transformers/output impedance

Al and Ralph, I reread Ralph's explanations in the other thread. I understand a little bit better why using the term "output impedance" is a bit of a misnomer. Maybe the industry could sex-it-up by referring to the attribute as "synthetic" or "apparent" or "hypothetical" output impedance.

I think the point that Ralph was trying to make is that using a meter to measure an amp's impedance/resistance at the output taps is **not** what the term "output impedance" is speaking to. Instead, it's referring to the amp's operational behavior in delivering current/power to a speaker load **as though** it's output impedance was a specified number.

Where an amp's "apparent" output impedance is close to zero, one could expect that the amp would double down current/power if speaker load is halved, but only if it is acting like a true constant voltage source, and **only if the amp is performing within its operational limits.**

And ... as Ralph said, using NF only permits the amp to perform in such a fashion. Ergo why the use of the term "output impedance" without an additional adjective is a bit of a misnomer, almost misleading.

I guess in a goofy kind of way, if a designer kept adding NF to the circuit, at some point, the NF could be greater than the current/power being delivered by the main output circuit. This may be what Ralph meant when he said that NF could also be viewed as a voltage source.

Well, maybe I still don't get all of it, but maybe a little bit more than I did before. This still makes amp/speaker matching a tricky business.

Output impedance and ability to drive low impedance are two different things. My amplifier has DF=4000 at low frequencies but cannot drive speakers below 3 ohms.

I cannot speak of tube amps but in SS amp feedback always reduces output impedance. I made this small example a while ago as a proof:

Let’s take amplifier that has gain of 30 (31.6dB). When input voltage is 1V output voltage is 30V. Output voltage drops (for whatever reason) 1V under 1A load to 29V. That's 1ohm output impedance (DF=8).

Now, let's build this amp with gain of 300 but feed 3% of the output voltage back to the input in opposite phase. As a result amplifier’s output is the same 30V as before but input is the difference between 1V and 3% of 30V = 0.1V Let’s verify (1V-0.03*30V)*300=30V

Let’s load this amplifier with 1A. Our voltage drop inside is still 1V under 1A load, but output voltage will be higher than 29V because we subtract less from the input. Output voltage will be 29.9V and output impedance will be 0.1V/1A=0.1ohm (DF=80). Let’s verify. (1V-0.03*29.9V)*300-1Vdrop=29.9V.

Output impedance dropped 10 times. Expression 1+B*Aol is known as “Improvement Factor”. In our case B (“Feedback Factor”) = 0.03 (3%), Aol (“Open Loop Gain”) = 300 thus Improvement Factor = 1+0.03*300=10

Adding NFB WILL add more power to lower load impedance because output impedance is lower. I'm sure Ralph has maximum power in mind.

It is important to realize that it doesn't matter in above example why voltage initially dropped by 1V at 1A load. It could be output impedance or bandwidth limit (or anything else). NFB will reduce output impedance, increase bandwidth, improve linearity - hence THD and IMD.

I think the point that Ralph was trying to make is that using a meter to measure an amp's impedance/resistance at the output taps is **not** what the term "output impedance" is speaking to. Instead, it's referring to the amp's operational behavior in delivering current/power to a speaker load **as though** it's output impedance was a specified number.

Bingo!

I'm sure Ralph has maximum power in mind.

Correct. The maximum output power will be a function of the internal impedance of the output stage. Its Ohm's Law after all :)