The amp makes a squealing noise when I remove an input lead

[FAQ Team]

When an amplifier has its input open circuit, i.e. no signal lead connected, it acts as a radio receiver and will pick up and amplify anything passing by in the ether.

As it is an AUDIO amplifier you will hear the result as possibly a squealing sound or maybe hum or you may even hear a local radio station.

The higher the gain of the amplifier the worse it gets, with things like disc pre- amps being very susceptible to all sorts of AF (audio frequencies) and RF (radio frequencies). This is why disc amps and pre-amps require careful screening, and the signal leads usually earthed at both ends.