Global Feedback Question

[Ben Reginato]

How important is the quality of the Global feedback cap and resistor? I have a Fisher 800C, would it help to put a SilverMica cap instead of a ceramic disk cap?

 

[rayma]

How important is the quality of the Global feedback cap and resistor? I have a Fisher 800C,
would it help to put a SilverMica cap instead of a ceramic disk cap?

Very important, since it has the amplifier's entire output voltage across it.
Virtually any other capacitor type would be better than the X7R or Z5U
ceramic that's in there. Also, it should be a non-inductive type.

 

[PRR]

C21 is 1,000pFd which is unlikely to be a X7R/Z5U.

_I_ don't think the 500C is, or can aspire to be, the Ultimate Amplifier. It is a fun piece just the way it was made. Well, but down to a price.

But sure, knowing you have Silver Mica is always good.

In NFB theory, the guts of the amplifier hardly matter at all. The NFB implements the inverse of the resistors (and any caps) in the NFB network and that is what the amplifier gives you. In telephony, a repeater (working many thousands of hours) amplifier guts could fade-away with age (up to a point) but the critical Gain could be made rock-solid with two high-stability resistors.

 

[6A3sUMMER]

PRR,

You remind me of another type of 'fading'.
The people who maintain optical fiber in the ground, call it 'Backhoe Fading".
That is why you see signs: "Do not dig here, buried Optical Fiber".
How much Ma Bell's world has changed over the decades.