how to split 9v PSU into 2 to avoid issues?

[daz]

I've done this before but often i ran into noise or other issues. One chorus pedal and a tube screamer. I plan to have 9v batteries for each one but at home i'll be using a wall wart. No issues with current, as total is only 40ma. But is there a way to split the power into 2 with diodes or whatever to isolate the pedals from each other?

[Mick Bailey]

Diodes introduce a voltage drop and won't isolate the split supplies - the only real way to do this is to use an isolated DC-DC converter, but these have other issues such as radiated HF noise. I have used an elevated ground in an automotive ignition circuit where the supply was spilt and noise from one section was getting into another via the common ground. I split the ground and used a pair of back-to-back diodes and parallel cap to give an pseudo-elevated ground to the second section. This worked well because the noise was below the forward voltage drop of the diodes. The issue with applying this to stompboxes is the DC ground is effectively bypassed by the signal ground as they're connected. When you get noise with shared supplies it's very difficult to resolve and I've tried additional bypass caps, resistors, inductors and diodes with only partial success.

[daz]

How does the simple one spot wall wart do it? Or DOES it?

[eschertron]

I have stompboxes that do this. My only solution is to use a 1-spot for everything else, then battery (or a separate PS) for the offending unit. If there's a clutter- and hassle- free solution, I'm ready for it

[Mick Bailey]

How does the simple one spot wall wart do it? Or DOES it?

I don't think there's anything special about the One Spot - it's a daisy-chained SMPS, so the same problem exists with sharing the supply. Mainly the common ground is the problem when it comes to noise. Jack Orman has a neat design to isolate supplies;
https://www.muzique.com/lab/9v_iso.htm

[daz]

I don't think there's anything special about the One Spot - it's a daisy-chained SMPS, so the same problem exists with sharing the supply. Mainly the common ground is the problem when it comes to noise. Jack Orman has a neat design to isolate supplies;
https://www.muzique.com/lab/9v_iso.htm

but from the schematic it would still require daisy chaining so how are the effects isolated? Or does this mean the input source has it's + and - isolated?

[Mick Bailey]

As it stands the output is isolated from the supply and the main benefit is if the supply is a grounded 3-pin type (with a common ground to the -ve output) this design eliminates the ground loop buzz you sometimes get. You're right that it still daisy chains, but with a split supply as in your case, you can have one daisy chain coming directly off the wall wart and one branched off using this circuit - you then have two independent supplies off a single wall wart.

There's no real reason other than efficiency why a number of these circuits could not be used to give multiple isolated supplies. The downside is the circuit itself draws power, so this has to be factored into the supply requirements.