Thanks Uncle Jed,
Looked at Farnell and they do have quite many variaties.
Unfortunately I'm stuck now as I'm still trying to find out how to measure the quiescent current to balance (not to increase) the bias..............
Any idea? I'm using OA21s
I think I saw some with a knob that you can adjust with fingers.
offset is measured at the speaker terminals. the reansistor current is measured by measuring the voltage across the emitter resistor and dividing by the resistance. offset should only be 100mV or less (any more than that will give a turn-on thump). many amps use 0.1. 0.2, or 0.47 ohm emitter resistors. so if you are measuring 20mV across a 0.47 ohm emitter resistor, 20mV/0.5 (i always round the 0.47) ohms=40mA.
usually BJTs run anywhere from 5 to 100mA and MOSFETs run from 50 to 500mA for idle current depending on the amp manufacturer.
for an offset vontrol, go wirh plastic film or cermet. for an output bias control, any type will work, but cermet types are more reliable.
cermet is also a low noise resistance, and you definitely want to minimize noise sources in the input stage.