My new amp hums - how do I stop it?

[FAQ Team]

This can be very tricky or very simple.

Hum can be generated by a variety of causes all of, which may be interactive or completely self contained.

First measure the hum with a Digital MultiMeter (DMM) across the loudspeaker terminals and note the reading. With most WAD amps this will be around 1-4 mV depending on kit, and may be inaudible depending on the sensitivity of your particular loudspeaker.

Then re-check all your wiring particularly that you have taken all the screens (outers) of the screened cable to earth where specified in the instructions.

Second check the routing of the cables, particularly the feed back screened wire which can be very sensitive to hum pickup. Try moving them around to see if the hum increases/decreases.

Hum can also be caused by faulty smoothing capacitors and the only way to check is by substitution, or by faulty rectifier diodes ditto.

It is also possible for hum to be caused by the OP valves being overdriven or incorrectly biassed. Voltage readings will possibly show the cause here.

It may be caused by oscillation in which case an oscilloscope will certainly pinpoint the area.

Hum may be inaudible with the amp only and may suddenly become audible when a passive/powered Pre-amp is connected. In this case the most likely cause is an earth loop where both the pre and the power amps are both earth via the mains as well as the signal leads. Generally disconnecting the pre mains earth (quite safe to do as long as the signal earths are connected) will cure the problem.