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In Reply to: RE: Frequency, etc posted by fredtr on March 23, 2021 at 10:24:58
Thank you for your excellent ideas. It is interesting to me, because I have an older schematic, dated 1977, in which the value of that ceramic capacitor is 290 pF, not 470 pF. Both of my amplifiers in both positions have a 470 pF capacitor in place. In addition, that is the one part that I have not double checked in recent months. And I need to take another look at that. Also, I had planned to just try substituting R 201 with a 10 K resistor as shown on the 1978 schematic I provided. Right now I have 22K, as I mentioned. As to a lower noise diode, I am using UF4007's, instead of 1N4007's. What would you use after having tried that?
You could try a soft recovery diode, but I would be inclined to focus on the RC time constant, trying different values for C202 and C204 or the varistor RV201 and RV202. Regarding R201/R203, I had missed that. That is a good question why one is 10K the other 22K. I agree with you, I would experiment with changing those. I would guess that it was simply a mistake on the schematic, someone meant to change them both to the same value, and only one was changed. But geez I have seen stranger things.
As you say, I have been assuming that the value shown for R201 (10K) was in error on the schematic. Especially since on both of my actual amplifiers, R201 = R203 = 22K, with the attendant C202 = C204 = 470pF. On the 1977 schematic, C202 and C204 are 290pF. So they were clearly tinkering with that part of the circuit.
1977: uses 47K and 290pF, both sides
1978: uses 22K and 470pF, both sides (assuming 1978 schematic is in error)
It looks like the other thing they changed were resistors 206 - 210, which will affect amplifier loading and the feedback path. Are these resistors 1.0M or 2.2M in your amplifiers? Have you experimented with different values for these resistors?
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