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?

[FiveseveN]

THE circuit?! Which circuit?

[Zerikin]

It should apply to any circuit input I assume, but I was specifically looking at this one http://tagboardeffects.blogspot.com/2012/04/dunlop-eric-johnson-signature-fuzz-face.html

[Rob Strand]

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?

In general adding a series resistor adds noise but for guitar this extra noise extremely small.

The effect of adding a 1k resistor depends strongly on the details of circuit.

For a high impedance input the added resistor is usually to stop RF and possibly oscillation issues.  It does this by forming a low-pass filter with the input capacitance of the circuit.   The filter cut-off is a very high frequency.  It has no effect on the guitar signal.  In fact often it's not low enough for strong RF sources.  Many circuits like this do have the 1k (or 10k)  series resistors.

For some circuits the gain depends on the impedance of the thing driving it.  Adding a 1k series resistor can set a maximum gain.  In these cases it may help with noise and oscillation issue.  However, circuits where this case applies are not the normal case.

[Zerikin]

Thanks for the detailed reply! The pedal I'm currently building is a fuzz face variant with several fairly high gain BJTs (about 240 hfe 1st stage and 530 hfe 2nd stage). Sounds like it might indeed be useful for this one.