PCB Input 'Low-Pass' Filter Cap Noise?

[Steve Mavronis]

I was curious about this because the MXR Distortion+ features a .001uF cap at the PCB input going to ground. Some call it a low-pass filter cap. So I wondered since they are so similar (almost identical circuits) what effect would the same have on my DOD YJM308 Overdrive, which had been modded to 250-OD grey specs. I used test lead jumpers to connect a .001 cap between the board input and ground and listened before and after with the gain knob at max. The pedal is noisey to begin with, but there was much more louder noise with the .001 cap than without - why?

[electrosonic]

I think your test leads are acting an antenna and adding noise to the circuit. The cap is to shunt high frequency noise to ground to keep it out of the audio chain. It should be invisible to audio frequencies. Without looking at the schematic, there should be a series resistance before the capacitor to make the low pass RC filter.

Andrew.

[Steve Mavronis]

Hmm that could be it, the test lead wires are pretty long about 2 feet each. I'll try to do a better test by soldering the cap ends to the first pin of the regular input cap and pot ground pin.