RF interference?

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MrUserNameIV

MrUserNameIV

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I've been having some minor issues when recording with my condenser. It sounds like a motor powering up and down very faintly in the recording-it doesn't do it a lot, but once in a while if I'm mid session, it'll start up and then go away. I tried a dynamic mic without phantom power and it didn't seem to do it. I read something somewhere that said this could be some sort of RF interference. Has anyone ever experienced this?
 
My guess would be that it might be your fans from your computer? I know that computer fans tend to sound like that, and if you're anywhere near your computer, the condenser is likely going to pick up the fan noise.

It's tough to say without hearing it. Maybe post an audio sample of the issue you're having?
 
I have had that happen - with no mic in the system, and with both my AI and with the OEM sound card, and my wife's computer will get it on occasion, too - so it's nothing to do with the actual computer or recording system. Is your computer hooked up to a DSL/FO/cable router? If so, try disconnecting it when the noise is happening and see if it goes away - and report back!
 
I picked up a radio station once for no particular reason. Move all your leads around while it's making the noise, one at a time, and see if anything in particular stops it or makes it worse...
 
It seems to do it when I have a mic with phantom power on, but doesn't do it when I have a dynamic plugged in, with the same cable, in the same input on the same interface. I was thinking it might be the computer fan or maybe a drive spinning up and down. I'll try the router trick, it's placed fairly close to the recording interface (small room) so who knows. The thing is that it only does it once in a while (once every 10 sessions maybe, and it usually doesn't do it for very long), so if it were something that is always on interfering wouldn't it be present all the time? I'll see if I can find a clip of it to post.
 
Same thing was happening with me - it didn't happen frequently or long enough to pinpoint so I lived with it. But it would happen whether I was using a mic or not, and could happen on playback just as easily as during recording. Hasn't happened in a while, but the only changes I've made were unplugging and plugging everything together when I redid my desk, and I'm now using a different USB cable to my AI.
 
It used to be called motor boating as it sounded a bit like a marine diesel ticking over in the background. Sometimes power supplies are the cause when capacitors get into a charge discharge heavy cycle. Could be the mic taking excessive phantom current? Do you have another condenser mic you can try?
 
rob aylestone said:
It used to be called motor boating as it sounded a bit like a marine diesel ticking over in the background. Sometimes power supplies are the cause when capacitors get into a charge discharge heavy cycle. Could be the mic taking excessive phantom current? Do you have another condenser mic you can try?
Click to expand...

Interesting. Yeah, that would describe the sound perfectly. I can try another condenser and see if it does it. If the mic is taking excessive phantom current, what would the remedy be? Should I be worried?
 
Some condenser mics draw more power than others. A hungry microphone connected to an only just adequate power supply can cause problems like this. If you try another mic and the problem goes away, then if you want to use the problem one, you need to do something - which could be as simple as in in-line phantom unit, which need not be very expensive - Stagg do some cheap ones. What mic and mixer/interface are you using - we can look up the two and check what is going on. Some phantom power supplies don't even put out the full 48V, because most mics will work on less - but some just won't.
 
rob aylestone said:
Some condenser mics draw more power than others. A hungry microphone connected to an only just adequate power supply can cause problems like this. If you try another mic and the problem goes away, then if you want to use the problem one, you need to do something - which could be as simple as in in-line phantom unit, which need not be very expensive - Stagg do some cheap ones. What mic and mixer/interface are you using - we can look up the two and check what is going on. Some phantom power supplies don't even put out the full 48V, because most mics will work on less - but some just won't.
Click to expand...

It's an Apogee Duet with a Blue Bluebird mic. The interface is USB powered, so I think you may be on to something.
 
Hmm... I did find another person via google complaining his blue wouldn't work with one mixer, yet that mixer powered a different mic fine, so it adds support to the power supply being a little inadequate for a mic that perhaps needs just a little more current than the average.

Plugging it into something else, using your existing cable and hearing it working will exonerate the mic, and trying another mic in that cable and hearing it work on your apogee will rule out the problem being that end. You could verify what is happening with a meter - the usual thing is that the extra current the mic takes can't be supplied so the voltage goes down. Just an unlucky combination.
 
This..
BLUE Bluebird
tells me that the mic draws a modest 1.8mA so that is an unlikely source of problems.

The review does however comment that the supplied cable bonds the XLR shells to pin one both ends. This is not standard practice but it is hard to see how this could cause this particular problem. It might however be worth trying a conventionally wired XLR lead, i.e. one with floating shells?

If indeed the problem continues try some ferrites on the cable. If you can prove the mic is at fault (or indeed the interface) get onto the company(s) and moan like a drain. RFI is not something we should have to put up with these days!

Dave.
 
Interesting. One thing to consider is that the interface is USB powered (doesn't run off the AC for computer operation, technically - but it's an iOS compatible device that uses AC power for that purpose). It does come with an AC cable as well, so I'm wondering if I plugged both in, if it would make a difference.

I'm not using the original cable anymore: Just a standard XLR. I did use the Blue cable years ago with a MOTU interface and never got that, so maybe it's something to do with the Apogee.
 
" so maybe it's something to do with the Apogee. "

That is certainly a possibility. One of the things the makers of some of this high end kit do is boast a super wide bandwidth for their mic pre amps. -3dB points out to 200kHz and beyond. Maybe in this case this very dubious practice has come to bite their arse?

Only in the field of audio engineering do designers go for bandwidths ten times or more than the wanted signal. They don't design an ocean liner's steering systems to respond to a 1mSec step input cos there ain't none!

Try those ferrites.

Dave.
 
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