building a fuzz with pnp and negative ground?

[Michael]

Im not sure... im building the GGG fuzz and on their page it says...

"Options are to build it with PNP or NPN transistors. There's also an option to build the PNP with Negative ground wiring. The Negative ground is more common in stompboxes and makes it easy to hook up the Fuzz Face to a common power supply. There is absolutely no tonal difference between the PNP positive ground and the PNP negative ground. Whichever version you chose to build, they all use the same Printed Circuit Board layout."

But when i read here it seems that people are talking about positive ground?

So,,,, is it really just a matter of choice, or is there a difference in noise and problems?

If its completely indifferent i dont understand why people dont make PNP negative ground...

Anyway... what would you advice me? (im using PNP AC128 transistors).

Michael

[mac]

The problem is motorboating. I breadboarded some pnp neg gnd and it sounded like a Honda XR 250 I had.
Go pos gnd and use a 9v bat to feed it as it draws less than a 0.7mA - 0.8mA.

mac

[petemoore]

So,,,, is it really just a matter of choice, or is there a difference in noise and problems?
 I haven't heard of any intractable problems with NPN Neg Gnd., or, PNP pos Gnd. FF's when they're wired right.
 Many have converted to one of these two types^ after encountering intractable problems with the PNP -Gnd FF.
  I like the sound mine makes better with this battery anyway, so, I baby it. Actually I baby a couple of my Fuzzes. I never know until I know, then I'll change the order of other of the efkt. chain, and everything changes again.

[southtown]

cant you just switch the wires on the dc jack around?

[zachomega]

I don't understand why everybody avoids positive grounding.  Personally, I prefer it.  In the event that a coupling cap fails at the last stage, the following stage (a tube amp usually?) will have a negative voltage applied to the grid shutting the tube off rather than putting it positive and burning out the tube and its surrounding circuitry. 

Granted, this scenario doesn't appear very often...and since I use batteries, I don't run into the power supply problems of positive and negative grounding. 

-Zach Omega

[R.G.]

Im not sure... im building the GGG fuzz and on their page it says...

"Options are to build it with PNP or NPN transistors. There's also an option to build the PNP with Negative ground wiring. The Negative ground is more common in stompboxes and makes it easy to hook up the Fuzz Face to a common power supply. There is absolutely no tonal difference between the PNP positive ground and the PNP negative ground. Whichever version you chose to build, they all use the same Printed Circuit Board layout."

But when i read here it seems that people are talking about positive ground?

So,,,, is it really just a matter of choice, or is there a difference in noise and problems?

I'm sure. There are really, truly intractable problems with wiring a nominally PNP positive ground pedal for negative ground. What confuses things is that it works OK somewhere between 3/4 and 9/10 times. But when (not if) you hit one of the situations where it does not work the only cure is to change the circuit back to positive ground.

cant you just switch the wires on the dc jack around?

You can. But it won't work right if you do.

I don't understand why everybody avoids positive grounding.

I believe it's because they want to use only one power supply, not one for positive ground and one for negative ground pedals.

[dachshund]

If you're using a power supply to power everything, then you can't mix & match positive and negative ground pedals, am I right?
I want to add some DIY builds with my Boss pedals, so I think I have to stick with the negative ground circuits. Correct?
Sorry if this is such a basic question. I was about to search on this when I saw this post.

[R.G.]

If you're using a power supply to power everything, then you can't mix & match positive and negative ground pedals, am I right?

You cannot successfully power both positive ground and negative ground pedals from the same 9V power supply. It shorts out the power supply.

I want to add some DIY builds with my Boss pedals, so I think I have to stick with the negative ground circuits. Correct?

No, this is not correct. All you have to do is make or by a second power supply for the positive ground pedals. You are perfectly free to use as much as one power adapter per pedal. At minimum, you must use one power adapter per group of pedals that are all either negative ground or all positive ground.

You are opening yourself up to a very difficult time by making normally positive ground pedals work with negative ground. You might get away with it. You also might not.

[mac]

RG, what makes the motorboating in the FF? The noise I heard were always similar, a fixed low freq as if there were some cap charging and discharging repetedly.

mac

[petemoore]

  You can get 9v+/9v- from a MAX1044 and run your pos gnd effect from an inverted 9v+ supply?, the conversion chip taking a volt or so to run..check data sheet.

[Meanderthal]

 I'm right there with ya when it comes to not liking the positive ground. For exactly the reason stated by R.G.- I want all my stuff to be run off the same power supply.

