Positive and Negative Ground in Same Enclosure

[eldanko]

Howdy friends -

I've done some Googling on this, but wanted some second opinions in here. I've built up another Dual Fuzz Face, and I'm having noise issues. The first time I did this, I used a PCB for the PNP side that had a bipolar charge pump built in. This time I did both on stripboard, still using a bipolar pump on the PNP side. The PNP side works fine - great actually. The NPN side is extremely noisy at all settings... unless I dial the PNP knobs to zero (even if that side isn't engaged). Some questions:

1) I'm guessing my issue stems from the + and - sides sharing the same ground. Agree or disagree?

2) Assuming that's true, would replacing my current bipolar supply with a Road Rage solve the issue?

3) Is there a way to solve it without adding a RR board or a second, independent power supply?

Sorry for the blurry pic. PNP side is on the left. Thanks!

[eldanko]

Anybody? 

[gtr2]

My vote is Option 2, road rage.  If I understand the description of how your powering it.

[eldanko]

Thanks for the reply, Josh.

Currently, both boards share the same 9v tap and ground. The PNP board has an onboard MAX1044 that I'm using as an inverter. It powers that side just fine, but I'm suspecting that it's creating some problems with the NPN side.

I had guessed that Road Rage would be a better option, but I'm not sure I understand why. How is the RR able to power both + and - ground effects without noise issues?

[RobA]

What kind of noise are you hearing? Is it a whine or shot or thermal noise or normal 60/50Hz hum?

The RoadRage is a charge pump. It does have noise, but the board uses filtering caps to take out the noise. It does this quite well. Depending on the charge pump used though, even it can get a high frequency whine if the charge pump isn't in working at a high enough frequency. I did some experiments with it a found that the LT1054 charge pumps work the best by a wide margin.

Also, the RoadRage docs specify that you should use only the 1044's with the SCPA identifiers because others can introduce noise.

[eldanko]

I don't think the noise is coming from the pump. I've definitely experienced the whine you describe with LT1054s before, but I'm using a MAX1044. It's best described as a static/radio/interference, and it only occurs when using the NPN side. It goes away if I set the PNP vol/fuzz pots to zero.

Correct me if I'm wrong here: when the PNP side isn't engaged, the board input gets connected to ground through the 3PDT and the board output goes... nowhere. Since the controls on that side DO have an effect on the noise level (even when that side isn't in the signal path), am I correct in assuming it must have something to do with the shared ground?

[RobA]

First off, it's kinda weird that you've heard the whine with LT1054's because they have a much higher frequency than the 1044 varieties. But, it sounds as if that isn't your issue.

It does sound to me as if there is a problem with the ground and injection of noise from the PNP side to the shared ground. There are different ways of wiring the 3PDT, but yes most of them seem to take the path of grounding the input and leaving the output floating when not connected.

Is there a pulldown resistor on the output of the PNP side?

[eldanko]

I play through a Matchless. They're somewhat known for their ability to amplify high frequencies. 

This is the layout I used (hope it's ok to post this here). I do not see a pulldown, unless I'm reading it wrong!

http://tagboardeffects.blogspot.com/2010/11/fuzz-face-pnp-negative-ground-with.html

[RobA]

I put all of the various charge pumps I could get my hands on on a scope. The 1044's all had whine down around 8 to 12 kHz even when boosted. The LT1054 was running over 30kHz unboosted. That's why I thought it was weird. But, it doesn't sound like that has anything to do with your issue.

I don't see a pulldown on there either. So, the output is completely floating (as is the input). These original designs didn't take into account the idea that you would use the pedals with anything other than guitar and amp. So, they depended on the guitar output and amp input to provide the pulldowns. it doesn't always work when mixed with other effects. This could be the problem.

I'd try putting a 1MΩ resistor between the input and the ground and the output and the ground of each effect. You could test it by just clamping a 1MΩ from the output to the ground of the one you suspect as the source of the noise (PNP side?) and then see if it helps.

[eldanko]

I'll give it a shot, but I've only been testing this with guitar -> pedal -> amp, not other pedals!

[RobA]

Well, you really have two pedals there, even if they are in the same box. It is likely that they could interfere with each other even in two boxes. Their grounds would still be coupled.

[eldanko]

It is likely that they could interfere with each other even in two boxes. Their grounds would still be coupled.

Yep, my thoughts exactly. That's what's sort of baffling with this thing.

Just tried the pulldown resistors. No perceivable change. It's the fuzz knob (PNP side) in particular that has a pronounced effect on the noise. Fuzz at zero = NPN side works as it should. Fuzz anywhere else = lots of noise.

[warriorpoet]

Have you tested the PNP circuit by itself?  That sounds very near an issue I've had with a noisy transformer in a FF Q2.

Edit: looking at your setup, it's also possible you have a short on one side.  Best thing to do is check your circuits out of the box.  I personally hate doing this, so I can understand your reluctance

[RobA]

If you havn't got this worked out yet, one thing I thought of to check is for ground loops. In particular, look at how the fuzz pot goes to ground. It's possible a ground loop on the PNP side got introduced between the way the fuzz pot comes off and is attached to the volume pot on the strip board layout. Maybe a ground loop is introduced when the input of the board is connected to ground in bypass?

It might be worth it to take all the grounds from both sides and bring them to a star ground point to see if that reduces the noise.

[eldanko]

If you havn't got this worked out yet, one thing I thought of to check is for ground loops. In particular, look at how the fuzz pot goes to ground. It's possible a ground loop on the PNP side got introduced between the way the fuzz pot comes off and is attached to the volume pot on the strip board layout. Maybe a ground loop is introduced when the input of the board is connected to ground in bypass?

THIS. Nailed it. I redid my ground scheme and it eliminated the noise. Good call!

I ended up putting pulldown resistors on both circuits to get them to play a little nicer with other effects. Seemed to do the trick with everything except my Vibe, which isn't a big deal. Volume rolloff seems to be unaffected, so that's nice.

Thanks for all the help friends. Until the next snafu... signing off.