Yesterday I changed my workspace and breadboard setup for a better ... or so I thought :mrgreen: I'm getting an annoying background noise now. I'll be going through all the changes to trace the culprit but maybe this is a known issue : I get a low frequency beat (<5Hz) that sounds like a purring kitten. Happens only if the breadboard circuit is powered up (9V battery) AND the guitar input is connected. Dead silent if guitar cable is removed. So I suspect the guitar cable is making for an antenna ??? Yet I don't pick up radio stations, only that purring sound. Is that what's meant by heterodyning maybe (just throwing words around here) ??