DC-blocking Capacitors

Guitar pedal circuits usually have two capacitors sitting on the signal input and output — so-called DC-blocking (or coupling) capacitors. These caps prevent direct current (DC) from entering or leaving the circuit, allowing only the guitar signal (AC) to pass through.

You generally don’t want any DC entering into or escaping a pedal. DC is used intentionally within the pedal but it’s a DC voltage deliberately defined by the circuit and not some stray DC.

DC on the signal input can originate from broken pedals or not properly shielded guitar cables earlier in the chain. The DC blocked at the output is the input voltage that the effects circuit mixes with the guitar signal. It’s needed only to operate active components in an effects circuit.

How it works (simplified)

Capacitors can block (or rather attenuate) low frequencies, such as DC, and pass high frequencies, such as AC, because they’re reactive devices that offer different resistance (or impedance) to signals of different frequencies.

To low frequency signals such as DC at 0 Hz they offer very high resistance. As the frequency of the signal increases they offer progressively less resistance. At audio frequencies a coupling cap of 0.1 μF is basically a short circuit and offers almost no resistance.

The exact resistance offered for AC signals of varying frequencies depends on the capacitance value and dielectric material of the capacitor.

Picking cap values

Coupling capacitors form a high pass filter together with the input impedance of the next stage of the effects circuit. The exact cutoff frequency depends on the specific pairing of capacitance and resistance values — higher frequencies will pass, lower frequencies will be attenuated.

Here’s roughly how this works for the Linear Power Booster:

• C1 forms a high pass filter with the impedance of the voltage divider formed by R1 and R2, in parallel with the impedance looking into the base of Q1.
• C2 forms a high pass filter with the shunt resistor of the volume attenuator at the output, in parallel with the input impedance of whatever is next in the chain.

The bigger the capacitance values, the less the filters will attenuate lower frequencies. So if you want more bass choose e.g. C1 = C2 = 3.3 μF and you’ll get a Hog’s Foot Bass Booster. Pick C1 = C2 = 0.002 μF for a Screaming Tree Treble Booster.