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Quick question regarding input resistor theory in pedal input circuit

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  • Quick question regarding input resistor theory in pedal input circuit

    Hi there,

    I've been trying to learn a bit more about the fundamentals and I've returned to a nice little pedal I've built a couple of times from the valve wizard website.

    I think I've finally got my head around input impedance and the necessity to have the greatest amount of signal voltage reach the circuit hence the large resistance to ground in the input section. I'm not too sure about the purposes of R2 and R3 though. I appreciate the need to have a large resistance from the +4.5v bias voltage to the signal, opamp buffer positive, node (I think, at least from simming and noticing the odd behaviour if this resistor isn't there). But I'm curious as how R1 helps that much if the AC signal already has a lower resistance path to ground through R2 and R3, surely R1 just very slightly REDUCES signal impedance to ground by adding another resistor? And I'm not sure at all why we need that R2 100 ohm resistor. I suspect the answer to my first question may include this one, R2, and possibly something to do with rejecting RF interference or something like that?

    Anyone with 2 mins on a coffee break that fancies clearing the fog for me would be appreciated!

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  • #2
    The input impedance of the circuit is essentially given by R3, which also lifts the DC potential of the buffer input to 4.5V. The lower end of R3 is AC-grounded by C2.
    R1 makes sure that C1 is charged to 4.5VDC to avoid popping when bypassing the pedal. Its large value lowers the input impedance by only 10%.
    R2 seems to be unnecessary, but may help to reject RFI.

    In my experience a guitar input impedance >= 500k absolutely sufficient and there is no benefit from impedances greater than 1M.
    Last edited by Helmholtz; 02-01-2021, 10:54 PM.
    - Own Opinions Only -


    • #3
      Exactly the description I was looking for, thanks as ever, I'll chew on that!