Quote Originally Posted by Light Dependant Resistor View Post
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.
Thanks Chris & although the hum does disappear when no cables are connected to the input, just speaker cables to the speakers, as soon as I attach any cables (tried about 6 different ones) , the hum comes back so don't think it's cable related in this instance.