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Wanting a microphone that picked up less noise and was more sensitive to soft sounds than the mic built into my laptop, I picked up the cheapest condenser mic I could find on Amazon, the Nady SPC-15 — in fact, I bought two as there was a special. It is connected to a M-Audio Fast Track Pro USB audio interface.

However, there is a constant noise distinctly audible when the gain is turned high, on one of the microphones. This noise varies in pitch or disappears entirely depending on:

  • the physical layout of the USB cable and hub (noise still present without the hub) connecting the audio interface to the computer and power source,
  • the setting of the gain knob on the audio interface and whether one or two microphones are connected, and
  • physical disturbance of the microphone.

I would guess based on this evidence that the microphone is picking up the noise from its phantom power.

Here is a recording of the noise, and its absence with the other microphone. In this recording, I swapped only the mic, using the same cable and input jack. (The other input jack gives the same results.)

I want to know whether this noise's presence in one microphone

  • indicates a defective unit?
  • is normal manufacturing variation in cheap condenser mics?
  • is a consequence of abuse, and if so what kind? (Mechanical? Hot-plugging? Leaving the mic powered for long periods?) I am especially concerned about this because the microphone without the noise is the one I have not used as much.
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What happens if you switch the mics' inputs on your Fast Track? It could be an issue with the interface and not the mic. –  Warrior Bob Sep 26 '11 at 18:17
    
I tried both inputs and the results are the same. In the sample recording, I physically swapped the mic and left everything else the same — input, cable, and even stand. I hadn't tried connecting both microphones at once — I have now, and the results were still specific to the one mic, but the positions of the gain knob resulting in the noise were different. –  Kevin Reid Sep 26 '11 at 18:35
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I don't know enough about microphone construction to really give you a good answer, but I would initially suspect a defective or damaged microphone. I would not expect something as drastic as that from a manufacturing variation. Do you leave phantom power on when you "hot-plug" mics? I've heard this is bad but can't say for sure that it is or isn't. I always turn it off (possibly superstitiously). –  Warrior Bob Sep 26 '11 at 18:47
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Hot-plugging should not be a problem for properly designed modern condenser mics. It can be harmful to old ribbon microphones, and perhaps vintage or otherwise unstable FET designs. But I doubt this is the reason here. — Regardless of what the principle problem is: the microphone is faulty, so if it's new I'd send it back. –  leftaroundabout Sep 27 '11 at 12:27
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Thanks for your input. I called Nady and described the problem and they directed me to return the microphone. This question itself still has no answer, though. –  Kevin Reid Sep 30 '11 at 17:30
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2 Answers

My best guess is that something in the mic or the mic cable is picking up RF interference.

The reason I suspect this is that you said that moving the USB cable affects the noise.

The problem isn't in the digital domain, so the USB cable isn't picking up interference. Therefore it must be emitting RF.

You only get the problem in one mic. Therefore the design of the mic must be sufficiently shielded/balanced to cope with the level of RF coming from your USB cable and elsewhere. But some manufacturing flaw means the shielding or the balancing isn't sufficient on the other mic.

Open the mic up only to the extent your warranty allows. There may be an obviously broken connection (which would defeat the balance on your XLR cable). If so, it's up to you whether you call in your warranty, or try to fix it yourself. Otherwise, get it fixed on warranty.

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Your mic is broken, simple as that, even the manufacturer says so. Mic noises are a constant problem though, I've seen all kinds of weird things cause noises. One old cheap trick is to phase reverse the noise if you really liked the take. Record the your take through the mic, then record the take again just recording the noise itself - then play the two together and phase reverse the noise.

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That won't work. Have you actually tried it? Noise that's correlated between the two recordings (like mains hum) will cancel, but only if you manually align the phase. Otherwise it may increase in level instead. The uncorrelated white noise component will always increase. –  endolith Dec 16 '11 at 16:07
    
phase reversing doesnt work ? what ? its a very old engineers trick. been a sound engineer for 20 years and yes ive tried it and it works. –  user1736 Dec 16 '11 at 16:39
    
Phase reversing only works if you make both recordings at exactly the same time. For example, the Zoom H4 has extremely noisy XLR inputs, so if you plug a mic into one input, but record both inputs and then invert the signal from the second input, you can cancel the noise out of your live input. If you have aperiodic noise, then trying to phase cancel with two recordings made sequentially isn't going to work. –  ObscureRobot Dec 16 '11 at 17:21
    
obviously if the sound isnt the same on both the signal isnt going to cancel out, but its not a requirement to make both recordings at the same time at all, done it loads of times. works fine for me! we used to use it for recording vocalists without headphones for a better take, cos you know how headphones change things for performers, if you are one. we used to record a vocal take in front of the speakers with sound up just enough to do a decent take, then just run it all again without vocal and without touching any levels on the desk, then reverse. works fine. –  user1736 Dec 16 '11 at 17:28
    
heh, two people eager to say something will not work who have never tried it! ever seen this phenomenon on the internet before ? :) –  user1736 Dec 22 '11 at 14:40
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