Input Serial Resistor

[Zerikin]

I saw someone suggest adding a 1k resistor in serial with the input to reduce noise on the circuit. I tested it on my bread board and it works BUT. What is this doing to the circuit, what are the side effects? Presumable there is a reason all pedals are not designed this way?

[Zanius]

Not that rare at all. You'll see many circuits with a resistor before the first stage. Not a theory guy, I suppose it weakens the signal a bit, depending of how much it will be amplified later. Never heard/read of it as a noise solution but I suppose it's a valid method.

[reddesert]

The resistor increases the input impedance by 1K. This limits the input current.  (The amount by which it's limited/decreased also depends somewhat on the output impedance of the source.)

Does this matter?  If your circuit has a high input impedance already, like input to the gate of a FET or an op-amp, then it doesn't matter very much. Sometimes you may see serial resistors before the gate of a FET or op-amp input anyway, in part for current limiting for protection against static charge, I think.  But the amount of current that can go into the gate of a FET is essentially zero. This is why they make good buffers.

On the other forum you mentioned that the circuit was a Fuzz Face.  This is one place where it could matter because the input to the transistor is low impedance.  So if you have some small noise voltage the transistor can sink current and amplify it (because that's what transistors do) amplifying the noise. Adding the input resistor probably lowers the gain as well as lowering the noise, but maybe you have enough on tap in the circuit that it won't matter.

[Zerikin]

I tested the circuit with the serial resistor and it did not seem to have an impact on noise when the gain was high and the guitar on, only when the guitar input was dialed all the way down so I'm leaving it out. Filtering the power did make a difference so I'm planning to use that as part of the circuit.