DC jacks on positive ground pedals

It seems that at the same rate the "boutique" effects manufacturers make themselves and their pedals more and more common knowledge, more and more people are complaining about how their shiny new fuzz pedals won't work with regular power adapters... This is from a thread on the Harmony Central Effects forum, and it's just one of many on the same subject:

Was using my newly-acquired [insert the name of your favourite PNP fuzz/booster here] this weekend, and tried to power it through my SKB board. Didn't work -- in fact, when I plugged in the Xxxx, the power went out for all of the other pedals (4 others) on the board. So I thought maybe I overloaded it, and tried it with just one other pedal - same thing happened.

Is this typical of the Xxxx? Or do I just have a bad one? (Which is weird, considering it's new...)

I do think there's a definite connection between this phenomenon and the fact that most small-time effects builders sell clones of old-style positive ground PNP designs... As more and more of these clones turn up on our pedalboards, more and more of us are having problems powering them - since they can't be powered the same way as your usual Boss pedals. This wouldn't be a problem if manufacturers of PNP pedals took the time to actually explain how to use the DC jacks installed on them. But do they? Not really.

This is a polarity issue, and before we do anything else, we need to get aquainted with the term "polarity". Most people seem to think that the reason these fuzz (and germanium booster/octavia) pedals won't work with their power supplies is because the

 

Jack/plug polarity

The polarity of the DC jack really has nothing at all to do with the polarity of the circuit - so there's no rule to force everyone to wire their jacks the same way. Some manufacturers wire the DC jacks "backwards" (center positive, instead of negative) on PNP pedals, while others seem to use the regular (center negative) wiring style. I'd probably end up doing it "backwards" too, since it will let you switch the battery in/out from the adapter jack the same way as you would on a normal pedal. But my point is that you don't have to...

But hey - why is the jack wired the way it is in the first place? It does seem a little odd to carry the positive on the sleeve of the plug, doesn't it? Especially since it means that for a regular negative ground design, you can't use a metal-sleeved jack (as it would send the positive straight to the chassis, and short the power out). Well, this is all Boss' fault... before they started using the barrel type plug, almost all pedals had the 3.5mm (1/8") mini plug instead. That plug is still used by Electro-Harmonix and on the RAT, by the way. But Boss chose the 2.1mm barrel plug/jack for their compact pedal line, and others have since followed (for instance, Ibanez started using them in '83, when the 9 series was released). The 2.1mm jacks that were available had a switch function, which was (and still is) used to switch the battery positive out when a plug is inserted in the jack. Since that switch was/is located on the sleeve connection, the positive voltage needed to go on the outside of the plug.

 

Ground polarity

In short, a positive ground (PNP) design can't share its power with negative ground designs - that will short out the power supply. This is because the pedals share a common signal ground - through the patch cable - which in both pedals is connected to the chassis. The chassis is also connected to the power's ground. In a "regular" pedal the adapter/battery negative goes to this common ground, but in a PNP pedal the adapter positive is connected to the same ground. If you combine them, the result is of course a short-circuit in the power supply. The solution is to run the PNP pedal(s) from a separate supply, or to use batteries with them.

What bugs me is that the manufacturers don't seem to realize that most people don't know these things, and will inevitably get it wrong, without proper instructions.

 

Which adapter works, then?

Most times, you need a separate adapter with a reverse polarity plug (center positive instead of negative). But you do need to verify the polarity with the manufacturer before you try - don't just plug in an adapter to see if the pedal lights up...

The reason why most PNP pedals have center positive DC jacks (wired backwards compared to "normal" pedals) is not to force you to buy a special adapter from the pedal maker, even though it might look that way. Rather, it is simply easier to wire the jack that way, to tie it in with the "input jack power on/off" scheme. As mentioned earlier, the DC jacks has a switching pole on the sleeve/outside connection. On a "regular" pedal, the battery positive is wired through this switch, so that the adapter plug can interrupt the battery feed to the circuit. The battery negative is wired to the "ring" connection on the (stereo) input jack. This allows you to turn the power off by pulling the plug out of that jack, by denying the battery negative its connection to ground. When you insert a mono plug into the stereo jack, the ring connection hooks up to ground, turning the power back on.

When you reverse the power connections for use in a PNP/positive ground pedal, the battery negative is now wired to the DC jack (to be switched out by the adapter plug) while the battery positive is wired to the input jack (to be connected to ground through the plug). Since the adapter jack is the same (the switching part is still on the "sleeve/outside" connections), the jack's connections will also have to be reversed. Simple, eh?

Fulltone sells a special reverse-polarity adapter for their PNP pedals ('69, Soulbender and Octafuzz) - but they don't really mention why it needs this adapter. And to make matters worse, they sell it as a direct fit for positive ground pedals - which just is plain wrong. Even though there is a strong likelyhood that your non-Fulltone PNP pedal has its jack wired that way, you can never be sure.

Other manufacturers - like Prescription Electronics and Voodoo Lab - have their jacks wired the "normal" way, so you can use a regular Boss-style adapter. Which is nice - you don't have to buy a special adapter for the pedal. But since you will still have to buy a separate adapter, not much is gained. This wouldn't be so bad if the user was properly informed of the situation. Prescription Electronics does mention it in the manual for their PNP pedals, so kudos to them (even though a sticker on the pedal itself would be even better). But Voodoo Lab doesn't say anything... I guess the strategy is that people will try their regular adapters first, and since it's wired to work with them, all is well. Judging by the number of people questioning me on why the Proctavia won't work with their power setup, I don't see it as a very successful strategy...

 

 

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