Wednesday, 13 August 2014
Negative and Positive Ground Effects
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.