presence ala. peavey

[Andy Le Blanc]

I had a conversation with a guy at a jam about the presence and resonance
controls on his peavey...... that got me to look into the circuit......

now its just as one would expect...... a 5k (4.7K) resistor at the end
of a long tailed inverter with a cap(s) going to ground controled by a pot.
the resonance is a 1M pot in series with a small (.0068) cap in the feedback
feed to introduce a bit of phase shift and color the responce.... etc.....
all have been used before and there are many examples in many differnt amps
and makers...etc.....

but the peavey circuit uses an electrolytic cap to couple these to the top of the
5k resistor....... its a 22/25v with the positive lead connected to the bottom of the inverter ......

why?........ does it provide a bias?...... a phase lead?.....or lag?

[Jana]

i have experimented with resonance circuits like you described. My guess for the 25 uf cap is that it is there to block DC. In the circuits that I experimented with I always had a scratchy sound when turning the resonance pot because it is changing a dc voltage which is eventually finding its way into the inverting input of the PI. I forget the voltages but there is a DC voltage of something like 10 volts or so (just guessing) at the 4.7k resistor and that is also present in the NFB loop, eventually being referenced to ground of the OT.

I never bothered blocking the DC because I knew that the scratchy sound on the pot was due to what I described and wasn't a bad pot. I can imagine though that from a marketing standpoint Peavey would not want people to be turning the pot and thinking it has a bad pot in it.

The early presence controls in plexi's with the .68uf cap connected to the wiper of the 5k presence, with the 5k being the "4.7k" to ground for the PI, they did the same thing, sounded like a bad pot. It was because the .68uf with the stored DC voltage was being moved to a different dc voltage along the pot when it was turned.

edited for a typo

[drz400]

I had a conversation with a guy at a jam about the presence and resonance
controls on his peavey...... that got me to look into the circuit......

now its just as one would expect...... a 5k (4.7K) resistor at the end
of a long tailed inverter with a cap(s) going to ground controled by a pot.
the resonance is a 1M pot in series with a small (.0068) cap in the feedback
feed to introduce a bit of phase shift and color the responce.... etc.....
all have been used before and there are many examples in many differnt amps
and makers...etc.....

but the peavey circuit uses an electrolytic cap to couple these to the top of the
5k resistor....... its a 22/25v with the positive lead connected to the bottom of the inverter ......

why?........ does it provide a bias?...... a phase lead?.....or lag?

Correct, only there to block DC so the pot doesn't sound scratchy

[Andy Le Blanc]

that makes perfect sence...... Im so used to that scratch in a presence knob
that I thought it just had to have another purpose...... one of those traditional
conflicts...... if you dont do it this one way it cant be good..... it sounds good