Question about caps on 4PM filter board

There are four caps on the 4PM filter board that are supposed to be polystyrene (220p) and one that is supposed to be tantulum (10u). I’m wondering why that is? What happens if I just use your basic, garden variety, everyday capacitor in these positions?


The LT1054 requires a low-ESR cap for the charge pump (hence: tantalum). If you don’t use a tantalum capacitor there, the DC-DC conversion circuit will be less efficient and won’t be able to deliver as much current. This is not a problem because the circuitry on the 4PM board is not very demanding; but if you add additional circuitry (CV processing stages? an additional FX?) running off the -5V rail you’ll hit the limit.

The filter requires closely matched capacitors so that each pole is at the same cutoff frequency. Hence, polystyrene with a low tolerance (C0G NP0 ceramic would work too). If you use capacitors with widely diverging capacitances (due to large tolerances), each stage of the filter will be at a different cutoff frequency and this mismatch might prevent self-oscillation.

Thanks Olivier, quick and useful reply as always. I had a spare 4PM board laying around and was going to build it up from parts I had around, but I think I’ll order the extra few special caps. Not sure what I’ll do with it once it’s done but at least it will be ready to go for something.

Sounds like a great opportunity to use it for the upcoming “Programmer Digital Board” Pichenettes is getting ready…

Perhaps. Thanks for the idea.