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I'm designing a guitar pedal circuit. I've received feedback that my input impedance is way too low in regards to my output impedance and I was wondering how I could go about increasing it.

Guitar Pedal Circuit Schematic To keep the diagram simple, I've omitted things like the power supply and the internals for most of the stages. I've kept the pre-amp stage just to give an idea of what the other stages also look like.

I know guitar pedals usually have about 1 MOhm impedance looking into the amplifier, and I believe mine is currently nowhere near that. 1MOhm input impedance for the circuit would be a much better counterbalance from the 90kOhm output impedance of my guitar's pickups as well.

Is it really as simple as changing that first 10kOhm bias resistor (R1) to a 1 MOhm resistor? I read that higher resistances have higher noise and a 1MOhm resistor would introduce a lot of noise into my signal path which I would like to avoid.

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  • \$\begingroup\$ Small review comment. OP1, OP2 and OP3 can probably all be combined into a single opamp stage. \$\endgroup\$
    – Graham
    Aug 16, 2023 at 17:43

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To increase the input impedance it really is as simple as increasing your 10k resistor to 1Meg. This is how the vast majority of guitar pedals do it.

Noise is a complex subject, and the total noise will not only depend on the impedance of the 1Meg loading impedance inside the pedal, but also the characteristics of whatever is connected, which will add in parallel. Op-Amps have an input current noise specification, which is generally the culprit for input noise for higher impedance sources like guitar pickups. A small noise current flows from the Op-Amp inputs through your total input loading resistance/impedance, creating a noise voltage. The higher the total loading impedance, the larger this noise voltage becomes.

The way to reduce this noise, whilst still retaining the high input impedance is to choose a FET or CMOS input OpAmp (e.g. TL072, OPA1642). These types of OpAmp have much smaller input noise currents compared with BJT input Op-Amps.

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  • \$\begingroup\$ Yeah I forgot to mention the op amps I'm using are the TL074. So by increasing it to 1Meg, does that make the overall input impedance in the 1Megaohm range? \$\endgroup\$
    – Da Mahdi03
    Aug 16, 2023 at 17:30
  • \$\begingroup\$ Yep, the Op-Amp input impedance is very high, in parallel with a 1Meg resistor, so the input impedance is approximately 1Meg \$\endgroup\$
    – Graham
    Aug 16, 2023 at 17:41

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