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guitar amp LC lowpass filter

What does this calculator do?

A resistor-capacitor filter is simple and inexpensive. When it needs to supply lots of current, however, it creates a large DC voltage drop, requiring a higher input voltage to achieve the same output. In high-power guitar amplifiers the power tube screens, phase inverter, and preamps draw enough current at full power to make a choke-capacitor filter more economical (at least in the early days of the Fender Bassman and the Marshall JTM45).

An LC filter with the same amount of ripple attenuation and the same size capacitor creates much less DC voltage drop than an RC filter. The key difference is that the inductance of the choke reacts to AC signals but presents, ideally, a short circuit to DC. A real-world choke creates only a slight DC voltage drop due to internal winding resistance.

The calculator computes ripple attenuation in dB and DC voltage drop for an LC low-pass filter with a zero-impedance source and an infinite impedance load. Relatively speaking, this is approximately the case in a typical guitar amp supply with large electrolytic filter capacitors.1 For a high-voltage plate supply with a full-wave rectifier, the fundamental ripple frequency is double the AC line frequency, i.e. 100 or 120 Hertz, depending on location.

The default values are for the LC filter in a Fender Bassman 5F6-A, which delivers about 11 milliamps to the power tube screens, the phase inverter, and the preamps at idle, and about 27 milliamps at full power. 110 ohms measured across a 10-henry choke with a volt-ohm meter, for example, represents a winding resistance of 11 ohms per henry.


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