2-in-1 pedal, NPN and PNP - Possible?

[steveyraff]

As the title says,

A guy wants me to build him a Foxx Tone Machine and a Sunface, both in the one enclosure, but the Sunface has a positive grounding and MP20B/GT402B transistors.

Is this possible, using one 9v power socket for both circuits?

Cheers.

[Fancy Lime]

Yes, it is possible. You can, in principle, turn any PNP circuit "on it's head" and thus turn it into a negative ground circuit. I have done this many times and never had any problems with it. However, when I asked this same question many years ago, I think it was Jack Orman who said that "sometimes it does not quite work for some weird reason" (I'm paraphrasing), especially with Ge fuzzes. I never built a Ge fuzz, so I would not know. With Si transistors or p-channel MOSFETS, I never had any problems. But Jack tends not to pull such things out of thin air, so he probably has a point. Trying it out on breadboard won't hurt, though. All you need to do is flip the whole circuit (from the perspective of gorund) by turning what used to be ground into +9V and what used to be -9V into ground. It looks unfamiliar in a schematic but identical (apart from the supply wires) on breadboard or circuit board.

Hope that helps,
Andy

[m_charles]

It can work just fine. You have to add a bit of an extra chunk to the circuit using a 7660S IC.
Patch that in to your circuit and send the positive ground section through there.
Just look up "reverse polarity 7660s" or something like that on google.

You can do this the quick n dirty way without the IC, but it is a little hacky, can have weird results, and I'd imagine some weird wall hum issues since you are placing into a circuit with NPN. 

[Fancy Lime]

I never fully understood the reason for using a voltage inverter (like the 7660) in a new pedal. If you have an existing circuit that you want to adapt it may be necessary but with a newly built one, you are just as likely to create new problems than you are solving old ones. I would always always always try the simple "flipping the rails" technique, explained in it's simples form here:
http://www.muzique.com/lab/fuzzface.htm

Like I said, that apparently sometimes does not work and even people who know way more about these things then I do are not entirely sure about why. See here for one of may posts on the topic:
https://www.diystompboxes.com/smfforum/index.php?topic=119943.msg1121583#msg1121583

For me personally, it has always worked, but not for everyone. I would hazard the gueass that if R.G. says, there is no universal solution, then there is no universal solution. Although really good power supply filtering is likely to prevent the more obvious problems. The circuit between the input and output AC coupling cap is completely agnostic of what "ground" ev en is, so in theory it really should not matter at all. But apparently, sometimes it does anyway. The only thing that I can think of that may be causing the noise problems in the negative ground PNP circuits, is the ever so slightly different impedance of "real" ground and the "other" (or supply) rail. No amount of power supply filtering can ever make them completely equal but you can get very close. Still, Ge PNP's are strange beasts, so it may not be enough. If that technique just does not work, try the voltage converter but take care to flter that one very thoroughly as well since they give off A LOT of noise. If all else fails, you can use a voltage isolator (http://www.muzique.com/lab/9v_iso.htm) internally to have a "porper" positive ground effect in the same housing and with the same single power supply connector as a negative supply one.

Andy

[m_charles]

Because running a pos and neg ground circuit within the same circuit, getting power from the same source is much different than flipping the rails in an independent circuit. It creates a whole new set of problems. One being, wall hum from power supplies.
Easy way to try. Put an NPN fuzzface and a PNP fuzzface the same circuit, flip the rails on one with the quick n dirty trick, and try it.
Maybe it'll work fine for you, but I'd guess you'd hit some issues like I did years ago. 

[steveyraff]

So, if I just build them normally, as two separate pedals, what are the power requirements for the circuit with the negative -9 power and positive grounding? A 9v centre positive supply?

Its because he supplied me with MP20B and GT402B  PNP transistors to use, so I have to use this layout: http://tagboardeffects.blogspot.com/2013/03/analogman-sunface-nkt275.html

There is also a regular +v layout, but it just uses BC108's.

