Corner frequency calculator
A resistor and a cap, or a resistor and a coil, make a filter with one corner. Enter any two of the parts and the corner frequency to get the third, as a high-pass or a low-pass.
Parts and corner
Around it
How one resistor and one cap make a filter
A cap’s reactance, how much it resists a signal, falls as the frequency rises: X = 1 / (2π f C). A coil’s rises: X = 2π f L. The corner is where the reactance equals the resistance, at f = 1 / (2π R C) with a cap and f = R / (2π L) with a coil. At the corner the level is 3 dB down and the phase has shifted 45°.
Past the corner the level falls 6 dB per octave, or 20 dB per decade. It’s 1 dB down at about twice the corner on a high-pass and about half the corner on a low-pass.
The time constant τ = R C, or L / R with a coil, is how long the cap takes to charge 63 % of the way through the resistor. It’s the corner written another way: f = 1 / (2π τ).
In an amp or pedal. A coupling cap into the next grid resistor is a high-pass. The stage before it is the source: about the plate resistor in parallel with the tube’s plate resistance, roughly 38 kΩ for a 12AX7 on a 100 kΩ plate resistor. A grid stopper into the tube’s input capacitance is a low-pass, and so is a treble-cut cap to ground. The load is whatever hangs on the output: the next grid resistor, a volume pot or the amp’s input.
A pickup. With R and L, a pickup’s coil into the volume pot and the amp is a low-pass. The pickup’s own capacitance and the cable’s turn that corner into a peak. They’re in the guitar tool.
How this calculator works. The response comes from the formulas for a first-order filter, from 1 Hz to 100 kHz, with the source and load as plain resistances. Stock values are E24 for resistors and E12 for caps. For a stage with a tube around it, try the gain stage tool. For networks with more parts, try the tone stack tool.
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