I am using a pair of km184s as piano spots. They have always been dead quiet with phantom power off. All of a sudden, both mics now have a humming sound with phantom power off. The hum goes away when i turn on phantom power. Same problem with different cables and with different preamps, so it seems to be something to do with the mics. I am confused…
Any ideas or thoughts on what this might be, or other troubleshooting ideas to further pin it down. Fortunately, it is not affecting recording with them, so it’s not the end of the world, but it still seems odd…
Thanks.

Just to be clear, the mics make this noise with P48 off? How do they behave when they are powered?

Hard to say what you preamps are seeing from a terminate mic cable but with P48 turned off.

D.

Doug - the mics behave and record properly with phantom power turned on. The hum is only there when phantom power is off. Strange.

I mix FOH in a video-infested venue that's an RF nightmare, and this place has two SM81's that do that a little (barely audible) and an active phantom-powered DI box that does it a lot. But they've never NOT done it, that's just the way they've always been.

When would you be using these mics with phantom turned off?

I don’t think I’ve ever listened to my pair of km184s with power off, but it sounds like you’re picking up something that could be new to your environment, or a fault somewhere else down the line.

@ jnorman
Brent's comment about RF got me thinking. Have you tried different cables and experienced the same buzzing? I wonder if either the cable or the mic output stage is acting as an RF antenna when it's not being actively driven. If there's no change, try different preamps.

I experienced something similar with my old studio monitors when I mistakenly powered them up before my interface.

Volt - yes, I tried different cables and different preamps, and had the same issue.

In my case I noticed it because the venue in question has an FOH console and a livestream console, both fed by a transformer-isolated splitter. The FOH console (the non-isolated side) supplies the phantom power, and I just happened to have the livestream console powered up but not the FOH one, which is how I came to hear the buzzing. It was one of those interesting-but-who-cares things.

Yes my instinct is there is something new in the near environment.

Your cables are giant aerials till you power them up.

Hell, even different light bulbs have made a difference in my workplace.

do you also get noise when using transformer isolated mic preamps? some weird rfi? - anyway, i wouldn't bother if there isn't a problem when the mics get powered via 48V.


[all my km-18x mics are digital mics which are pretty much immune to rfi]

Have you substituted the KM mics with another pair of mics (leaving cables, mic-pre’s etc unchanged)? It’s unlikely to be a simultaneous circuit fault in both KMs together, typically what you describe would implicate a cable or preamp fault….perhaps a room light dimmer switch left halfway through its travel, or even a bad LED lightglobe ? Try switching off other mains powered devices in the room or house…and presumably you don’t live near any RF (or microwave) emitting foreign embassy buildings ?

Have you check to be sure the capsules are screwed on all the way?

I recently had a similar problem with a KM184 and the capsule had come unscrewed just slightly.

A lot of energy being spent investigating a non-problem.

What exactly do you think we're doing here on the Internets of the Web?

When the P48 is off, could it be that the earth is also disconnected somehow?

I don't know, I'm just sliding down this grassy bank with you!!