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Measuring preamp output impedance

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Everyone has covered this pretty well, the large current swing at the output modulates the transconductance of the cathode follower and results in a varying output impedance and affects harmonic spectra at the output as well.. I'd say my suggestion to change the input impedance of the unity gain power amplifier stage to 50K would be a reasonable mitigation, provided that it actually dominates the input impedance. (With bipolar transistors unless heavily degenerated this will not necessarily be the case, so are you using fets or bipolars?) Can you post the actual circuit the Aikido is intended to drive.

I didn't check before posting the comment about the white cathode follower, in fact this circuit could potentially provide better performance than a CCS loaded CF. You'd have to test it to be sure.
 
The small signal model is fine provided you have small signals. Part of the art of engineering is knowing which approximation to use, and not complaining that 'theory is wrong/useless' when the wrong approximation has been used instead of the right approximation. It is not the fault of the approximation if an engineer naively uses it outside its domain of applicability.

Exactly, and not realising this leads to the common erroneous assumption that (small signal) output impedance and drive capability are the same thing.

It is easy to design two different cathode followers operating at the same gm, one using a 12AX7 and another using a 12AU7. The small signal output impedance of both will be very much the same but the drive capability of the 12AU7 into, say, a 10K load will far exceed that of the 12AX7.

Cheers

Ian
 
ruffrecords said:
It is easy to design two different cathode followers operating at the same gm, one using a 12AX7 and another using a 12AU7. The small signal output impedance of both will be very much the same but the drive capability of the 12AU7 into, say, a 10K load will far exceed that of the 12AX7.
Yes, and into a 100K load it could be that the 12AX7 has lower distortion. Hence asking things like "what is the best valve for a CF?" is in the same category as "what is the best length of string?", yet people continue to ask "best?" type questions.
 
To a first approximation output impedance will be 1/gm. gm varies with current, and therefore will vary with signal. All circuits have this problem to some extent. Good design minimises it, but cannot eliminate it, therefore output impedance will vary with signal.

An alternative point of view is that AC impedance is a small-signal phenomenon (strictly, for infinitesimally small signals only) and is undefined for large signals where non-linearity is an issue.

But the cathode follower works strictly in class A single-ended mode, so where is that current variation?
 
What is meant here with current is the anode current, internal to the tube. Anode current changes value over the tubes' operation in amplifying the audio signal. It's a dynamic property, versus the static property you're thinking of (resulting from biassing, setting the tube working point).
 
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What is meant here with current is the anode current, internal to the tube. Anode current changes value over the tubes' operation in amplifying the audio signal. It's a dynamic property, versus the static property you're thinking of (resulting from biassing, setting the tube working point).

Still don´t get it, class A means that there is no current variation within the signal.
 
Well, the preamp is intended to drive 100W class AB unity gain power buffer, so preamp output should be around 40V at full 100W output.

I do not know how to measure it properly, I mean why so different results? Is it the problem with DMM?


The output impedance is NOT constant over such a HUGE range. Your two test points are 15K and INFINITY. That is to large. Try two loads that are spaced closer like 15K and 150K or 15K and 20K

The output impedance depends on the operating point. Try 10K, 15K, 25K, 50K and so on and make a plot.
 
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