TRL Dude or Joule 150 MKII for Major Pre Upgrade

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.

This was extremely bizarre to me...

To me as well.

Have the preamps been located close to a possible source of emi/rfi, including perhaps the amplifier itself? If that seems like a possibility, try moving the preamp to different physical locations.

Beyond that, I'm out of ideas at this point.

Best regards,
-- Al

From the descriptions above, IMO the preamp is off the hook- this is a problem with the power amp.

Also based on these descriptions, its my theory that the amplifier has a problem with noisy rectifiers, which is getting picked up at the input if the source impedance is too high. This is why it is quiet with a shorted input and a 1K resistor, but not with the Dude (which has a relatively high output impedance as the volume control is at the output of the preamp), and likely a few other tube preamps as well.

Noisy rectifiers can be tricky as the commutation noise they produce acts very much like RF (which *is* part of their noise component), in that it can easily get to other parts of the amplifier, and use input connections as an antenna.

However this is only a theory, another possibility is a layout problem where the AC power is somehow too close to the audio input. Either way though- things are pointing at the amp.