I apologize for my bad english. Quiescent DC is not AC of course! Still, It would be interesting to clarify if a CCS improves AC balance in a PP power stage.
Since the total cathode current is fixed by the CCS in a differential amplifier (voltage or power), then when one tube's current increases, it must cause the exact current decrease in the other tube, just like a see-saw.
Wrong translation of the term "quiescent"
Please don't bother...Thanks! I'm aware of that. What is knew to me is how residual 2nd order distortion interacts with the function of the differential amp. Does this only work with low distortion? Is it OK when distortion is high but equal for both tubes? When we say that a CCS enhances AC balance, do we mean fundamental frequency or the total harmonic sum?
Yes, where the signal level is low the bias shift effect is small so CCS does little harm in that respect. Hence you can take advantage of the CCS other useful properties. Note that for an LTP it is often not necessary to maintain a fixed DC quiescent current - the CCS is used because it has high AC impedance, which is precisely the opposite reason from using a CCS in an output stage.MagicBus said:
LTP CCS: want high AC impedance (for good balance), not too bothered about DC impedance
O/P CCS: want very low AC impedance (so need to bypass), want high DC impedance (to maintain average current - but that is the wrong current to maintain).
So CCS is a good thing in an LTP tail but a bad thing in an output stage.
My query is about the imbalance between the two halves in a differential stage, mostly in a PP power stage. It reminded me a Lynn Olson's article ETF Presentation where he says
"[FONT=Verdana, Helvetica, Arial, sans-serif]All PP or differential amplifiers are really two amplifiers operating in parallel: a perfectly balanced PP amp, and a secondary SE amplifier running 26-30dB down from the main PP amplifier. This secondary, "shadow," amplifier is the sum of all imbalance terms in the tubes and transformer primary, typically 2~5%. You can try various nulling schemes, but this makes the problem worse, not better, because as the residual approaches zero, you start to see frequency and phase-dependent terms appear. In other words, the null will only be at one frequency and power level, and will skip around on both sides of the null under real-world conditions."
[/FONT]That makes me think a CCS may not do what it was intended in a PP stage.
So, the conclusion is that a PP output stage should not be seen as an LTP. Thanks DF96.
One of the best vacuum tube amplifiers uses just such configuration, e.g., Vaccum State Electronics DPA-300B. And there are many differential power amplifiers from Japan... You just got to know what you are doing.
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