DOD250: what's this 25pf cap do?

[rhys]

I fired this up for the first time this morning, but I didn't have a 25pf cap (C5) so I left it out.  How would adding the cap change the sound?

http://www.generalguitargadgets.com/diagrams/dist_250_lo.gif

[tungngruv]

From what I understand (I could be wrong), I think it's supposed to tame the "highs" of a dual op amp. Supposedly a lm1458 or 4558 has harsh sounding highs associated with it.

[Mark Hammer]

It's for taming fizz.  In conjunction with the 1M feedback resistor, it forms a lowpass filter rolling off highs starting above around 6.4khz.  Although there is an additional fizz-taming cap in parallel with the diodes (C7), it is advisable to feed the diodes with a signal that is already less prone to fizz since the diodes will add harmonics, and you don't want harmonics of harmonics.  C5 will also help in keeping hiss low.

You can probably tolerate values for C5 as low as 15pf, though don't go any higher than 47pf.

[Stevemas]

I've tried a 101 ceramic cap and it works wonders.

[Groovenut]

From a purely stable design standpoint, that cap is there to prevent high frequency oscillation (ie frequencies so high we wouldn't be able to hear them).

From a guitar pedal design stand point, it's as mentioned earlier. A low pass filter for taming fizz. Primarily when the opamp starts clipping the rails, which happens when the gain of the stock pedal is at about 60%.

I like a little bigger cap here as well. Anything from 100p - 220p.

For a little different flavor of clipping you can also try offsetting the bias voltage. This will give asymmetric rail clipping in the opamp. You can try a 10k and 33k and see if you like it. As long as the 2 resistors total around 44k, you should be good.