I built a PNP neg ground FF, used smallbear trannys, and it works fine. Through using it to test other PNP germaniums, I've found that it can indeed oscillate with certain combinations of transistors, turning your guitar into a cheesy theremin, complete with pitch control using the volume and tone knobs on the guitar. Fun, but not good.
However, some combinations of trannys(including the smallbear) are working fine. The fact that their gain characteristics change with temperature makes me nervous about relying on it though, as it might just decide to up and oscillate if I use it somewhere else- outside or even just another room.

Sooo...

I just got done building a NPN fuzzface using OC140s. I had 10 to sort through, and the first pair I tried sounded horrible, but once I found a good pair(and a good orientation- those trannys work backwards with lower gain too), it sounds great. Just flip the polarized caps and battery polarity for converting to NPN. It'll work.
Oh, and npn trannys prefer neg. ground of course...

[R.G.]

RG, what makes the motorboating in the FF? The noise I heard were always similar, a fixed low freq as if there were some cap charging and discharging repetedly.

I have to admit to being a bit embarassed here. This is hard for me to say, but... I don't know.   

That's not to say that I don't have things I think it could be. I have a lot of experience at killing oscillations in various setups, both theoretical and practical. When you say - or I hear - motorboating, the words "power supply impedance" and "decoupling" appear in my mind as automatically as breathing. "Gain-phase", "Nyquist", and "relaxation oscillation" conjure up images of both theoretical explanations and how when I moved that ground connection from ...here... to ... there it quit screaming.

But I have expended my best efforts on figuring out a simple, direct, clear explanation of how to stop a positive-ground circuit from oscillating in a negative-ground connection once and for all, and failed. There are specific examples where I've built these and simply could not get them to quit oscillating by anything short of changing back to the original ground. I've tried decoupling, star grounding, ground planes, star distribution of power, rerouting leads, power regulation, RF-style layout, all the way up to burning incense made by Icelandic virgins at the crossroads at midnight while throwing salt over my shoulder.

I could always cure some of them. But there were a few that were intractible. Reconversion to the "correct" ground always worked.

What keeps this alive is that many attempts work fine. Maybe most of them. Which leads to my observation that there are two kinds of people in the world: those that have had an inverted ground setup oscillate, and those that have not -yet- run into one. The next one could be it. That's why I advise beginners not to do this. Advanced builders can take their own chances, of course. But when they come back here for advice, they're going to get this.

Case in point:

I built a PNP neg ground FF, used smallbear trannys, and it works fine. Through using it to test other PNP germaniums, I've found that it can indeed oscillate with certain combinations of transistors, turning your guitar into a cheesy theremin, complete with pitch control using the volume and tone knobs on the guitar. Fun, but not good.
However, some combinations of trannys(including the smallbear) are working fine. The fact that their gain characteristics change with temperature makes me nervous about relying on it though, as it might just decide to up and oscillate if I use it somewhere else- outside or even just another room.

What's likely going on is that the change in impedance in the power supply vs ground lines when you swap them changes the susceptibility to several different kinds of oscillation, and there may be no closed-form solution.

I'm right there with ya when it comes to not liking the positive ground. For exactly the reason stated by R.G.- I want all my stuff to be run off the same power supply.

You can get 9v+/9v- from a MAX1044 and run your pos gnd effect from an inverted 9v+ supply?, the conversion chip taking a volt or so to run..check data sheet.

And these two quotes probably contain the best advice - put a MAX1044 inside the box and invert +9V to -9V to run the effect. The MAX1044 prevents you from having to disconnect the effect ground from the signal ground and rely on the power supply impedances to shuttle the signal down to signal ground, which is what happens in the inverteg ground case.

A 1044 costs about a buck, is a single DIP and the whole circuit uses three caps and the 1044. It's pretty cheap insurance.

[Michael]

i really appreciate your help!

Thanks to all.

Michael

[mac]

I have to admit to being a bit embarassed here. This is hard for me to say, but... I don't know. 

"And in knowing that you know nothing, that makes you the smartest of all."  Socrates, 470 BC-399 BC

I've tried decoupling, star grounding, ground planes, star distribution of power, rerouting leads, power regulation, RF-style layout, all the way up to burning incense made by Icelandic virgins at the crossroads at midnight while throwing salt over my shoulder.

Maybe you had to put the salt on your hand and forgot the tequila after the salt... ¿?

I wonder if the FF motorboating I hear is the same you hear. I mean, every time I tried with different Ge and gnd it was the same noise. If the noise is different for each of us then it is more likely a problem of gnd or construction. If it is the same noise for all of us maybe it is the circuit. Just guessing...

mac

[R.G.]

Since I've never been able to come up with a definite cure and since you can get any mix of motorboating, audio oscillation, and RF oscillation, I suspect that there are several demons lurking behind the one situation.

It's one more reason for beginners to be advised not to try it.