Pedal Power Supplies

Reverse-polarity protection and daisy-chaining are covered on the wiring side in Assembly — this chapter covers the circuit-level power supply work that happens inside a pedal itself: charge pump ICs that double a 9V supply to 18V (or generate a negative rail) for extra headroom, and the LC filtering that keeps a noisy switching supply's whine out of the audio path.

Power Supply Conventions and Polarity Safety and Daisy-Chaining Multiple Pedals cover the wiring-level power decisions — polarity convention, reverse-protection diodes, and how multiple pedals share one supply. This chapter is about a different layer: what happens to power once it’s inside a specific circuit, and the two problems a circuit-level power design has to solve that pure wiring can’t.

Why some circuits want more than 9V

An op-amp running on a single 9V supply has roughly ±4.5V of output swing available around its bias midpoint — plenty for most gain stages, but a real ceiling for a circuit chasing extra clean headroom before clipping. Doubling the supply to 18V roughly doubles that available swing (to around ±9V), which is exactly why some overdrive designs — a JRC4558D-based Tube Screamer-style circuit is the standard example — specifically benefit from running on 18V rather than 9V: more headroom before the op-amp itself starts clipping means the circuit’s own diode clipping stage, not the op-amp running out of room, stays in control of the pedal’s character.

Charge pumps: getting 18V (or a negative rail) out of a single 9V battery

A charge pump IC (the ICL7660 and similar parts are the standard choice) uses a pair of capacitors switched at high frequency to move charge around and generate a voltage the input supply doesn’t directly provide — commonly inverting a 9V input to produce -9V, which combined with the original +9V gives a circuit access to a ±9V dual supply. Getting a true +18V single rail out of the same starting point takes an extra step: the charge pump’s inverted -9V output gets summed with the original +9V through a small diode-and-capacitor doubling stage, since the charge pump IC alone only inverts, it doesn’t double the positive rail on its own. Either result — ±9V or +18V — solves the same underlying problem: more voltage swing available to whichever gain stage needs the extra headroom, generated entirely from the same single 9V input a normal pedal supply already provides, with no separate higher-voltage supply required.

Switching supply noise: a real, filterable problem

Modern pedalboard power supplies are commonly switching designs rather than old-style linear transformers, and a switching supply’s own internal oscillator — running well above the audio band — can leak a high-frequency whine into a pedal’s power rail if that rail isn’t adequately filtered where it enters the circuit. An LC filter (a small inductor in series with the incoming supply line, paired with a capacitor to ground after it) forms exactly the kind of low-pass filter covered in Frequency Response and Filters: it passes DC through with negligible loss while presenting real impedance to the switching frequency’s AC content, keeping that noise from reaching the audio circuitry at all. This is a genuinely different fault from the daisy-chain ground-loop hum covered in Assembly — switching noise is a whine or hiss riding on the supply rail itself, present even from a single, properly-grounded pedal, not a low-frequency hum that depends on how multiple pedals share a ground.

Common mistake: assuming a voltage-doubled circuit is safe to power like a standard 9V pedal

A charge-pump-equipped circuit generating an internal ±9V or +18V rail still presents a standard external 9V input to the world — but the parts around that internal higher-voltage rail (electrolytic capacitors especially) have to be rated for the actual internal voltage present, not just the pedal’s external 9V input spec. Building or modifying a circuit around a charge pump without checking that every capacitor on the doubled rail is voltage-rated above what the charge pump actually produces is a fast way to have a capacitor fail under normal operation, not from any wiring mistake but from a straightforward component-rating oversight.