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Old 07-12-2015, 04:07 PM   #1
mikeypee
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Default Phantom-Power noise: Bad usb, or humidity??

Hey guys,

Yesterday while recording, my condenser mics started doing this weird thing where they made that swooshing sound that you hear when you first turn on/off phantom power. They only seemed to do it when I sang a loud note into the mic, after being silent for a few seconds or more.

At first I thought maybe my mic was broken ... so I tried my 2nd one, and the same thing happened. (MXL 770 and MXL V67).

Then I thought - maybe it's just too hot/sweaty/humid in here, but the (cheapy) little meter I have in here only read 80 degreees F and about 60% humidity (it's usually around the same). Plus one of the mics had been inside a foam case the entire time before recording, and the other always has a ziplock plastic bag over it when it's out.

So now I'm wondering ... the only other thing that was different, is I used a different computer to record. Usually I use my laptop - no problems whatsoever. Yesterday I used the PC in my bedroom. Could it be a bad USB port? A crappy voltage regulation thing going on with an older motherboard? I'm using the onboard port (no hub), but the PC is ancient-as-can-be. Only 4GB RAM and probably one of the first ever $20 processors to go 64bit, lol.

Have any of you experienced something similar? Would a USB port fix it?

I'm going to record again later today and I'll try to A/B test between my laptop and that PC again, with both mics... and see if I can get something to misbehave in a regular pattern.
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Old 07-13-2015, 01:39 AM   #2
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I have a mic, an AKG C451EB that draws a little bit too much current from the phantom power supply. No problem with most mixers. With a USB interface that takes it's power from the USB bus however, it's a different story. It might take a minute settling down. During that minute, it produces swooshing sounds, it motorboats and behaves as if it's broken.

That's with a well-built bus fed interface from the likes of RME or Sound Devices...

When it comes to the lesser gods, it might not function at all.

In your case I see two possible problems:

- The older PC will have voltage levels that are a bit lower, adding to the problem. Also, the USB socket might be connected with very thin wires to the motherboard. These add to the voltage drop. Once you go below 4.85V all sorts of funny things start happening.

- The desktop is grounded. The laptop is not. If any bad interference is happening on the ground wires (and that could be coming from a mile away), it will infiltrate your interface's power supply and it just might make the mics behave strangely.

I once did some trouble shooting in a studio where there was up to 150V DC on the ground. It took me a while to notice, as you'd expect AC, not DC. All my power analyser tools are AC based. Some can't even measure DC at all. It wasn't until I looked at a map. There was a small industrial railway station just behind the studio. These trains had a 550V DC power supply. And they only used that once in a while.

When the DC appeared on the ground, no interference was heard, as it was DC. But lots of equipment stopped working. Some even broke, once in a while.

The studio moved...

In your case, with an older desktop, I'd bet the PSU is a bit off. And when the mic's internal PSU can't cope because of voltage drops, most mics will woosh, or produce other strange sounds.

If you have a DMM and a spare USB cable (and who hasn't got several spare USB cables?) you could cut on in half and measure USB voltage. Usually, that's the red and black wires.

If your interface can use it's own power supply, that's also a thing to check. And trying another USB port might just work too. USB ports usually have automatic circuit breakers. Sometimes, these don't age very well and add some resistance to the chain. And an interface's PSU might just be too much for this port, while it may work on another port with the same computer.
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Old 07-13-2015, 04:09 AM   #3
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Thank you so much for the thoughtful reply. I think you've helped me understand just a bit more.

I think I'm going to buy this little gadget I just found on amazon that will allow me to test the voltage on all USB ports I own. I probably have a DMM laying around somewhere but I don't want to scissor a usb cable and I'd rather buy a cool new gadget anyway ... just so I can show my brother in law and the general populace "Hey look what I have that you don't have ... and therefore I am cooler than you"

Derp, so maybe I should have been using that powered USB hub sitting on my desk. I thought for some reason the interface would work better being plugged directly into the motherboard. I suppose I was thinking more in terms of signal length or perhaps any delays that a USB hub might add (I haven't researched if they do or not).

I will say that the mics work with my laptop both when it's unplugged (for shorter sessions), and when it's plugged into its power supply (3-prong grounded) for longer ones. I'm thinking grounding is probably not the issue.

Wow ... come to think of it... the PSU is probably an extra-generation older than my old crappy PC, as I love re-using parts to save money whenever I build things. I'm even pretty sure I'm using some of those molex-to-sata power adaptors, which means the PSU could be a decade old or more loooool.

My primary interface is the Scarlett 2i2, so not external power option available... but I'm still hopeful, based on your advice and my recent observations.

Thank you again.
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Old 07-13-2015, 04:17 AM   #4
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Oh I wanted to add that I'm finding a lot of people talking about similar problems, and basically going over the same bullet points you went through with me (underpowered usb ports, leaky cables, certain mics, etc).

I think another solution I can try, if my efforts fail, is just to use an external phantom power supply. That or maybe I can just start using my little Art preamp thingamajig again.
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Old 07-14-2015, 09:24 AM   #5
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A Behringer external phantom power box is what I use when all else seems to fail with power from a USB bus powered laptop setup.

And that's usually when a dynamic mic seems to work fine. They're only 30 € or so. And the Thomann house brand, Millenium even has a stereo one for that price.

Mics that don't work wit these probably need restoration anyway. Leaky capacitors start drawing too much current after some years...
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Old 11-10-2015, 09:50 AM   #6
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I just wanted to do a quick bump and tack this info on, for anyone who may be searching this type of issue in the future.

The problem appears to have been the phantom power supply ; not enough juice coming in from certain USB ports.

At first, I bought a USB voltage checker but as my luck would have it, the audio interface refused to work through it, so I was unable to verify or debug.

So then I just bought an external phantom power supply, and my problems immediately went completely away, on all USB ports. Woot.
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