help calculating treble shunt cap value

[Groover]

This is actually a question about some FX pedals, but it is a theory and design question and the effects forum doesn't seem to get as much traffic as the amp section, so I'm posting here.

I just built an Orange Squeezer and a TubeSoundFuzz/Red Llama. I would like to roll off the highest frequencies of the outputs of both. They both use 10k volume pots as the last part of the circuit, so I am thinking that if I bypass the vol pot with a small cap that would work, but I don't know what value cap to use. I can use trial and error, but I would like to know how to work the math too.

For the cap in parallel with the 10k pot can I just use the f=1/2piRC formula? I also don't know how to calculate the source or output load impedances concerned, especially since I am dealing with ss devices, which I know less about than tubes. Links to the schematics:

http://www.generalguitargadgets.com/pdf/ggg_whrl_sc.pdf

http://www.generalguitargadgets.com/...bc35a68d4e7b6d

thanks!

[überfuzz]

It would be slightly easier to help you if you told us what frequencies you like to bypass.

[Merlinb]

For the cap in parallel with the 10k pot can I just use the f=1/2piRC formula?

Yes, but if you're connecting the cap between wiper and ground then the resistance varies with the pot setting. The resistance is the parallel combination of the upper and low parts of the pot track, so it can vary from zero to 1/4 the value of the pot.

Also, opamps will usually oscillate if you hang a cap right off their output, which is exactly what you're doing if you put a cap on the wiper and turn up the volume! Strictly there should be a 100R resistor directly in series with the output to protect against cable capacitance and that sort of thing, but then those pedals aren't exactly masterpieces of design.

With the Orange squeezer, at least, you can roll off the highs by putting a cap in parallel with R9 (feedback resistor). Use the 1/(2 pi RC) formula as usual.

[Groover]

Actually I was connecting the cap to the input/hot lug of the volume control and ground, not the wiper.

[Merlinb]

Actually I was connecting the cap to the input/hot lug of the volume control and ground, not the wiper.

Unfortunately you can't do that, because firstly the output resistance of an opamp is minuscule, and second, it will probably oscillate. In parallel with the feedback resistor is fine though.

[Groover]

Unfortunately you can't do that, because firstly the output resistance of an opamp is minuscule, and second, it will probably oscillate. In parallel with the feedback resistor is fine though.

I already did it! 36pF - it didn't make much audible difference. I suppose the fb resistor you are referring to is R3 on the above-referenced schematic, and I could increase the value of C5, right?

[Merlinb]

I suppose the fb resistor you are referring to is R3 on the above-referenced schematic, and I could increase the value of C5, right?

In the case of the red llama, yes.