Originally Posted by
Light Dependant Resistor
The usual cause of hum like this in a unbalanced audio system is a cable with a ground connection missing or broken - like a RCA cable looking OK externally but actually having a
broken connection for its ground. Suggest disconnect input cables to power amp... does hum disappear ? If Yes then fault is a input cable.requiring repair or replacement.
Opto coupling with a LDR attenuator enables total silence for audio attenuation- which is a amazing feature of LDR's that other preamps cannot match.