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Deciphering 70's era RC filter circuit - guitar foot-pedal schematic

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Jan 15, 2013
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hello all, i have some questions about the attached schematic. I believe this is a low pass filter, it comes from an old guitar foot-pedal schematic.

my question relates to how to calculate the critical frequency for this circuit. I know how to compute it for an RC filter... fc=(1 / 2pi*R*C). After the AC source the first R / C (10k / 2.2n), we would expect a Fc of 7234.3 Hz. However, I don't know what to make of the next part, another 10k but this time with the larger cap tied to the emitter. Nor do I know why you have yet another R / C component following that... with it's small 330 pF value it's critical frequency is way up at 48kHz, a frequency I thought would have been taken care of by the first R/C element.

One important word about this schematic... the AC source is about 1 V, but is biased up +2.84 V. My simulations suggest that the transistor would be forward conducting. the base voltage ends up being 2.83 V, the emitter voltage 2.15 V (simulated) and another thing to be aware of, the "battery" voltage is 8 volts, not 18 like it looks on the schematic (drawn in TinaTI). I've included the AC analysis of it, it seems to have a sharp drop off after the -3db point at around 3.9 kHz.

Any and all mathy explanations welcomed. Thank you in advance.


At frequencies where Xc3 is large the signal at the emitter = signal on the base, so C2 has the same signal at both ends so has no effect. When the impedance of C3 falls, the signal on the emitter falls, now C2 has a voltage across it so it takes current, causing a volt drop across R2. This makes the fall off steeper. Its a standard top cut circuit.

In Google look for Sallen-Key Lowpass Filter. A second order filter uses two resistors and two capacitors and the input of this circuit uses an additional third RC filter. Then it has a slope as shown of 18dB per octave that is produced by a third-order filter.

EDIT: I think the sharp lowpass filter was used because the guitar pickup inductance resonated with the cable capacitance producing a peak at about 5kHz and the awful speakers used back then also produced a peak at about 5kHz. Also the very loud guitar players were deaf to higher frequencies anyway.;-)


  • discrete Sallen-Key lowpass filter.png
    discrete Sallen-Key lowpass filter.png
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1st cap controls the Q or peaking at LPF breakpoint, smaller increases boost from 7.5dB, bigger flattens it.
2nd and 3rd cap work together as 2nd order LPF, 1st cap also increases rolloff to 3rd order or 18dB per octave.

Same in Q or Op Amp, but Op Amp is lower Z filter for guitar.jpg

If you want simulation text for Falstad, let me know.

ah okay, that is interesting, it's like an active filter with an op amp, but just with an transistor. thanks!

Based on my experience with gadgets like that back in the 60s and 70s, the chances are someone went through their collection of parts, trying each in turn until it sounded right. You have probably done more analysis and calculation than the designer ever did!


ah okay, that is interesting, it's like an active filter with an op amp, but just with an transistor. thanks!
Yes, the transistor is an emitter-follower with a voltage gain of only 1 like the opamp that has 100% negative feedback so its voltage gain is also 1.

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