Guitar pickup / PhotoMOS off-state leak
1uA is the maximum leakage over temperature and voltage. At zero volts the typical leakage is 10-13A (less than a picoamp). However the capacitance at zero volts is about 250pF, which may be significant.



Most guitar preamps have a very high input impedance, typically ranging from 1 to 10 MegOhms. The PhotoMOS capacitance and preamp input resistance form a high pass RC filter with cutoff frequency of $$fc=\frac{1}{2\pi R C}$$ For 250pF and 1M that works out to 636Hz. However the pickup inductance and cable capacitance also have an effect. A typical 15ft cable has a capacitance of ~450pF, so the complete equivalent circuit might look something like this:-







simulate this circuit &ndash; Schematic created using CircuitLab

Cable capacitance causes some attenuation in the pass band, but the cutoff frequency is also greatly reduced. The result is you will still hear a fairly strong signal even when the PhotoMOS relay is turned off.

To combat this you need to swap the PhotoMOS relay for one with much lower capacitance. Since the pickup impedance is very high you don't need a high current relay, so choose one with higher resistance and lower capacitance.