Power Supply Filters

Most guitar pedals require DC voltage to operate. If not using a battery, a power adapter will convert AC from a mains supply to DC. The DC output of an adapter is not perfect and there will often be a small AC component in the signal. That noise is called ripple.

Ripple is a variation in voltage. Its frequencies are harmonics of the fundamental frequency which is usually the original AC line frequency at 50 or 60 Hz. For switching-mode power supplies the fundamental frequency can be tens of kilohertz to megahertz.

Once ripple has entered the effects circuit it’s very hard to separate from the actual guitar signal. So it needs to be filtered out before the DC reaches any active components or biases the input signal.

Like DC-blocking capacitors, there are AC-blocking capacitors that — as the name suggests — remove AC components. They’re also called decoupling capacitors.

Filtering AC components

If a high pass filter can be used to block (attenuate) most DC and pass AC, then a low pass filter can be used to block most AC and pass DC. Here’s an example of such a power supply filter consisting of a single resistor-capacitor pair:

C1 offers almost no resistance (impedance) to high frequencies — effectively a short circuit — and drops them to ground. Its resistance to low frequencies is high — effectively an open circuit — so they pass. With R1 = 47 Ω and C1 = 100 μF the cutoff frequency is at about 33.9 Hz.

Without R1, C1 would form a low pass filter with the output impedance of the power supply. However, as the output impedance of power supplies can be very low, the cutoff frequency might change significantly and noise might no longer be filtered out.

R1 thus sets a minimum resistance. A smaller resistor decreases the voltage drop (that itself depends on the current drawn through it), a larger resistor on the other hand allows smaller caps. 10 - 100 Ω are good starting points.

You can think of the cap as a rechargeable battery that smooths out voltage drops in the DC supply. The bigger the better, but bigger caps take up more space, are more expensive and draw a lot of current when the circuit is powered up — so there’s a tradeoff. 47 - 470 μF are generally good values for experimentation.

High-frequency noise

A single electrolytic capacitor filters lower frequencies like the infamous “mains hum” and its harmonics. It is not very effective against high-frequency noise as caused by switching-mode power supplies or radio frequency interference from poorly shielded cables.

Multiple capacitors allow taking advantage of different dielectric materials. An electrolytic cap provides a lot of capacitance to lower the cutoff frequency, a ceramic or high-quality film cap offers better ESR and high-frequency response — sending any radio frequency interference directly to ground.

A single ideal capacitor would theoretically be sufficient on its own. In the real world though we usually have to add a smaller cap next to it to handle what the big one misses.

C1 and C2 are connected in parallel, meaning their capacitances add up. That small increase in capacitance doesn’t alter the cutoff frequency but, depending on the choice and quality of C2’s dielectric material, the filter’s high-frequency response improves significantly.

The exact cap values aren’t actually that important. As a general rule there should be one large electrolytic cap as a power supply filter and another much smaller ceramic or good film cap to dump high-frequency interference.

The RAT power supply

Finally, let’s have a look at the power supply stage of the RAT pedal. Besides the parallel diode D1 for reverse polarity protection there are two capacitors that smooth ripple in the incoming DC — a large electrolytic and a small film.

R2 and R3 form a voltage divider establishing a 4.5 V reference voltage for the op-amp. C3 is yet another AC-blocking cap that filters out potential noise originating from the divider itself.

Higher value resistors in a divider network can produce self-noise; adding C3 across the divider’s output shunts that to ground. It also prevents crosstalk between other points in the circuit connected to the same reference voltage.

For C3 you should be able to use anything in between 1 and 100 μF. R2 || R3, 50 kΩ in parallel, and C3 form a low pass filter with a cutoff frequency of about 3.1 Hz.