Need advice

[Leo_Gnardo]

True, good idea. Also, what about wiring of the NFB as far as noise....would shielded cable be a good idea since it's pretty long run with the variable NFB pot and all?

Consider the typical NFB run, from the impedance selector switch to one end of the 100K or whatever Rfb, then to the presence pot. As long as the high-signal (speaker) wiring is kept at reasonable distance from the rest of the pre circuitry, generally no problem. If you keep most of your pot/switch/R's & C's near the back panel, you'll be staying away from the pre circuits & less likely to bring on ultrasonic oscillation whether constant or in chunks riding the output waveform.

[daz]

Consider the typical NFB run, from the impedance selector switch to one end of the 100K or whatever Rfb, then to the presence pot. As long as the high-signal (speaker) wiring is kept at reasonable distance from the rest of the pre circuitry, generally no problem. If you keep most of your pot/switch/R's & C's near the back panel, you'll be staying away from the pre circuits & less likely to bring on ultrasonic oscillation whether constant or in chunks riding the output waveform.

There are fairly long runs. Goes from imp. selector to NFB resistor on the turret board. Other side of resistor goes 4" to the NFB pot. From there a 6" run back past the 100k to the PI tail.

[Leo_Gnardo]

There are fairly long runs. Goes from imp. selector to NFB resistor on the turret board. Other side of resistor goes 4" to the NFB pot. From there a 6" run back past the 100k to the PI tail.

The end with the large (speaker) signal on it is the one to worry about. So if you can keep that where the speaker signal already is, you're going to be OK. Again consider the typical FB signal location, one end of it's near the input to the power amp, granted, but not enough gain there to raise hob with unexpected oscillations. Now the other end of the FB R is a low level signal, not likely to interact with nearby circuit points, and a fairly low-impedance to ground besides, considering its path thru the regular presence control. Mostly you want to keep speaker size signals as far as practical away from earlier stages of preamplification.

I think you can skip the use of shielded cable for your feedback circuit.

If you want to look at the circuit model involving shielded, you'll have to think of the cable capacitance which "shorts" high frequencies to ground, and would tend to tip up the audio spectrum at the hi end as in turning up the presence control. This effect may be darn near negligible considering the Z to ground for the FB circuit points but at worst you get more HF sizzle. Not where you want to go. Use your favorite kind of plain ol' wire.

[daz]

Got it, thanks.