[Fancy Lime]

To what m_charles said: With NPN and PNP circuits like that in the same enclosure you definitely need to filter the +9V rails of the two circuits separately. Else there will almost definitely be hum and whatnot. ANd like he said: try it, it may work.

If you put them in separate boxes, you want to get a power supply with isolated outputs. I have a Palmer PWT 12 Mk 2, which has eight 9V outs and four that can be regulated between 6V and 18V. The key point is that *all* outputs are individually isolated, meaning you can run up to 12 pedals with 12 different polarities. In a different Universe at least where there are more than two possible polarities. More importantly, the isolation also takes care of a certain class of hum problem. It is expensive but for me it was worth it because it cures many problems in one simple box. There are smaller and cheaper supplies that only offer a few isolated outs, mostly in groups (like 3 outs isolated from the other 3 for a total of 6 outs). For most problems or smaller pedal boards, these will suffice.

Andy

[steveyraff]

To what m_charles said: With NPN and PNP circuits like that in the same enclosure you definitely need to filter the +9V rails of the two circuits separately. Else there will almost definitely be hum and whatnot. ANd like he said: try it, it may work.

If you put them in separate boxes, you want to get a power supply with isolated outputs. I have a Palmer PWT 12 Mk 2, which has eight 9V outs and four that can be regulated between 6V and 18V. The key point is that *all* outputs are individually isolated, meaning you can run up to 12 pedals with 12 different polarities. In a different Universe at least where there are more than two possible polarities. More importantly, the isolation also takes care of a certain class of hum problem. It is expensive but for me it was worth it because it cures many problems in one simple box. There are smaller and cheaper supplies that only offer a few isolated outs, mostly in groups (like 3 outs isolated from the other 3 for a total of 6 outs). For most problems or smaller pedal boards, these will suffice.

Andy

Thanks - but am I still correct in my wording of my previous reply? In that the PNP transistor circuit as shown above with a negative -9V supply will need a 9v centre positive supply?

[Fancy Lime]

What you make center, + or -, is entirely up to you. You can wire your effect up either way to make it match the power supply you happen to have. BUT ONLY IF you use a kind of power jack that is isolated from the housing. In my opinion power jacks should always be isolated to avoid problems (most manufacturers do that). If you use non-isolated jack, then the PNP circuit indeed needs to have the opposite polarity than the NPN circuit.

Andy

[steveyraff]

What you make center, + or -, is entirely up to you. You can wire your effect up either way to make it match the power supply you happen to have. BUT ONLY IF you use a kind of power jack that is isolated from the housing. In my opinion power jacks should always be isolated to avoid problems (most manufacturers do that). If you use non-isolated jack, then the PNP circuit indeed needs to have the opposite polarity than the NPN circuit.

Andy

Hmm, bear with me while I try to get my head around this (sorry). So, say I am wiring up my negative -9v pedal... and I want to use a standard centre tip negative supply... I could just solder the -9v power to the lug on the DC socket that I'd normally use for ground, and then solder any grounding wire to the DC lug I'd normally use for +9v ? So I'm just switching what goes to which DC socket lug? I hope this makes sense!

If that is the case, I can understand better why it doesnt work wiring both +9v and a -9v circuits to the one DC socket, because you'd have a single DC lug sharing both the positive ground of one circuit, and the negative power of the other... is this correct?

I feel like I may be speaking complete gibberish, so I hope you can understand. I am just thinking, the easiest most straight forward option to offer the customer might be to just make two completely separate pedals, but wire the DC socket to each pedal the opposite way around so that he can use centre tip negative supplies to both?

Sorry if I am getting this entirely wrong - and thanks for the help!

[Fancy Lime]

Hi Steve,

you are making perfect sense, no worries. Yes, as long as the power socket is an isolated type, it does not matter which lug is plus and which is minus. You only need to make sure that how you wire the lugs matches the power supply, which you are going to use.

