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Old 5th October 2012, 04:25 PM   #21
kevinkr is offline kevinkr  United States
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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.
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Old 5th October 2012, 04:28 PM   #22
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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.

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Old 5th October 2012, 04:30 PM   #23
kevinkr is offline kevinkr  United States
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Old 5th October 2012, 07:09 PM   #24
DF96 is offline DF96  England
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Originally Posted by ruffrecords
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.
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Old 6th October 2012, 09:32 AM   #25
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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?
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Old 6th October 2012, 09:45 AM   #26
disco is offline disco  Netherlands
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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).
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Old 6th October 2012, 10:21 AM   #27
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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 donīt get it, class A means that there is no current variation within the signal.
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Old 6th October 2012, 11:03 AM   #28
jane is offline jane  Norway
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Still donīt get it, class A means that there is no current variation within the signal.
No it doesn't.
Draw a loadline over the plate curves and you will see. As long as there is a load the loadline will not be horizontal and the plate and cathode current will depend on the signal.
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Old 10th October 2012, 04:25 PM   #29
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Still donīt get it, class A means that there is no current variation within the signal.
In class A the average (supply) current is constant, equal to the quiescent current and independent of the signal level, but the instantaneous current is not.

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Old 10th October 2012, 04:57 PM   #30
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Originally Posted by kacernator View Post
Still donīt get it, class A means that there is no current variation within the signal.
If there was no current variation, there would be no (ac) output current.
The ac output current is the difference between the instantaneous currents in the two output halves.

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