Dc bias cap in tone circuit?

[Diablothecat]

I was wondering if using dc bias cap to decrease capacitance for finer tone control would work. I drew up a quick schematic for ilustration.

[curtisa]

What's your circuit trying to achieve?

[Diablothecat]

Basicy by applaying dc to capacitor you can lower its capacitance which would change cut off frequency.

[curtisa]

Applying DC to a capacitor doesn't change its capacitance. Its value is constant irrespective of the applied voltage across its terminals. 

Your circuit looks a little like a capacitance multiplier, but the implementation looks odd. Normally a cap multiplier circuit increases the apparent capacitance by the gain of the transistor (hFe) and can't be adjusted by a resistor. In your circuit all the pot is doing is providing a variable resistance for AC signals (ie the guitar signal) to bypass the cap/transistor through the battery. I'm not even sure the transistor will even be doing anything as the base appears to be grounded.

[Diablothecat]

I redrew schematic and got rid of the transistor instead going with switch, which is meant to discharge cap (if i applied lets say 5vdc and wanted to go lower i would need to discharge it) and from what i read here it should work:

https://www.murata.com/en-eu/support/faqs/capacitor/ceramiccapacitor/char/0005

R1 is there just to limit current, P1 is to change dc voltage to cap. I think it should work but if im wrong please correct me

[curtisa]

Interesting - I wasn't aware of that phenomena, although it appears to be largely a property of multi-layer ceramic capacitors with particular dielectrics.

P1 won't alter the voltage to the cap. The capacitor will still charge to whatever the voltage of the battery is; it will just change how quickly it gets there.

You shouldn't need S1/R1 to get the cap to discharge to a lower battery voltage. The cap should follow the battery to whatever voltage it is set to (notwithstanding my observation that P1 won't adjust the battery voltage).

Where does 'in' and 'out' connect to in the guitar?

[Alex M.]

On 5/17/2023 at 10:43 AM, Diablothecat said:

I was wondering if using dc bias cap to decrease capacitance for finer tone control would work. I drew up a quick schematic for ilustration.

 

Long story short — nothing useful.

[Alex M.]

Especially when applied to guitar electronics.