Sudden 60 cycle hum from tube condenser

Tadpui

Tadpui

Well-known member
I'm starting to think that there is some sort of Twilight Zone electrical situation in my house. The latest piece of gear to be affected is my Avantone CV-12. It's been quite a soldier in my home studio for the last few years...dead quiet, great sounding mic for its price range. Tonight I sat down to do a scratch track, turned on my usual vocal and monitoring chain, and there was a horrible 60 cycle hum coming through the headphones. Maybe 60 cycle hum is incorrect here...it sounds like when you unplug the cable from your guitar while the amp is still on. That distinct hum, not a buzz. This hasn't been a problem before, and I just noticed it tonight. Last time I used this mic was maybe a month ago.

I tried different cables, preamps, etc. where I could. Other mics are dead quiet at idle. The only cable I couldn't swap out was the 9-pin cable that goes from the power supply to the mic itself. That's not something that most people have a spare at hand. I even tried swapping the tube for an old ECC83 I had laying around (stock tube is a EHX 12AY7). Still hummed. FYI, the ECC83 sounded like shit in this mic ;)

What in the world would cause a sudden ground loop in a mic whose routing and cables haven't changed or moved in a month? I leave everything plugged in and turned on all the time, including the mic's power supply (KCP&L hates me). Seriously this mic probably hasn't been unplugged or turned off in almost a year. But it doesn't get hot or anything, so I can't imagine that the heat from the tube would have loosened or damaged anything. A visual inspection didn't show any obvious damage, and the mic still produces a strong signal. It's just a strong signal with a strong hum to it.

Just a couple of weeks ago, my new (to me) Distressor went on the fritz after only a week of use. The next day my home theater receiver lost its L and R channels (OK, it was a 10 year old Sony entry-level receiver, but still...). And that same day my network switch gave out. I'm starting to suspect that this issue started during the same time frame, while the mic was idle but not in use. I'm no conspiracy theorist, quite the opposite. I'm really a skeptic. But damn, the gremlins seem to be working overtime around here lately.
 
What you're describing is definitely a 60hz hum and probably due to a faulty ground. It could be the special 9-pin cable and maybe all you have to do is tighten the clamp on the connector housing. But with all the other stuff having problems at the same time, I'm thinking you had a power spike. Is everything on a surge protector? I would wonder if the power supply took a hit. Any storms in the last few weeks? (You're in Kansas, of course there were storms in the last few weeks)
 
Thanks guys, I appreciate the input.

While I was trying to narrow it down last night, I noticed that the buzz would totally stop if I unplugged any cable in the mic's chain: mic->power supply->patchbay->interface. It would still hum whether the power supply was on or not. And I couldn't get the hum to stop by touching anything (mic, power supply, patchbay, rack, interface). I plugged the mic/power supply directly into the interface and through an external preamp.

I'll give it a once over with a multimeter and see if I find anything.
 
[Edit: Misread your post]

So maybe it's the mic itself? Could have been dropped or knocked about?
 
Good question Chili...it's not impossible, but improbable. It's just me, my wife, the dog, and sometimes the stepson. The wife never goes into the studio, the dog wouldn't have repositioned a toppled mic stand, and the stepson doesn't seem too interested in the studio room. But I do leave the door open and mic on it's stand. Somebody could have knocked it over and not told me.
 
Excellent news fellas! I plugged the power supply into a different outlet in my studio, and the hum is gone! It's dead quiet once again.

The weird thing is that the other outlet is still on the same circuit as the previous one, but I just put it on a power strip/surge protector. I have 2 circuits in my studio: 1 for computer and audio path, and the other for amplifiers and lighting. There are 4 4-outlet receptacles behind my computer where everything is plugged in, and they're all on the same circuit. Maybe I had something else on somewhere that was causing the hum. I'll have to start flipping switches and lights, HVAC, ceiling fans in other rooms...I'll find the culprit. But I also removed the power supply and cabling from its original resting spot, so maybe you were right arcaxis, maybe some interference crept in from somewhere. My cable management is pretty much nil in this room, there are signal cables and power cables all mangled together in a horrible blob behind my computer and patchbay.

Or maybe it was an intermittent thing and it'll return as soon as I try to record vocals again :(

Thanks again for your suggestions fellas.
 
Funny, that was what I was going to suggest when I changed my post.

I've got the same problem in my studio. Two outlets. Everything runs off of one except my keyboard, which plugs into the second one. They both go back to the same panel, but different breakers, but the panel is literally 3 ft from the room. I'll get a hum when I use the keyboard plugged into my interface. I have to run an extension cord across my studio to power the keyboard when I want to record. The studio isn't big at all, it's just a hassle to have an extension cord in the middle of everything.
 
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