A few things to try:

1) Don't test with open-circuit inputs. Plug An XLR connector with all 3 pins shorted together into a humming channel - do you get the hum then?

2) Physically rotate the pre-amps in 3 axes - does the hum change amplitude or character?

3) Plug the humming pre-amps into the same mains outlet as the MOTU.

4) Do the Big Knob/headphone test that you suggest.

5) Daisy-chain your humming pre-amps through a 512C in the Lunchbox set to overall 0dB gain (20dB pad, 20dB gain, phantom off) - hum then?

Since two different types of pre-amp give the trouble, this is likely to be a ground loop issue. The Lunchbox outputs are floating transformer coupled, so no ground connection.

Boswell wrote "1) Don't test with open-circuit inputs. Plug An XLR connector with all 3 pins shorted together into a humming channel - do you get the hum then?"

Why do this? I mean what does this test show and how are the results different?

Boswell wrote: A few things to try:

1) Don't test with open-circuit inputs. Plug An XLR connector with all 3 pins shorted together into a humming channel - do you get the hum then?

2) Physically rotate the pre-amps in 3 axes - does the hum change amplitude or character?

3) Plug the humming pre-amps into the same mains outlet as the MOTU.

4) Do the Big Knob/headphone test that you suggest.

5) Daisy-chain your humming pre-amps through a 512C in the Lunchbox set to overall 0dB gain (20dB pad, 20dB gain, phantom off) - hum then?

Since two different types of pre-amp give the trouble, this is likely to be a ground loop issue. The Lunchbox outputs are floating transformer coupled, so no ground connection.

Excellent suggestions. Not exactly sure what option #1 means, but I'll get on this...

jg49 wrote: Boswell wrote "1) Don't test with open-circuit inputs. Plug An XLR connector with all 3 pins shorted together into a humming channel - do you get the hum then?"

Why do this? I mean what does this test show and how are the results different?

An external hum problem like this is usually caused by one of three things: electrostatic hum fields, magnetic hum fields or conducted interference. My guess is that this particular case is due to conducted effects, but it is necessary to rule out the other two possibilities. Note that equipment failure (e.g. a smoothing capacitor going high resistance) can cause a similar trouble, but it would not be duplicated on different pre-amps.

Using a short-circuiting XLR plug on the input conducts any electrostatic pickup to ground. If fitting the plug makes no difference to the hum, it's not electrostatic. Rotating the equipment changes its orientation in any magnetic field, so, again, if there is no change in the amplitude or character or the hum as the pre-amp is rotated in all planes, the pickup is probably not magnetic. That leaves conducted interference as the likely trouble, and we move on to step 3.

Boswell wrote: [quote=jg49]Boswell wrote "1) Don't test with open-circuit inputs. Plug An XLR connector with all 3 pins shorted together into a humming channel - do you get the hum then?"

Why do this? I mean what does this test show and how are the results different?

An external hum problem like this is usually caused by one of three things: electrostatic hum fields, magnetic hum fields or conducted interference. My guess is that this particular case is due to conducted effects, but it is necessary to rule out the other two possibilities. Note that equipment failure (e.g. a smoothing capacitor going high resistance) can cause a similar trouble, but it would not be duplicated on different pre-amps.

Using a short-circuiting XLR plug on the input conducts any electrostatic pickup to ground. If fitting the plug makes no difference to the hum, it's not electrostatic. Rotating the equipment changes its orientation in any magnetic field, so, again, if there is no change in the amplitude or character or the hum as the pre-amp is rotated in all planes, the pickup is probably not magnetic. That leaves conducted interference as the likely trouble, and we move on to step 3.

Wow, exhaustive, informative, but scary that I actually understand what you explained.

Thanks so much, Boswell!

Thursday night I get to troubleshoot hum... Yay!

Thanks

PROBLEM SOLVED! - or at least identified...

With some minimal sleuthing, I found the problem and it should be easy to remedy, easier still to circumvent.

I unplugged everything except my preamps and Mackie Big Knob. This was the most minimal setup which would allow me to hear the hum. Sure enough, it was still there. Then I began pulling the mic cables out of the preamp. Each cable disconnection brought a change to the hum (not always a reduction) until I'd unplugged the last cable into the preamps. Suddenly, the silence was deafening.

I had built a snake/breakout box to extend preamp inputs out into the tracking area. There's obviously a grounding problem with the soldered connections I made in constructing this box.

I ran mic cables straight into the preamp, connected my Neumann U89, juiced the levels super high to listen for any remaining hum and all I could hear was a butterfly clearing its throat two Counties to the west.

I should be able to identify which connection on the breakout box is faulty and then effect a repair.

Thanks to all, particularly Boswell, who helped give me the hints necessary to get to the bottom of my hum problem!