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(This is about to get even further off-topic. Hope that's OK. bI am also not starting a new topic as this is just as off-t. as the original)
More interestingly, maybe, though, is that the DC offset seems to be haunting me now. I have a Sony TA-3200F I've recapped, then adjusted and measured via AP box. Measured very well (whoever's so inclined, can read the measurements here:
However, after a few hours of flawless operation, POP!, both channels stop outputting signal. A burned smell accompanies all this (no smoke, though), traceable to the speaker protection board, which is designed to trigger upon either low frequency output (maybe targeting motorboating?), or DC. The latter triggers mine, as I measure 0.85V and 1.38V at output L/R, respectively.
I've been suspicious of the pre/final transistors, but finals being soldered, I came to reconsider deconstructing the amp too much before thinking this through. Thank you very much in advance for thinking on this with me.
On Wed, Mar 11, 2020 at 9:02 PM Radu Bogdan Dicher via Groups.Io <firstname.lastname@example.org
Thank you both.
Interestingly enough, it mirrors part of the modification I've made to the Triode Electronics board I have in my Dynaco MKIII monoblocks - namely, in my case, a cascode
MOSFET CCS far more linearly loading the long tail pair pre-driver to the PP finals. It's been so long since I put this together it completely slipped my mind (in all honesty, I need to spend more time reading the schematic to get all this - thank you Bob for expanding on the details of the circuit). I enclose the schematic for my monoblocks for reference.
If anyone can point to (a source of) Marsh' article in AA, I'd love to see it. I'll try to locate it too.
I guess then the pot is to be adjusted to null the DC balance on the PP as seen at the output of the stage - thank you Herb for clarity. I'll measure what DC I see on there first, to calibrate the effort.
Yes, Herb is right, that is a common amplifier adjustment as well for 0 DC at the output.
Those pots are for adjusting current from the current source, to balance the long tailed pair, but also correct for other circuit imbalance from downstream stages. The long tailed pair circuit itself should already be pretty balanced, as it uses matched-pair-in-one-die JFETS (2SK68A) and cascode transistors (2SC1775).
This is a really premium preamp phono gain stage design, as it uses both a current source on the tail (versus a resistor) and cascode circuit on top. It may look like a current mirror on first glance, frequently used there, but it's cascode, some feel the best architecture you can have there for audio stages used w/ feedback.
Marsh detailed the cascode circuit advantages in Audio Amateur way back in the old days, with plenty of
example circuits and measurements.
More background here
from the wiki on cascode
"The major advantage of this circuit arrangement ... the upper transistor permits the lower FET to operate with minimal negative (Miller) feedback, improving its bandwidth."