Problems with FuzzFace, PNP-Transistors NEGATIVE ground!

[dr.angus]

Hi there! Just signed up for this great forum.

I'm having this problem with Fuzz Faces and was hoping one of you guys might have an answer.

Goes like this:

I've built 2 FFs so far, both using AC128s (PNP) and both utilizing negative ground (since I find batteries annoying and like to chain many FX to one powersupply). Both FFs hummed exessively and would start to oscillate when I rolled down the volume pot on my guitar. Since a FF has a LOT of gain, I figured my powersupply might induce a tiny bit of hum that I never noticed before and the FF would just magnify it. After trying some heavy filtering in my FF (which didn't help at all) I built a powersupply with the LM317T, filtered properly and BINGO: no hum, no oscillation (..and a grrrreat sound btw.   )

Yesterday, I built a little booster-device, using one AC128 and negative ground. It's just a little emitter circuit, nothing fancy, has a gain of about 20 and still, with the regular powersupply (not the selfmade one) it hums.

So, there must be more to it than just poor filtering of the pw-supply, or the huge gain of a FF, because when I wire it to positive ground the hum is gone (both FF and the booster). And what about the oscillation?

Maybe someone knows what's going on. Also, I wonder if I'm the only one to ever try to build a FF with PNP devices and negative ground. Has anyone ever built one that worked just fine?

Thanks for taking the time to read.

dr.angus

[H.Manback]

Well, from what I've read, the general consensus on the negative ground (with PNP) fuzzes is that they are more trouble than they are worth...

It seems as though the modification to negative ground brings a lot of unexpected and weird problems with it, which usually are very strange and very hard to get rid of...

Don't have experience with the negative ground version myself though, so I'm just writing what I read. Try the search function, neg ground fuzzes come by pretty often.

[mojotron]

The deal with the oscillation is that the current for Q1 and Q2 is drawn through the internal resistance of the power supply. On the emitter side this signal gets added to the signal from the guitar. I've filtered this before with a really huge cap - but it wasn't pretty :oops: ..

I'm guessing the power supply that you built is cleaner and perhaps has less of an output/internal impedance.

[dr.angus]

Alright, thanks! Good to know it's not just me but the circuit as well.

I guess I'll start building NPN fuzz faces.