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previous: andyfuchs Dynaco's Pas-3 phono preamp used a ... -- 5/20/2000 4:02 PM

Re: positive feedback musings...

5/20/2000 4:16 PM
Ken Gilbert	Re: positive feedback musings... btw andy i read your email you sent to me a little while ago re: cascodes. i can't say that i've got a lot of ideas as to what's going on, or why it's happening. on the other hand, i think i HAVE seen something LIKE what you're referring to, principally the strange bursts of noise when a large input signal transient is cranked into the thing.. it boiled down to a problem with the bias voltage on the JFET. make sure that you've got sufficient negative bias to account for the largest signal that may be injected. even though the jfet will not draw grid (oops, GATE) current, i have a feeling things get a little wild when it's voltage exceeds that of the cathode (oops, SOURCE). one thing to try is a bunch of different jfets, if you've got them on hand. the wide variations of Idss may just get you a unit that will bias up nicely, and avoid the noise. as for my own experience, finally i got frustrated with all the fet bullshit and i went back to the 2nd generation cascode, which was all tube, 6n1p, self biased upper grid. i lost a good chunk of voltage gain, but the sound is much more organic, and better behaved. ken

5/20/2000 4:16 PM

Ken Gilbert

Re: positive feedback musings...

btw andy i read your email you sent to me a little while ago re: cascodes. i can't say that i've got a lot of ideas as to what's going on, or why it's happening.

on the other hand, i think i HAVE seen something LIKE what you're referring to, principally the strange bursts of noise when a large input signal transient is cranked into the thing.. it boiled down to a problem with the bias voltage on the JFET. make sure that you've got sufficient negative bias to account for the largest signal that may be injected. even though the jfet will not draw grid (oops, GATE) current, i have a feeling things get a little wild when it's voltage exceeds that of the cathode (oops, SOURCE).

one thing to try is a bunch of different jfets, if you've got them on hand. the wide variations of Idss may just get you a unit that will bias up nicely, and avoid the noise.

as for my own experience, finally i got frustrated with all the fet bullshit and i went back to the 2nd generation cascode, which was all tube, 6n1p, self biased upper grid. i lost a good chunk of voltage gain, but the sound is much more organic, and better behaved.

ken

Replies:

andyfuchs Thanks for the response Ken. I adde... -- 5/20/2000 4:41 PM