Audire Diffet 1 hum

My Audire Diffet 1 preamp recently developed a hum audible through the speakers. Still sounds very good at volume, but probably like all of you, it bothers me. Could it be a transformer, capacitor or otherwise?

PS Audio had a nice article on isolating a hum, but didn’t mention the possibility of equipment. I guess my question is it probable that the problem is external to the preamp, like interconnects, or line voltage? (My other system doesn’t have a hum).  Oh, and it produces a thump when the amp and preamp are turned off together (I don't do this anymore).

See if you can open this.  There is a little button cap right across the wall power in the preamplifier.  If it is a capacitor, I'm hoping this is the issue.  What do you think?
Listen to the 60Hz Hum and then the 120Hz Hum. Does either of them sound like your hum?

Also I forgot to ask you earlier, does the hum vary with the volume control?

60Hz hum

120Hz hum

120Hz hum

It sounds like a 60Hz hum and its constant, does not vary with volume.  The Audire amplifier and preamplifier are both old enough to have just a two prong electrical plug. 
The Audire amplifier and preamplifier are both old enough to have just a two prong electrical plug.

Do you have a multimeter?

I assume the AC power plugs on the preamp and power are not polarized? They will plug into the receptacle outlet in either direction.

If you have a multimeter you should measure the voltage from the metal chassis of each piece of equipment to the ground contact of the wall outlet.
** Interconnects must be disconnected/removed from the equipment.

With the equipment powered up measure the AC voltage from the ground contact on the wall outlet to the metal chassis. Make note of the AC voltage reading. Turn off the equipment wait a few minutes then unplug the piece of equipment, being tested, and rotate the plug 180 and plug it back in. Turn it back on and measure the voltage from the ground to the chassis again. The lowest voltage reading of the two readings is the correct plug AC polarity orientation for the primary winding of the power transformer of the piece of equipment tested. Mark the plug for identification. Test both pieces of equipment.

Reconnect the interconnects from the preamp to power amp. Plug in the preamp and power amp plugs with the correct identified plug orientation. Check for hum.

An appearance of hum in old equipment is most likely due to aging power supply capacitors! An easy and affordable fix!