A guitar signal is alternating current (AC) but the vast majority of effects pedals require direct current (DC) to operate. So how do they work together in a pedal? It’s mostly simple sine wave arithmetic.

Combining waves

When mixing multiple sine waves their amplitudes add up. Adding two equal sine waves with a voltage peak of x volts will result in a sine wave with a voltage peak of x + x = 2x volts. Adding a sine wave to a 180-degree phase-shifted version of itself will cancel out completely as x - x = 0.

[Adding equal signals]

[Adding equal signals]

[Signals cancelling each other out]

[Signals cancelling each other out]

Biasing AC signals

The same holds true when adding a DC component to an AC component. The whole AC signal will be shifted by y, the DC offset. Positive voltage peaks will increase by y to x + y whereas negative peaks will decrease by y to -x - y (the absolute peak voltage increases).

[AC signal biased by a steady voltage]

[AC signal biased by a steady voltage]

The DC component is also called a DC bias and we can hence us it to bias an AC signal, for example to establish operating points for transistors or op-amps. DC is also used to power these transistors or op-amps directly and, last but not least, the LED.

Before the signal leaves via the output jack it’s important to again filter out any DC components introduced for active components. That’s the role of DC-blocking capacitors.