TRL Dude or Joule 150 MKII for Major Pre Upgrade

I wouldn't expect that shorting the preamp inputs would make any difference, since the problem occurs even when the preamp is turned off and unplugged from the AC.

Sure is a baffling problem :-)

Best regards,
-- Al

Grannyring, what kind of speaker cables are you using, and how long are they? If by any chance they are one of the kinds that has extremely high capacitance, such as Goertz or Ocos, and you are using them without a Zobel network, I'm wondering if that may be resulting in the amp being on the edge of having some sort of low level oscillation, that could somehow be put over the edge by conditions at the inputs.

Also, did the positioning of the speaker cables change significantly when you changed speakers? Perhaps they are picking up rfi/emi which is entering the feedback loop of the amp, if it has one.

Just some shots in the dark.

Best regards,
-- Al

Granny has put an integrated amp in place of Dude preamp and Atlas amp, with the same CD player, ICs, power cords, and speakers. It was quiet. No hum.

The Atlas amp has balanced differential input gain stage and followed by driver and output stages in bridged configuration. We measured the RCA ground to the IEC ground pin to verify that signal ground is connected to AC ground.

We also tested the RCA and XLR inputs and concluded they are correctly wired, by measuring the resistance of pins to the ground and RCA center to XLR Pin-2. We got new XLR connectors, short pin 1 and 3, and connect them to the amp.

Then we use a pair of RCA sockets to simulate inputs. First, we shorted the RCA sockets. With ICs between shorted RCA sockets and amp, the system was quiet. So we changed the RCA sockets from being shorted to being loaded with 1K ohm resistors. This was to simulate if the amp could pick up noises and then hum/buzz. But it was quiet too.

Then strange thing happened as soon as we connected anything else to it. They don't even need power cords to cause hum. We also measured Dude preamp and concluded that it was also wired correctly too. On the Dude preamp, if we adjust the volume all the way down, it is like shorting the output. (We verified it by measuring the output with a multimeter and got 0.3ohm.) But the system would still hum, without power cord. This was extremely bizarre to me...

What was even more bizarre to me was that we connected CD player to the amp directly, without any preamp. One channel was humming loudly and one was humming a little. When we switched cable the hum pattern stayed.

I found a bunch of unused RCA cables and connected the ground and center wires to short the jacks. I places these on my 3 sets of unused preamp inputs. Still have the buzz. Still think this may be a good idea however.

Vett93 summed it up very well. My speaker and other cables really did not change position. My speaker cables are simply solid core copper runs and very straight forward. Changing power or ic cable brands did nothing to change the hum.