Twin Face Input Cap

[phi1]

I've been studying the twin face schematic among other online resources in preparation of a PNP Ge Fuzz Face build.

There is only one input cap that both PNP and NPN circuits share. It is + on the input, and - toward Q1 Base. For PNP, this is good since the Q1 Base will have negative DC, about -0.2V according to the electrosmash article. But, for the NPN circuit, the Q1 Base will be positive, so the input cap is backwards of what it should be.

I understand that it's already up to a 4-pole switch, so it's preferred not to add another switch pole for this capacitor.

Is the bias so low on the Q1 Base (around +/- 0.2V) that the electrolytic capacitor polarity doesn't matter? Or, is it better to use a 2.2uF MLCC cap that is not polarity sensitve? Is there any downside to the MLCC in this case?

 

[Robert]

Yeah technically it's not "correct", but hasn't really caused any issues. Back when the circuit was designed I planned to make a modification if it was needed, but it didn't really seem to make any difference, so I never revisited.

There are a few ways to address it:

1) Ignore it
2) Use a non-polar cap (2.2uF electrolytic or MLCCs exist)
3) Replace it with back to back 4.7uF electrolytics
4) Re-orient the cap so it gets switched with the rest of the fuzz circuit (would need to add another cap)

I'll see about doing #4 in the next revision of the PCB, just because it's the "correct" way to do it.

I know plenty of folks are against MLCCs in the signal path, personally I have no problem with them and see them used in big name production pedals every day.

 

[Chuck D. Bones]

Another consideration is to reduce the capacitance for a less muddy tone.