"Arny Krueger" wrote in message
...
"Iain M Churches" wrote in message



Hi Arny. Good to be discussing something of mutual
interest with you. I have rather lost track of whether
you though Zo and DF were important or unimportant
factors in the evaluation of a valve amplifier.

Can you please confirm your position?

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)

(snip. my methodology)

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

Please elaborate. If there are shortcomings in this method
I would be interested to hear of them. I know you have
considerable experience in this field.

I know of three static methods (there may be more)
The original classic way seems to be to inject a low level signal
into an amp, and then measure the output voltage with no load.
Then add a variable wirewound resistor and adjust it until
the Vo falls by 6dB. This resistance is then equal to the
Zo of the amp. This is a simple method, but assumes that
all amplifiers are stable into an open circuit.

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 did that, but into my 4 and 16 Ohms resistive load.
There was very little variance. I feel that the same
measurements into a real-world load would yield
different results (more of that later)

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.

I know of a static third method, which involves using both channels
of a stereo amp. The dummy load is connected between the
output of one channel which is driving with a smallish input
signal, and the output of the other channel which has no signal
at its input.

I would be interested to hear about your dynamic testing,
Arny. I have access to a broadcast facility lab.

I thought your PC plots were good. I use Excel and enter the
data into two columns and let the software plot the chart,
in four colours on log paper which looks even better.

I am interested in what you term loudspeaker-like loads. By
this I presume you mean simulated loads. I am in the process
of building one of these from the circuit published in Stereophile
and designed by Ken Kantor

http://www.stereophile.com/reference/60

It represents a smallish two way speaker. I am interested also to
find a schematic that simulates a large studio three way
design.

Lets's forget Mikey falling down for the moment.
I have renamed this part of the thread.

Cordially,

Iain