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Old 17th November 2006, 11:09 AM   #121
SY is offline SY  United States
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Might that be an argument for doing it the other way around? Generally, the stage that clips first will be the output stage. Placing the shortest time constant there would minimize blocking time after overload.
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Old 17th November 2006, 05:44 PM   #122
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SY, must you be difficult?

Seriously, let us see ........

One would expect the power stage to overload first, wouldn't one? Take, say, 440V h.t. and 100Vsat for a triode driver stage, and at the high side of the cycle another 100V minimum voltage drop across the load resistor for limited second harmonic distortion (the latter very subjective). That leaves a maximum signal Vpp of 240V or 120Vp. Yes, that is unlikely to be reached before power stage saturation, so point conceded!

Guess I was influenced by my use of the semi-cathode feedback output circuit (Quad style), where driver overload could be encountered at the same time as output stage saturation.

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Old 17th November 2006, 06:10 PM   #123
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Quote:
SY, must you be difficult?
Sorry, I was born that way. When I bought Allen Wright's preamp book last year at ETF05, Morgan Jones warned him that I'd start peppering him with quibbles and to perhaps reconsider the sale. Allen, always the wise entrepreneur, made me agree that I would bundle things and only send him one letter, or at least know that he would only read one.
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Old 17th November 2006, 10:14 PM   #124
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Keeps me on my toes, though at my age, please keep my rheumatism in mind .........

(Yah, JP, perhaps you should have spelt that rheumatism-in-mind.
Ah! my always truthful alter ego.)
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Old 18th November 2006, 06:08 AM   #125
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Quote:
Originally posted by Johan Potgieter
The attenuation of subsonics, indeed the attenuation of any products falling outside the audible spectrum, is something that enjoys too little attention imho. One must remember that amplifers are not perfect. The design criteria might be met regarding the audio spectrum, but somewhere outside at both ends, any amplifier will exhibit reactive phase angles as the NFB goes to 0.
I very much agree ... while many audiophiles are opposed to any sort of signal filtering, keeping inaudible signals out of the amplifier frees-up power for the important (audible) part of the spectrum. IIRC, this was why the Marantz 8b had "test" inputs -- the "test" input excluded the rumble filter present on the main input.

Regarding the issue of coupling caps in a Williamson... it has been done both ways throughout history, and certainly each configuration has merit given different design criteria. I believe the Electrohome PA-100 placed the smaller couplers first, while my Craftsmen 500's put the big couplers first (.47uF, then .1uF). I suspect some testing would be necessary to confirm which offered better performance given specific output transformers, feedback circuits, and power supplies.
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Old 18th November 2006, 01:46 PM   #126
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Speaking of feedback, I suspect that the feedback resistor in either the Williamson or H/K amps are tuned to the characteristics of the TO-300 OPT. Should I start using this value with my Edcors or would it be better to buy a couple linear pots and find a good-sounding feedback level? Would I even be able to tell what was a sweet spot for feedback?
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