Electronics > Projects, Designs, and Technical Stuff
Op amp guitar buffer noisey when volume down
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dfox416:
I built a battery powered op-amp buffer for an electric guitar, and I am having a problem that as I turn the volume knob down on the guitar, I hear the noise/hum of the buffer increase. Basically, the guitar pickup is connected to a 500k volume potentiometer, which is connected to the input of the buffer. The noise is worst when the volume is about half way, presumably because this is where the input source impedance is largest. However, I cannot wrap my head around exactly what causes this, and why the same does not happen when I plug my guitar into a normal amplifier or effects pedal.

dzseki:
Your input impedance is high. When the volume pot is half way your output impedance also rather high, therefore your guitar cable and even the guitar picks up noise easier. Try to lower your input impedance on the amplifier.
Yansi:
What opamp have you used and what was the circuit supplied from?
Yansi:

--- Quote from: dzseki on February 12, 2020, 01:22:56 pm ---Your input impedance is high. When the volume pot is half way your output impedance also rather high, therefore your guitar cable and even the guitar picks up noise easier. Try to lower your input impedance on the amplifier.

--- End quote ---

A guitar requires high input impedance. In this case the 1M is good enough.
dmills:
Resistors generate noise in proportion to their value.

Opamps have current noise which (if you pick the wrong part for your impedance value) can get converted into noise voltage by the source impedance.

You probably want a jfet input opamp (There is a reason TL071s are everywhere in guitar circuits), and if noise is a real concern you might do better to buffer the guitar and then use a much lower value pot (Think 10k) at the output of the buffer.   
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