"Iain M Churches" wrote in message


Hi Arny. This DF business is something that interests me
in trying top find out why amplifiers that have similar FR
and PBW perform differently into a less-than benign load.

I made up a resistive box of 4R and 12R in series, with a
switch across the 12R to short it out. I then measured
the voltages across R1 and R2 (calling them V1 and V2)

Then using the formula: (V1-V2) / ((V2/16) - (V1/4))
I calculated the output impedance Zo of the amplifiers
in which I was interested. From this, using 8 / Zo, I
could calculate the DF.

That's one way to do it. Pretty classic and of course very
good as far as it goes.

I am told that various companies measure Zo at 400Hz
700Hz or 1kHz.

That's typical. Those frequencies aren't far apart for an
amplifier test. If you want to see Zo vary, go to the
frequency response extremes.

I got very similar figures at all these
frequencies. I also found that there was no variation
with power either which confirms what you have stated.

Thanks for the confirmation, as your methodology is classic.

Here are examples of some of my Zo measurements:

http://www.pcavtech.com/pwramp/macrot-5000VZ/zout.gif

http://www.pcavtech.com/techtalk/wir...4BSTleft-z.gif

http://www.pcavtech.com/techtalk/wir...ST-left-ph.gif

The methodology I use is dynamic. I can change the operating
conditions and the plot data on my PC screen in real time. I
can test Zo using loudspeaker and loudspeaker-like loads. I
can use actual recordings of music to drive the test rig.
I've collected a lot of informal data about how Zo varies
under different operating conditions. For good amps, it
doesn't.