Mic problem?

[Alex Richards]

*Different mic to my other thread, might not belong in PC Music*

I have a Roxdon EM1 that has a buzzing sound when it is plugged in to a Fast Track Pro(FTP). The signal path is Mic > FTP > PC. Monitored on headphones.

I thought it may be a ground loop so I unplugged the FTP from the mains. No change.

I tried an AKG C1000 and there was no buzzing sound (same cable). Success.

Returned mic to seller who says it has been tested, there is no problem and returns it saying the problem is on my end.

What are the possible problems?

[Alex Richards]

Update: I managed to get rid of this buzzing noise by turning the -20dB pad ON. When it is off the noise returns. Again, no other sources are affected this way.

Any ideas why this is happening?

[Andy Le Vien]

Switching the pad probably just reduces the buzz by 20dB. It's possible that the connector on the cable you are using is not a perfect fit on the mic, so try another lead. Is the cable the one supplied with the mic?

[Alex Richards]

No, it removes it altogther, trust me. I have recorded 'silence' at max gain and then normalised it in software, there is no buzzing noise when the pad is switched on. Yes it is the cable supplied with the mic.

Also, it is the pad on the interface, not the mic, that I am referring to.

[Alex Richards]

OK, so I looked up how pads work
http://www.uneeda-audio.com/pads/
You add resistors of a certain value depending on the impedance of the mic. Could a difference in the impedance of the two mics cause this problem?

When I googled the mic I found this sentence on their website "we don't believe in paper specifications", so I have no idea what impedance it is, or current or anything else.

[Alex Richards]

Final update: tested a 3rd condenser mic (sE Electronics) and still no noise. Contacted the seller who was very helpful and offered a refund. It seems it was a strange combination of the mic and interface, but I still don't have an answer if anyone wants to theorize why it might have happened.

[dmills]

Phantom power filtering (or lack thereof) would be my guess!

Sounds like there may be a DC/DC converter involved which is either just not stiff enough or is coupling noise into the preamp when heavily loaded.

Generally IMHO if a mic manufacturer will not publish good specs, that tells you everything you need to know..... Take the refund.

Regards, Dan.

[Alex Richards]

Thanks for the guess Dan! It was driving me crazy.

I googled phantom power filtering and all I could find was about removing it altogether, could you explain a little further how it may have caused it?

Thanks again.

[Alex Richards]

Ok, so the problem's back.

2 condenser mics (sE electronics X1 and Audio Technica 2035), both make a 'whining' ground loop-like noise when plugged in.
Again, turning the -20dB Pad ON removes the noise. Mics were tested on another interface (Alesis iO2), both worked fine. Tested FTP on a Mac, same problems. Tested on both bus power and mains power; no difference. Tested FTP (and both mics) in standalone mode; no problems.

Can anyone help?

[Hugh Robjohns]

Buzzing can only come from some form of interference.

It could be from an internal DC-DC converter within the microhone itself. Some mics use a DC-DC converter to raise the phantom power supply to a higher voltage (60-70 volts, typically) to bias the capsule.

If the problem was confined to only one mic, then this might be a possibility.

It could also come from external interference -- something radiating something nasty that gets into the mic's electronics and becomes demodulated and audible. Poor grounding is often a cause of this -- either between the cable's screen/pin 1 connection and the mic's socket, or because of the way in which the mic's metal body and/or grille is (poorly) connected to the cable's screen connection. Again, were this the case you'd expect it to be constrained to one mic or mic/cable combination.

Another possibility is that the phantom power supply in the preamp is poor and generating noise. Again, DC-DC converters are often used and these can generate out-of-band noises that can become demodulated and audible, especially if the mic is drawing a lot of phantom power current and straining the internal power supply.

Or it could be interference getting into the interface because of ground loops, poor grounding, Firewire chipset issues and so on.

Given that your problem seems to happen with several mics, the interface is looking more like the culprit to me.

Can you try those mics on someone else's system (or a known good mic preamp) to see if the mics are actually okay?

BTW, I'll move this post to the R&P forum.

hugh

[Alex Richards]

I have tried the mics on a different PC with the Alesis interface, no noise. I've tried different USB ports, same problem. Different USB cable, same problem.

Thanks for the reply and the detailed response Hugh.

[Hugh Robjohns]

Okay... then there are probably two possibilities:

A weird incompatibility with the interface on your computer -- possibly related to the mains distribution, ground loops, or an unfortunate interaction with your computer's hardware...

...or some local interference problem with your specific set up and/or the way it is installed.

If the noises really do go away when you switch the interface's preamp pad in circuit then I would support the earlier suggestion that it is a mic output driver issue... but if it happens with other mics too then perhaps sit is a phantom power problem with the interface itself.

Most odd...

Hugh

[Alex Richards]

I assume it can't be a ground loop because I had the same problem on an iMac when there was only one plug connected to ground.

I'm almost positive it's the interface now, I noticed that on the PC the noise isn't there until Pro Tools has finished loading (it's there as soon as I boot up on the mac). But yet there is no problem in stand-alone mode?!