Positive ground?

…also known as ”why does my power supply cut out when I try to power my fuzz pedal?”. This is a question I come across quite often, and in the interest of maybe not having to explain it too many times on the discussion forums, I thought I’d whip up an article about it. But since the question does appear quite frequently in the forums, let’s start with a quote:

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…)

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 is reverse polarity (center positive), and that you therefore can’t mix negative center (Boss etc) pedals with positive center ones. But it is a little more involved than that. We must learn to distinguish between the polarity of the jack (which only tells us where the pedal wants to see the positive and negative) and the ground polarity (which is what dictates if the pedal can be daisy-chained or not).

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 on some Electro-Harmonix pedals and 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 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. Simple as that.

So that’s why the jacks on normal pedals are wired the way they are. But the plug polarity is not why some pedals short the power out if you try to daisy chain them. That has to do with the…

Ground polarity

In normal guitar pedals, the battery/power negative is referenced to ground, with the positive sitting 9 volts above ground (hence the +9v term). But there are a few designs that are ”upside down”, where the battery/power positive is referenced to ground instead, with the negative sitting 9 volts below ground (-9v). These are called ”positive ground” (PNP), and they can not 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 they count on people using their power supply anyway, and since it’s wired to work with it directly, 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… 🙂