Hum with a specific mic (balanced), only when connected directly to my interface

I've got a Line Audio OM1. Recently I started having some trouble when I plug it directly into the interface (RME FF 400). It's mains hum, at 60Hz. It goes away/diminishes when I touch the cable (more so the closer I am to the mic) or the microphone body.

It's a fairly basic balanced circuit using an 8" electret (with built-in FET), a single transistor providing some gain, and a pair of BJTs for a transformerless output. I don't have the circuit traced, but everything is solid inside and the mic is working the same as it always has.

-If I plug the mic into ANY OTHER preamp on the shelf and then into the interface, I don't get the hum. This includes any number of other pres running off the same power strip, even the ones without three-prong plugs.

-I have changed cables.

-I have connected a different microphone using the same cable and interface input and I don't get the hum with other microphones of various stripes under the exact same circumstances (multiple schoeps style, transformer balanced, and transformerless mics).

Since it's not a buzz, I'm not suspecting a ground loop. The output appears to be balanced (there's no other reason to have that many devices in there if weren't), but the fact that I can kill the hum with the presence of my body has me wondering if I'm not overlooking something. Since it only happens with this mic and this preamp (direct to the interface), it's not unusable, but I am definitely stumped.

Can anyone think of something else to test to identify the problem?

Um, use one of your preamps that works and live with it. Life is too short to try and solve all problems. It doesn't like the RME input. Get over it.

Seems in the evidence to be some incompatibility with the RME. The only odd thing I have come across is that the RME has servo balanced inputs, whatever they are.

Cheers

Ian

Jumper wire from mike body to RME case.

If that's the fix, cable is bad (no ground continuity).

radardoug said:

Um, use one of your preamps that works and live with it. Life is too short to try and solve all problems. It doesn't like the RME input. Get over it.

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That's what I've been doing. However, it (a) adds an inconvenience for live recordings and (b) solving problems isn't purely about fixing problems but also learning. It could have been something simple I was overlooking.

PRR said:

Jumper wire from mike body to RME case.

If that's the fix, cable is bad (no ground continuity).

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I verified ground continuity with a multimeter and tried multiple cables.

ruffrecords:
RG Keen's explanation of servo-balanced outputs was:

Outputs being servo balanced means that (1) the outputs are DC coupled, and so have a frequency response all the way down to DC; (2) the device has a circuit that senses any DC on the output and drives the input of the device to force the output voltage to be very close to zero; (3) this will have substantially no effect whatsoever on a pedal's input, as almost all pedals are NOT DC coupled on their inputs, so they mostly don't care, with the exception of switch popping on bypass if there is any DC level on the input. (4) the circuit used is an integrator which has the integration tied to the output, the reference to ground, and the output of the integrator to a DC coupled point inside the unit. The integrator will sense any DC voltage other than ground/0V and drive the DC coupled point on its output in a direction that nulls out the offset.  In general, this is just one integrated circuit opamp set up as an integrator.

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Not sure how that translates to inputs

midwayfair said:

I've got a Line Audio OM1. Recently I started having some trouble when I plug it directly into the interface (RME FF 400). It's mains hum, at 60Hz. It goes away/diminishes when I touch the cable (more so the closer I am to the mic) or the microphone body.

Click to expand...

have you tried talking to line audio about this?

Since the presence of your body seems to affect the problem, couldn't it be an EMI/RFI problem? Can you experiment placing caps across 1&2 and 1&3, 0.01uF like?
Or maybe the head basket has lost continuity with the cable's shield...when you checked the cable continuity, have you made sure it extends to the body and headbasket? It could be such a simple issue as plastic-body XLR vs. metal-body...

midwayfair said:

Not sure how that translates to inputs

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Exactly my point. It may be simply a typo or an over enthusiastic sales blurb writer or it might be some whizzy new circuit that just happens not to work well with the internals of certain phantom powered mics.

Cheers

Ian

I once had the same problem, although under somewhat different circumstances.
The cause what a little hum on the phantom voltage.
With a perfectly balanced source (microphone) the hum was suppressed by the common mode rejection of the input stage of the microphone preamp.
But... some microphones are 'quasi balanced',  with one wire carrying the signal, and the other wire coupled to ground through a capacitor.
At the moment the impedance on both signal wires was different, the common mode rejection was not perfect anymore, with hum (from the phantom power voltage) as a result.

I've been watching this thread "with interest" as they say...

midwayfair said:

I've got a Line Audio OM1. Recently I started having some trouble when I plug it directly into the interface (RME FF 400). It's mains hum, at 60Hz. It goes away/diminishes when I touch the cable (more so the closer I am to the mic) or the microphone body.
...
Since it's not a buzz, I'm not suspecting a ground loop. The output appears to be balanced (there's no other reason to have that many devices in there if weren't), but the fact that I can kill the hum with the presence of my body has me wondering if I'm not overlooking something. Since it only happens with this mic and this preamp (direct to the interface), it's not unusable, but I am definitely stumped.

Can anyone think of something else to test to identify the problem?

Click to expand...

That last post from RuudNL got me thinking (it's a plausible explanation for something like this happening), though I don't think your hum is from the phantom power as it was in that case.

The way you describe it, the problem is electrostatic pickup of hum (the hum volume may vary a little with your hand waving around the mic, kinda like a Theremin). I suspect this doesn't happen with other preamps because they're grounded to "ground" through the ground wire (NOT the hot or neutral) in the power cable, but the way you're powering this interface (it appears there are several options), it's not. Try connecting the line out of one of your other preamps (that you tested this mic on and didn't get hum) to the line in of the interface as you did to test the mic through the pre, but leave the actual mic and cable plugged into the mic input of the interface. This presumes you can still use the interface's internal pre with something else plugged into a line input. If the hum goes away, it's because your interface is "floating" rather than grounded, and the other pres provide a ground path.

And it happens only with this mic because, I presume, it has a high impedance point  (at the capsule and before the FET buffer) that's not as well shielded as in other condenser mics.

Reading through your post once again:

-If I plug the mic into ANY OTHER preamp on the shelf and then into the interface, I don't get the hum. This includes any number of other pres running off the same power strip, even the ones without three-prong plugs.

Click to expand...

At first thought this suggests my above suggestion may not work. But then, are all these pres in the same metal rack, or are their chassis/grounds all tied together some way, as through a patch bay? That would mean they're ALL grounded, even the ones without three-prong plugs.

No patch bay, not even a rack really.

The interface is floating ground, though, I think. I usually leave something that's fully grounded plugged into one of the line ins just incidentally, though.

I think I'm stumped and despite my previous post I'm going to accept that this one mic just interacts poorly with the inrface. If ever find a solution I'll update the topic. Thanks everyone or the help as always!