Guitar Effect Pedals: Question about powered/biased input jacks and modulation

[PomPom]

Hello,

Take, for instance, this schematic: https://www.muzique.com/schem/fuzzface.gif
First, why is this input jack boosted by 9V? It's often boosted or lowered by 9V and I can't really understand why.


Next:

I want to modulate a square or triangle wave with my input signal. So say my input (audio) signal is 100mV.
First, from the above, what will it look like biased to ground? Will it just be the same signal, lifted up by 9V?

Second, how do I modulate this correctly? I intend to multiple my audio signal by my square wave, with a AD633ANZ multiplier. But how do I need to bias the square wave, for this to work properly?

Thank you,

 

[FvM]

It's not "boosted by 9V". It's using the "ring" contact to switch-on the battery when the guitar cable is inserted.

I guess, the method is used for guitar effects since half a century.

 

[PomPom]

OH! that is incredibly helpful thank you very much. So for single supply opamps in effects circuits, the opamp must be biased up to, say, 4.5. And for modulation I'd have both signals biased to the same voltage?

 

[FvM]

Most audio circuits are working with AC voltage, there behaviour doesn't change when the bias level is shifted. When using non-linear circuits like multipliers, the bias level matters however. You'll usually implement a virtual ground at about mid supply and take care that all signals have the intended bias level related to it.