Check four things on the pedal and match them on the supply. The wrong voltage or polarity can damage the pedal. Too little current won’t, but the pedal will misbehave.
| Check | Most pedals | Where to find it |
|---|---|---|
| Voltage | 9V DC | Printed by the socket, or in the manual |
| Polarity | Centre negative | The small polarity diagram by the socket |
| Plug | 2.1 mm barrel | Manual, or the plug that came with it |
| Current | Analogue: often under 50 mA. Digital: often 100–500 mA | Manual or spec sheet, in mA |
Voltage
Use the voltage the pedal asks for. Some pedals are built to take more, usually 12V or 18V, for extra headroom, but only where the manual says so. Feeding 18V into a pedal designed for 9V can damage it. Use a regulated supply: a cheap unregulated adapter can put out well over its rated voltage when the load is light.
A few pedals run on AC rather than DC. The original Line 6 DL4 is the well-known one. Don’t plug a DC supply into an AC pedal, or an AC supply into a DC pedal.
Polarity
“Centre negative” means the pin in the middle of the socket is the negative side. It’s the standard Boss-style arrangement, and most pedals and supplies use it. Some older and vintage pedals are centre positive. Reverse polarity can damage a pedal that has no protection against it, so check before you plug in, and keep polarity-reversing cables labelled.
Current
Current is measured in milliamps (mA). The supply outlet has to give at least what the pedal draws. Supplying more than the pedal needs is fine, because the pedal only takes what it uses. Supplying less is the problem: the pedal may switch on and then crackle, reset or cut out.
When you add up a whole board, use the pedals’ listed figures and leave some spare capacity on the supply.
Daisy chains and isolated supplies
A daisy-chain cable runs several pedals from one outlet, so they all share a ground. That can work for a few analogue pedals. Add digital pedals, which draw current in pulses, and you often get hiss, whine or hum: the pulses ride on the shared ground and you hear them.
Never put a centre-positive or positive-ground pedal (many vintage-style germanium fuzzes) on a daisy chain with other pedals. Sharing a ground with them shorts the supply, and can damage the supply and the pedals. Give that pedal its own isolated outlet, or a battery.
An isolated supply gives each outlet its own separate output. It costs more and fixes most power-related noise. If a board is noisy, run the noisiest pedal from a battery for a minute. If the noise goes away, power is the problem.
Batteries
A pedal with a battery in it draws power whenever a lead is plugged into the input, even when it’s bypassed. Unplug the input when you’re done. Take batteries out of pedals you’re storing, because a leaking battery damages the contacts.