The reason why you cannot use a positive ground and a negative ground effect on the same supply, is that the cable between the effects is referenced to one of the rails at the output of (pretty much) all pedals. Usually, like in a fuzz face (https://www.electrosmash.com/fuzz-face), the volume pot goes to ground after the last AC coupling cap. This pot thereby references the signal to ground. That means that the shield of the following cable must be at ground, else you get some large DC voltage in you cable, which can cause all manner of problems. For noise shielding reasons, the housing of effects is usually also at ground and the shields of the input and output cables is thus electrically connected to the housing. So far so good, but if you do that with two pedals, one of which has a positive, the other negative ground, and you connect them bot to the same power supply, you simply short out the supply via the cable connecting the two pedals. But if you use two supplies or two electrically isolated outputs of one supply, then the problem goes away because the supplies only deliver to wires with a *relative* voltage between them. They neither know nor care about absolute voltages or where ground is supposed to be. In fact, if you connect the two effects as described above with two supplies, you can measure the voltage between the "non ground rail" (aka the power rail) of the one and the "non ground rail" of the other and will find 18V. The two supplies just float around happily at different "absolute voltages" (meaning compared to some immovable reference, like your house ground).

If you are building these things for a customer, yes, I would suggest two separate pedals. And make *very* sure to have good reverse polarity protection (a series 1N5817, 1N5819, or 1N400x diode in series with the power rail straight after the connection to the power plug is usually enough) on all customer pedals, as well as absolutely clear instructions on what power supply to use. "But the plug fits" is the explanation of almost all of my customers who managed to fry their pedal with too much or wrong polarity voltages.

Hope that helps,
Andy

[steveyraff]

Thanks so much for all the help,

So - I'm going to go ahead and build these as two separate pedals.

I'll build the one with positive grounding as normal, but I'll wire all the grounding going to the lug on the DC socket that I'd normally use for +v power, and I'll wire the circuits power going to the DC socket lug I normally use for grounding. That way I should be able to use a normal centre tip negative power supply...

Correct?

I hope so! lol. Educational post - thanks all!

[Fancy Lime]

Yes, the power supply does not know ground, only plus and minus. So you can do as you described as long as you make sure the customer does not use a daisy chain to connect other effects to the same supply. For that reason some people do not include power jacks in their positive ground effects but make those battery only. Just to trick the users out of being stupid.

Andy

[steveyraff]

Yes, the power supply does not know ground, only plus and minus. So you can do as you described as long as you make sure the customer does not use a daisy chain to connect other effects to the same supply. For that reason some people do not include power jacks in their positive ground effects but make those battery only. Just to trick the users out of being stupid.

Andy

Cheers Andy - superb help all round. Thanks.

[tubegeek]

Great thread, some excellent practical advice.

In theory, theory and practice are the same thing. In practice, they're not.

My 2 cents: I use a "flipped" Fuzz Face, for all the reasons described above, using the Muzique.com info. In order to get it to behave properly with an external supply and other effects, all that was necessary for mine was some extra power supply filtering, I think I put 100R in series with the "+9" input and 47uF to "ground" after that. Behaves itself now. I'd say trying the twofer pedal would be worth a look at least. It'd be a pretty cool item - I just love my Fuzz Face!

[m_charles]

Thanks so much for all the help,

So - I'm going to go ahead and build these as two separate pedals.

I'll build the one with positive grounding as normal, but I'll wire all the grounding going to the lug on the DC socket that I'd normally use for +v power, and I'll wire the circuits power going to the DC socket lug I normally use for grounding. That way I should be able to use a normal centre tip negative power supply...

Correct?

I hope so! lol. Educational post - thanks all!

you can do that, but you can also use RG's circuit here. It's pretty simple. Build that, take the neg ground from one, pos from the other. This works like a charm, used it many times. And it's literally one IC and a couple caps. Easy peasy.

[steveyraff]

Alright heres a question....

what way do I wire up the LED ??

I just reversed how I wired the DC socket and it works great, but trying to get my head around why the LED won't go on. I am using this diagram....

[PRR]

Reverse the LED.