RC Ripple Filter Calculator

RC Low-Pass Filter

What does this calculator do?

A typical tube amplifier power supply uses several resistor-capacitor low-pass ripple filters in series.

RC filters are simple and inexpensive. When they need to supply lots of current, however, they create a large DC voltage drop across the resistor. For the same amount of ripple attenuation and the same size capacitor an LC filter creates much less DC voltage drop.

The RC filters in a guitar amplifier perform more than AC ripple filtering. They also decouple preamp stages to prevent feedback that can lead to motorboating. For adequate decoupling, the high-frequency cutoff for each RC filter should be much lower than the low-frequency cutoff for the preamps. If large coupling capacitors connect the preamp stages to provide generous bass response, then better power supply filters are required for the preamp plate supplies to prevent feedback.

The calculator computes ripple attenuation in dB, the DC voltage drop, and the high-frequency -3dB cutoff frequency for an RC low-pass filter. The calculations assume a zero-impedance source and an infinite impedance load which, relatively speaking, is approximately the case in a typical guitar amp with large electrolytic filter capacitors.1 For a half-wave rectifier, such as in a DC grid bias supply, the ripple to be attenuated has a fundamental frequency of 50 or 60 Hertz, depending on location. For the high-voltage plate supply with a full-wave rectifier, the fundamental frequency is double: 100 or 120 Hertz. This calculator assumes a full-wave rectifier.


1Richard Kuehnel, Circuit Analysis of a Legendary Tube Amplifier: The Fender Bassman 5F6-A, 3rd Ed., (Seattle: Pentode Press, 2009).

Wire Capacitance LC Ripple Filter