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In Reply to: RE: ALT. 1. posted by cpotl on January 09, 2015 at 08:09:17
HINT: The current draw in a Class A amplifier is NEVER, EVER "constant" when the amplifier is reproducing musical dynamics.The THEORY that current draw is constant in ANY kind of amplifier is an old-wive's tale that needs to be put to rest.
Constant current draw DOES NOT EXIST in any circuit that is processing musical dynamics.
---Dennis---
Edits: 01/09/15Follow Ups:
"Constant current draw DOES NOT EXIST in any circuit that is processing musical dynamics."
Assuming perfectly linear tubes, a push pull Class A output stage will draw constant current on a moment by moment basis.
Assuming reasonably linear tubes, the current draw of a push pull Class A output stage will not change by very much at all.
So while you statement is in fact true, it's misleading.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Assuming reasonably linear tubes, the current draw of a push pull Class A output stage will not change by very much at all.
This comment always bothered me. Chris covered it when he included the "over a period of time" but when we look at the actual current through the output the picture is quite different.
this is a few watts out of a 2A3 with a DC bias of about 58.5ma
dave
That looks like the current draw for one tube playing music.Add to that another tube with an input signal out of phase but otherwise identical and the total current draw change on the power supply should go down.
If both tubes were 100% linear and if the drive signals were 100% out of phase with each other but otherwise 100% identical then the current draw change would be zero.
There's no such thing as a 100% linear tube so Dennis' statement "Constant current draw DOES NOT EXIST in any circuit that is processing musical dynamics." is true but in the context that he made it, IMO, it is misleading.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 01/09/15
What is all this PP talk in a SE forum.... what next those ideal transistors?
dave
And furthermore Dave, isn't this discussion about a B+ filter to a SINGLE ENDED A1 amp, and not a PUSH PULL A1 output stage ?? I am certainly no electronics whiz, but might that be significant ? If so, why would one anyone inject P-P examples in a SE amp design discussion ?
Jeff Medwin
" "Assuming reasonably linear tubes, the current draw of a push pull Class A output stage will not change by very much at all."
This comment always bothered me. Chris covered it when he included the "over a period of time" but when we look at the actual current through the output the picture is quite different."
That statement you quoted was for a push-pull class A amplifier. I think the plot that you displayed is for single-ended, right? The current as a function of time looks just right for single ended; fluctuating roughly equally up and down around the quiescent level.
For push-pull class A, the total current should be very nearly constant, since it should be like the sum of the plot you showed plus an inverted version of that plot.
Chris
"The current draw in a Class A amplifier is NEVER, EVER "constant" when the amplifier is reproducing musical dynamics..."
Indeed, of course it isn't. As has been said many times, for an SE amplifier the current draw fluctuates in step with the audio signal. But the average current draw, averaged over the timescales of the audio frequencies in the signal, is approximately constant. (The current oscillates up and down around the quiescent current. Assuming there is no gross distortion, the fluctuations above and below the quiescent level, on the positive and negative halves of the audio cycles, are essentially equal in magnitude, and hence the average current is essentially constant.)
Chris
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