Tonebender MKII Reverse Biased PNP Germanium

[epreuss]

Hello DIY-Stompbox Community,
first thread, so i try my best.
I have a problem with a tonebender MKII Circuit. I build them several times with a charge pump or just with positive ground. Now I tried to setting up one with a negative ground.
It woks, BUT: there is a really annoying white noise. i can not explain what's the reason. I tried the same transistors in a positive ground circuit with the same bias and there is not that level of white noise.
I tried different transistors for Q1 with different leakage from nearly zero to 0.1mA with an hfe of 70...100. So maybe there is someone out here, how has the same experience with that. Oh the noise reduces when i tried very low leakage transistor for Q1, but the fuzzy sound isn't that good. for biasing q3 i installed a trim-pot. R_C of Q2 is 80K. changing Q2 did not change the nosie level.
Maybe you have an idea? If you have any question about the circuit let me know.
Thanks Enrico

[midwayfair]

Negative ground with PNP is always noisy. Better to use the charge pump.

The higher leakage in Q1 has less noise because it's less gain.

[epreuss]

The thing with the charge pump is that i had sometimes poor quality of pumps, so that there is always a background squeal, although the boost of the chip (ICL7660S) was activated.... but i think the thing with negative ground and pnp doesn#t work that fine, as some reported around the web. so i have to make my decision. isn#t there any possibilty to lower the white noise?

[midwayfair]

so that there is always a background squeal, although the boost of the chip (ICL7660S) was activated...

Are you sure that wasn't just that you needed some extra power filtering? MKIIs are pretty notorious for oscillation with a power supply. It might just take a lot more filtering than normal.

I'm unaware of anything that will lower the noise you're experiencing without making serious sacrifices to the circuit. You can filter it out from the audio path, but then you're filtering out guitar, too. You can lower the gain so you don't amplify it, but then of course you're losing fuzz.

Except of course to just use NPN transistors with negative ground.

[LucifersTrip]

Most of my MKII's sounded different (worse) with power supply vs battery (simplest solution)...that is until I brought
the power in via a 100 ohm resistor. Then, there's little or no difference.

[epreuss]

i tried to filter the power section, but that doesn't work. filterin the audiopath is no option, because you suck guitar tone! yeah NPN should be the way to go, but i think i give the charge pump a second chance, i ordered a batch of 50 long time ago, and will test them. so the best solution will be to work with positive ground, no matter if people like it or not. thx for your advices

[DDD]

PNP stages with negative ground are extremely sensitive to power supply noise and ripple. In this case equivalent noise source is connected in series with guitar pickup hence it's subject to be amplified as input signal.
It's good to filter power supply positive rail with 500 Ohm - 100 uF Alu/0.047 uF disk ceramic cap or so.
*** I've had just the same problem with BC860C Si tranny. The above mentioned filtering solved the problem successfully.

[italianguy63]

I build gangs of PNP fuzzes with charge pumps.  The charge pump portion of my circuits are stone quiet.  It is filtering and chip selection.  My best experience has been with the TC1044SEPA.  MC