Using A Pot To Control Feedback Loop

[Tim C.]

I have seen articles relating to a mod involving changes to the feedback loop in an amp. Is using a 1/2 watt 25K pot to control the amount of feedback the phase inverter sees dangerous?

[Earl]

Changing the series NFB resistor to a pot won't hurt anything. You might consider using a smallish fixed resistor in addition to the pot.

[stokes]

Basically what a presence control does.

[Enzo]

Dangerous?

Remember, the load determines the current draw.

An example here. Let us say we have a 100 watt amp. That means about 28v RMS across an 8 ohm load. Now if I connect a feedback resistor to that, it will have 100v across it more or less. At most, depending upon where it goes. SO within that resistor, power is dissipated by the voltage across it and the current through it.

Power = voltage squared then divided by resistance P=E^2/R

So (28x28)/25,000 = 784/25,000 = 0.03

Your 25k resistor - or pot - would have to dissipate 3/100 of a watt, or 30mw.

If you turned it down almost to zero, then of course dissipation would rise, but you wouldn't want to do that anyway. AS EArl mentioned, you would probably want to make a minimum resistance in the circuit with a resistor.

[Regis]

The resonance control is a cool mod, I got this off the old Ampage, Enzo was it you that originally posted it?

Anyway it effects the low end of the NFB and allows you to dial some crunch and distortion in and out if you want to.

[Enzo]

Not me, I would have been more likely to point someone to a Peavey schematic that had the resonance control and say "steal this circuit."

[kg]

one caveat--from a stability standpoint, it's possible to induce oscillations from either too much or too little feedback factor.

this article might help to explain it.

if the amp has some components and/or operating points that are pushing the envelope, an unnoticed parasitic oscillation could be the last straw.

ken