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Negative feedback capacitor question.

Carbsandcaps

Carbsandcaps

Super Member
Working on my fisher x-202-b. I broke the leg off the capacitor in the negative feedback circuit. I can replace it with the same ceramic cap or I can put a film capacitor in there with a 1000uuf rating (which I would rather do) but I’m also trying to learn about this circuit. I understand that a capacitor going to ground can filter some high frequencies out but this one seems to just bypass the resistor then goes to ground. What are the affects of replacing this with a different value? IMG_9958.jpeg
 
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The cap parallel with the feedback resistor forms a low pass filter to prevent oscillation at high frequencies. 680pf in parallel with 1.8k gives a rolloff frequency of 130 KHz. A 1000pf cap would be 88 KHz.

I would stick with the value Fisher engineers chose. Since this is in the signal path use a polypropylene film cap or C0G (NP0) ceramic.
 
It will likely slightly over dampen the amp's HF response and will slightly de-tune the amp's transient response. This cap is important to keep at the correct size and was carefully chosen by the engineering team who designed that amp originally.

The other HF stability tuning circuit in this amp is C39 and R79--those two components are also critical to keep at the stated values.
 
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Its technically a high pass, but since its in the negative feedback loop the result is a low pass. As frequency goes up, the impedance of the cap goes down, so more signal gets through.

It applies more feedback at higher frequencies to limit the amplifier's bandwidth. It needs to be limited enough that the feedback can't go positive, but not so limited that it affects the useful frequency. Make it too big and it rolls off too soon, too small and it can allow oscillation.

so yeah, stick to the stock value.
 
Thanks for the responses. I’ll keep it the same size. The parts manual shows it rated at 1kv but I’m sure that’s because it’s what they had on the shelf. A little different with film caps. There’s no voltage written on that line. Would 100v work? Is this just speaker level voltages?
 
I would use a polystyrene, silver mica or ceramic NPO, in that order of preference.

Jack
 
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