This question comes up frequently and many of the articles you read about it online can be a little difficult to get your head around sometimes, so I thought I'd just do a short write up on negative and positive ground effects and the problems associated with mixing them. I am writing this in a way that I think will be easy to understand, but if I over simplify or miss out anything important then by all means post in the comments and I will edit where necessary :o)
One thing that makes this topic confusing for some is that we can often think of the supply having a +9V positive side and a 0V negative, but although this is often perceived to be true, it isn't exactly the case. With a floating supply like from a battery or isolated power supply there is no point of reference until it is in a circuit and so the only thing we know for sure is that the positive side is 9V greater in potential than the negative. In a common negative ground effect this makes perfect sense to us, ground is 0V and the positive supply is +9V, but when we build an effect and are instructed to connect what we think of as the +9V side to ground it doesn't compute.
When you have a chain of effects, ground is fixed at 0V by the negative ground effects, and more importantly your amp which is connected to all effect grounds via the sleeves of the connecting cables. So that "9V connection" to ground you made in your positive ground build is set at 0V by the local connections. This gives a reference point for the supply, and remembering the negative side going to the supply point of the effect circuit is 9V lower, that determines it must be -9V. This is why it is never a problem to mix negative and positive ground effects in your chain. Ground is always 0V with each pedal having either a +9V or -9V supply when in circuit depending whether the ground connection has used the negative or positive side of the source of supply.
The problem with mixing negative and positive ground effects comes when people try to power them with the same source of supply. In this instance the power supply isn't floating any more, negative is 0V and the positive is 9V and so making that non-computing connection from positive to ground creates an immediate short which will usually destroy your power supply unless you have one with a fuse or some other sort of short circuit protection.
So what can you do?
This is why power supplies with multiple isolated outputs can be very useful. Because the channels are isolated from each other, they find their point of reference independently and so you can have one powering negative ground effects at 0 to 9V and the other isolated side powering positive ground effects at -9 to 0V. Then there is no problem in daisy chaining all your positive ground pedals together from one isolated output channel, and likewise daisy chaining all your negative ground pedals together from another. Always ensuring of course that the current available from each output isn't exceeded by the pedals' combined consumption.
As an alternative, and something I always recommend trying with the layouts on this site is to use a charge pump IC to provide a negative voltage. The charge pump takes a 9V input and gives a -9V output, so instead of connecting the positive side of the supply to ground this allows you to keep all ground connections 0V, and supply the positive ground effect with the -9V from the charge pump. Common ground, common power supply.
Thanks for posting this! I'm beginning to wrap my head around it. This helps a lot.
ReplyDeleteNice explanation! It's a tricky concpt I did not fully grasp for 15 years whilst playing guitar and using power supply adaptors. It was only when I started building pedals and looking at schematics that it clicked. I think a lot of confusion is sown by the use of the term "negative ground" or "positive ground". Calling it "positive or negative SUPPLY" sums up the issue in a more accurate way, since ground is fixed as you point out, by the amp+guitar's shield connection, and the amp references to the mains supply ground (or certainly should), and so ground cannot be made to be positive or negative. Instead, we must decide if the OTHER part of the power supply system should be a positive or negative SUPPLY, relative to this fixed ground. It also allows the easier of understanding of bipolar supplies, where ground is still fixed, and two supplies are used, one positive and one negative relative to ground. I agree with the idea that in-built charge pump / voltage inverters for individual "specialist" pedals and one universal +9v supply is a good approach, since you never have to think about the risk of frying a protection diode or worse when plugging a pedal chain together, but I can see where people have vintage units that require different supplies, they might want to use an isolated supply.
ReplyDeleteYes the labels positive and negative ground really does cause the confusion trying to get it to make sense in your own head, and when you think about it from the other side it does all click into place easier. To make the builds easier to understand I considered simplifying the layouts even further to make the connections more obvious and show the red 9V wire going to the ground rail and ground going to the supply point, but thought in the end that would cause more problems than it would solve because it isn't emphasising an reverse polarity circuit which would mean a lot of people would just try to daisy chain it without thinking. At least the way it is done now makes people think more about it during the build which reinforces the separate power supply thing in their mind.
DeleteAnother alternative that may be preferable to the owners of vintage pedals is a small inline adapter like this, but taking the feed from the inverter pin instead of using it as a doubler. Then you could power a vintage pedal directly from a daisy chained supply without ever having to adapt it and potentially lose value.
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DeleteI like that inline adapter. Something to stash under the pedal board for those pesty fuzzes... Thanks!
An inline adapter is an awesome idea so you don't have to modify the actual pedal.
Delete*like* :)
ReplyDeleteNice explanation Mark.
ReplyDeleteA Positive Ground offboard drawing (with optional battery snap), on the Offboard Wiring page might be useful for beginners.
I could really use on of those off-board drawings for this since im struglling with a fuzz build atm. and need to know that tha power is ok. before troubleshooting.
Deletecheers and thanks for a greeat site!
/ Richard
The offboard is the same as it is with negative ground effects, the on,t difference is that you switch the positive and negative around. So the idea is where you would normally attach the positive lead, you attach the negative, and where you would normally attach the negative you attach the positive. Just switch the red and black wires around. Hopefully that helps.
DeleteI've built ToneMaster - Germanium Booster (www.diyguitarpedals.com.au) with on-board charge pump although I run with a battery. It's fine in the chain with other pedals I've built ( general guitar gadgets ) but won't let a signal through when I put my Boss DD-5 delay in the chain .... any ideas ?
DeleteIgnore that, I will build an inverter instead, and make sure I get the right power out.
ReplyDeleteThanks!
Well, built the Inverter and the inverter worked like a charm, great for future pos. ground. Didn´t make any difference to the Bonamassa Fuzz Face though. Still acting really strange, and I have tripple checked everything. Well guess its back to the drawing table :)
ReplyDeleteHey roccster, did you ever figure out the Bonamassa Fuzz Face with Inverter? I built one myself and the inverter didn't work. I kept thinking it was human error, but not so sure any more haha
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