Workbench / Power Supply

RC / LC Filter

Ripple attenuation, voltage drop, resonance.

Filter Type

Mains

Components

Input DC voltage

Input ripple

Series resistor R

Filter capacitor C

Load current

RC smoothing stage

Transfer (normalised to DC)

Output DC

306.0 V

drop 94.0 V

Output ripple

56.5 mVpp

-40.5 dB

Resistor power

1.88 W

use ≥ 5 W

RC corner

720 mHz

Engineering Notes

How it works

After the reservoir capacitor, valve amps add one or more smoothing stages to reduce ripple further and to decouple stages from each other. Each stage is a low-pass filter whose cutoff is far below the ripple frequency.

RC filter

|H| = 1 / √(1 + (2πf·R·C)²) ≈ 1/(2πf·RC) for f ≫ fc

Cheap and effective, but the resistor drops voltage proportional to current (V = I·R) and dissipates I²R. Ideal for low-current preamp nodes. Each stage gives roughly −20 dB per decade above its corner.

LC (choke) filter

|H| ≈ 1 / |1 − ω²LC| → −40 dB/decade above f₀ = 1/(2π√LC)

A choke offers high impedance to ripple while dropping only its small DC resistance, perfect for the high-current power-valve or screen supply. But an LC filter is a resonant circuit: near f₀ it can actually amplify disturbances. Keep the resonance well below the audio band. The load resistance (V/I) provides damping and is included in the full nodal simulation here.

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