frequencies are present at once and transient attacks are part of the normal program source. Stereophile frequency flatness measurements represent a different type of measurement and do not address the transient properties of the speaker-amplifier. I repeat, a low output amplifier impedance does not guarantee proper speaker damping. It often actually overdamps the speaker. The subject of overdamping, critical damping and underdamping is a well-studied topic in engineering. No one interested in this subject would ever assume that the most is the best without verifying with measurements. I have done this and I described the apparatus that I developed for these measurements in my previous post.

Oh by the way, Berning amplifiers have a nominal output impedance of less than two ohms, but the user can increase this in the ZH270 by setting the feedback switch to MED or LOW to best optimize system transient response and user preference.

By the way, Sean, what would think of an amplifier with negative output impedance? After all, the output impedance of the amplifier must be added to the dc resistance of the speaker cable and speaker voice coil resistance in the damping equation. An amplifier with negative output impedance would increase the true damping factor further. Building such an amplifier is not difficult, I once modified an old Heathkit tube amp by applying some current feedback to do this. I can't remember what it sounded like, as I did this over 30 years ago, but never pursued it further.

Thank you, Duke.

The 27 ohm maximum was at about 3 kHz, which is not a good place to have a response anomaly

Indeed, although rather benign compared to a (expensive) branded speaker I onced measured with a friend...! And you had few components in the signal path messing things up, which is a bonus. After all, a push-pull (or an OTL?) amp may work wonderfully with that... who knows...
Mr Berning sez

By the way, (Sean,) what (..you..) would think of an amplifier with negative output impedance

Not to hi-jack the conversation -- but hasn't Mr Pass tried/experimented with something like that?

I repeat, a low output amplifier impedance does not guarantee proper speaker damping. It often actually overdamps the speaker. The subject of overdamping, critical damping and underdamping is a well-studied topic in engineering

I agree that damping is well studied in engineering, however, in engineering "damping" is NOT the same thing as the audio "damping factor" term used to describe an amplifier. Actually "damping factor" is simply a reflection on the amp output impedance relative to to the speaker load. This is an "indication" of how well an amplifier can be expected to handle a reactive load. It should not taken as gospel ...there is a lot more to an amplifier than damping factor.

Once the output impedance is low relative to the load then the "damping" of the driver (suspension/magnet/coil)and box will dominate the response...The amplifier simply puts the correct voltage over the speaker terminals which always pushes the driver towards the proper position (as determined by the audio signal) by supplying the appropriate current.

In theory more current can be supplied for a given amplifier voltage if the output impedance is lower...i.e more "push" in the right direction.

Apologies if I have oversimplified things but I don't see how amplifier damping factor can be regarded in quite the same way as the critical damped response that is often saught in most speaker designs.