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Electricians wanted

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I have a annoying dilemma and I'm hoping that someone with an idea of electronics will have some sort of theory on whats causing it.


I keep my interface plugged in using its external power supply at all times. However, I have an annoying electrical rumble that shows up when using the phantom power on my interface. The interesting thing is, it goes away as soon as I unplug the external power supply and run the interface solely from the firewire bus.



Here's what it sounds like https://www.box.com/shared/dapcjkouz4


Here's a few examples


Interface plugged in with dynamic mic = No issue

Interface plugged in with condenser = Rumble

Interface unplugged with condenser = No Noise


Tried unplugging computer and running laptop on battery, no difference.

After I unplugged everything, disconnected all the other gear, shut down everything and just used the interface plugged in, the noise was still present.

This has led me to be almost 99% sure its the PSU unit for the interface.

Definitely not a grounding loop. I took care of those a long time ago. Those Apple PSU are notoriously noisy.




In the end, all this points to the PSU (IMHO). The fact that when its plugged in, its leaking some sort of current into the preamps and causing this electrical rumble that appears generally a few minutes into engaging phantom power.


I'd like to hear others opinions on this. :?


I'll be sending the interface back to Metric Halo for them to examine the analog board as they have stated.

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OK, that's a curious (pinkish) noise, to me this sounds like some electrical component is "semi-blown" - it doesn't necessarily have to be in the PSU though, could be any component in the IF that is affected by using the PSU. I would definetely contact Metric Halo, they should be able to identify which component is faulty and repair or replace it.
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So, you can run a condenser mic under phantom power when the unit is being powered by the FireWire bus.... but not when the unit is plugged into the wall. Hmmmmmm.


Well, first guess would be a geound loop, but that noise doesn't sound like a loop: it sounds like dirty juice. Since the phantom power spec is from 9v to 48v, I'm going to take a wild guess that when the unit is being powered by the Firewire bus it is outputting phantom power at the extreme low end of that scale, but when you plug it into the wall, the voltage is higher.


So, maybe one of the two matched resistors on the phantom power output (which have to be in really close spec to keep the circuit quiet) has gone bad, and stays close enough in spec when being fed the lower voltage... but when the unit is plugged into the wall, it gets much more voltage, eventually warms up and goes far enough out of spec with its twin that the noise comes through the line.


Disclaimer: WILD GUESS. I'm not a repairman, and I've never had a phantom circuit so bad, so I haven't really got a clue.

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Great ideas guys!


Gravity Jim, I think you NAILED it. That's the same conclusion I came to this evening while using the unit.


I had it plugged in for roughly 50mins or so before I noticed the "rumble" coming into the audio path. It started off slow, and then within a few mins it was at full peak. As soon as I unplugged the unit and ran on firewire it vanished, as always.


This time I plugged in both a sm57 and a condenser side by side to make sure it was only affecting phantom power before I send the unit back. I let them sit there, warming up, and since the sm57 held up fine, its almost certain something is going on with phantom power and regular line voltage as opposed to whatever firewire is putting out.


Its a strange phenomenon, but it makes sense to me. Something is taking its time to "warm up" or "heat up" and once it gets there it starts leaking in the signal path. Happens on both channels. (ULN-2 is two channel)


The only other theory I came up with was: I'm simply getting a dirty power source in the first place, and somehow phantom power is bringing it out to the top. But it never happened with any other gear... so this most likely leads to the ULN2.


Thanks for the input!

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