Please explain amplifier output impedance

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.

Hi Sean, I can't think of an amplifier that that qualifies as 'universal use'. For any example you can think of I can think of a speaker it does not work with. On that account your argument seems to fall apart. It is also not true that I 'prefer vented speakers', in fact what I prefer is speakers that sound right regardless of their technology. I use headphones (Stax, Grado) and master tapes for reference to avoid the pitfalls that you (?might?) have assumed that I have fallen prey to.

We've used a variety of speakers over the years including acoustic suspension ('sealed'), horns, bass reflex, magnetic and ES planars. On all of these speakers it was easy to demonstrate that our amps were making correct bass against the typical power house transistor amps which do not.

I should also point out that I do use a set of design rules based on reality and easily proven. Nor do we lack in the measurement department for them (see our website for details). IOW we respect the human ear and the rules that *it* uses, rather than made up rules that it does not use.