AC Noise (w/ audio sample)

Posted: Thu Jul 17, 2008 7:19 am

I moved into a new house about a month ago. The power in the recording area (a former garage converted into a bedroom) is really dirty: I can't record guitar amps right now because the noise is so bad. But I also found out this morning doing some acoustic guitar work that the noise is getting in through my microphones when using a high-gain preamp.

Here's a sample. The volume is soft, but I normalized into the transients so it shouldn't be too abrasive:

AC Noise (if the hotlink doesn't work, open a new window and copy+paste)

I'd first like help identifying this, whether it's EMI or radio interference. Secondly, I'd like strong opinions whether these less-than products will help: some solutions I'm looking at are the Monster PRO 2500 and Carvin AC120s. I'm only able to spend around $200 right now.

Posted: Thu Jul 17, 2008 7:39 am

Hello Patrick;

Right off the bat, it sounds like a simple ground loop or a bad cable, not nec. "Bad AC" per se.

You can easily track down the source of the hum several ways, but I'd suggest this: Get a good clean power conditioner ($150-500, depending on your budget, make & model, and power requirements) and start with that as your ONLY source of AC power. Check the conditioner's AC outlets with a tester to confirm proper voltage is present, ditto for grounding. Use only these outlets.

Do not use any other outlets or ground sources anywhere else in the room/house at this point. Plug everything into this, one at a time, starting with the power amp, and listen as you add devices for hum, etc. (You're making a "Star-ground" this way...all ground points are the same, and all power is the same phase, etc.)

If your amp is quiet, then you can assume the line source is doing ok., Add your mixer (or computer sound card, etc.) and bring the level up to normal working levels, and try that. Eventually, you'll find out that something is adding the ground loop via a chassis or wiring issue. It could be mic cables that are picking up some local hum, as well, although truly balanced lines should be ok, even in the presence of some strong external noises.

If the hum is indeed air-borne and omni-present, your guitar pickups will be the first to let you know - as you probably know, you'll just have to turn and move to find the "Sweet spot" where the AC radiation is minimal and out of phase enough to cancel. (I really doubt this is the case, though....)

In the worst of cases, you may find a hidden transformer or power unit somewhere in your home? (Is it a stand-alone dwelling, or could you be on a party wall to a distribution transformer to the block, or a big Air conditioning system?)

Hope that helps....

Posted: Thu Jul 17, 2008 9:47 am

Thanks, Joe. I'll be anxious to try your suggestions once I buy such a unit. Please speak freely: can I expect a power conditioner in the sub-$500 range to make a noticeable difference? If so, are there any models you'd recommend?

I hear two strong components in this noise: one is a steady, quickly oscillating ~G# (in the low 400 Hz range); the other is a more random, mechanical sounding pink that breaks in and out. I know very little about this kind of thing, but is it possible the latter is a voltage regulation issue?

Posted: Thu Jul 17, 2008 12:32 pm

I'd be hard pressed to tell you which one to buy, and I'm also not sure yet that it's really "dirty" power. Are these noises everywhere, omni present on everything you turn on, or do they appear on certain devices? Does the noise change in level or type when you add/disconnect certain items in the chain? Is it possible you just don't have a good ground scheme? (Make SURE you check with a professional electrician on that....if by some chance you don't have a good ground scheme going on, you may have some SERIOUS and DANGEROUS hazards there...so be careful!)

Most comsumer and pro-sumer audio gear should have enough rectifying and filtering built into its own circuitry to filter out most line problems. (It's true, although most power-filtering manufacturers don't want you to know that!) With that in mind, assuming each unit is working properly, as well as powered by a "clean" filtering unit and all grounded together properly, you should be fine. (SHOULD being the operative word here....)

Can you try your setup elsewhere in the house, or another place entirely?

I'm still waiting to hear what happens each time you add devices to the signal chain, starting with the power amp. I really suspect there's something odd going on with an individual device's power supply or grounding scheme vs everything else. The buzz in the 400-500 range would likely be 240 doubled, or a multiple of 120 hz, definitely in the power-supply range of filtering. The other noise.....weird. Possibly an open cap or something in a circuit that's misbehaving?

The trick is to find out which device is the one picking it up, and finding out if the device is defective, or just prone to noise, and if the latter, is the noise unique to your space alone?

Posted: Fri Jul 18, 2008 3:41 pm

I'll tell you most of that sounds like ambient room noise from HVAC.

One of the things I've used is to simply rotate the amplifier or rotate your guitar. There are numerous electromagnetic fields all over. What you need to do is to rotate either device to null out the offending noise. This is not unusual in other studios as well including NBC in Washington DC.

If you want cleaner power, I'd recommend a power isolation transformer from Signal Transformer Corp. and/or the EQUITECH balanced power conditioners. I mean you don't like static that much do you??

Remy likes silence
Ms. Remy Ann David

Posted: Sat Jul 19, 2008 6:40 am

RemyRAD wrote:

I mean you don't like static that much do you??

Adorable.

I now realize my initial post was unclear: in the above sample, the only elements are --

Soundelux U195 [mic] ---> Summit Audio 2BA-221 [preamp] ---> Mackie Onyx 400f [computer interface]

All the noise is coming from mic gain and balanced cables.

Posted: Sat Jul 19, 2008 12:18 pm

And your computer? I have found many computers, including laptops, which utilize a grounded power supply to the AC mains are frequently the culprit. Sticking on a 3 into 2 AC adapter can usually clear the problem. The problem seems most apparent on laptops but is not exclusive to those.

It's definitely pure 60 hertz so it's a ground loop. I don't hear any other harmonics other than the rooms own HVAC. Which I think is also a big problem here? You can hopefully, turn it off while you're recording? I find that more unacceptable than the 60 hertz. 60 hertz at least can be easily dealt with while HVAC noise is broadband brown noise.

Feeling in the pink
Ms. Remy Ann David

Posted: Sun Jul 20, 2008 2:53 am

Thanks, Remy.

Posted: Sun Jul 20, 2008 4:06 am

It probably doesn't need to be said, but if you are using a laptop, the easy way to check this is to run on battery for a short test.

Remy's suggestion of bypassing the path to ground through the AC cord makes me nervous even though the problem means that there is another path to ground. I think there is an old thread about safer (though more expensive) ways of getting around a computer ground loop.

Posted: Tue Jul 22, 2008 9:55 am

This:
http://www.palmergear.com/pli04.shtml
Was recommended to me for the dirty ground loop my laptop creates on playback.
Haven't used it though. If you get something similar, let me know how well it works for you.

Posted: Wed Jul 23, 2008 1:39 am

Thanks, guys. I use my desktop for audio applications.

I'll let you all know what I end up buying and whether it works.

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