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In Reply to: RE: Frequency Compensation posted by Triode_Kingdom on March 01, 2015 at 15:10:02
I deal with it by not using 6SL7's or 12AX7's.
Your circuit with no bypass cap has an output impedance of about 100K and gain of about 23dB, which is pretty rough. A full bypass of the cathode resistor cuts the output impedance in half and gives you 34dB of gain instead.
A 12AT7 with a CCS will give you around 30dB of gain and output Z under 15K. That's how I would deal with it.
Follow Ups:
I've always found the 'SL7 and 'SN7 to provide excellent sound, and I believe they're more linear than most other octals and miniatures. Also, I'm building this particular amplifier with pre-WWII outputs, and I wanted to keep the late '30s look to the tube complement. That - plus the gain requirement - precludes the use of most other tubes for this.
FWIW, I did try paralleling both sections of the SL7 and bypassing the cathode. Neither technique produced flat response to 20K (I didn't try both at the same time).
I might try your suggestion for a CCS on some upcoming projects. Thanks for your input!
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Buy Chinese. Bury freedom.
Try using a pentode input instead. If you can find something with decent transconductance, you can use a pretty low R plate load and drive that 6SN7 easily.
Realistically, as there isn't much content at 20kHz and since 98% of people who can afford to build and listen to such a piece of gear can't hear anything up there anyway, why put so much effort into it?
If it's 2db down at 20kHz then the -3db point can't be much above that.
The phase will be shifted a full decade below the -3db point, so a -3db point of 30kHz give phase shift all the way down to 3kHz.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Thank you for the further understanding!
Also, there are many articles about life above 20kHz, why do we even bother with hi res digital if not. Why does vinyl sound so good? Ever tried Supertweeters, they really will convince you.
Naz
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