Reducing noise (60hz electrical noise) when recording?

[Red Squirrel]

At our church we have a basic setup with mic, sound board, and computer to record the speeches. We also have 4 big spot lights that use a dimmer and it creates lot of noise on the electrical lines or so it seems. Depending on how they are dimmed it varies. The middle seems to be worse. The lights plug through a custom dimmer switch board which then plugs into a stove plug and uses both hots with the neutral for basically two 120v circuits.

I'm wondering what I can do to reduce the noise created by this. Would plugging all the sound equipment in an isolation transformer help? Where would I buy one of those anyway?

Here is a sample of what I mean:

http://gbcspeeches.iceteks.net/nov-22-09/Peter Kerr - Nov 16 09 am.mp3

 

[sdifox]

http://www.tequipment.net/BK1604A.html

and if you are really crazy

http://www.isobox.com/?source=google&gclid=CP6_l9SUoJ4CFQ8eDQodfUIwmA

 

[Modelworks]

Listening to the clip it sounds like either something is not grounded on the input side sound mixer or input to the pc or you have a ground loop .

I would start by trying to find out where it is entering the system. Start unplugging equipment connected to the audio from the AC outlet one item at a time till it stops. If everything is unplugged from the input and you still have the noise then you might need an isolation transformer.

The tricky thing is that it could be RFI noise from the dimmer switching on and off of the triac inside it. A transformer will not help much with that.

Isolation transformers can get very expensive depending on how much wattage you need, ebay has some listed in the 4 amp range for under $100 . You just need to find the exact source of how it enters the system then either use a transformer or a filter.

 

[Red Squirrel]

Hmm I was afraid it may be RFI. I'd have to experiment with the dimmers further away from the equipment. I'm pretty sure those dimmers are the culpit. If they're off, I don't get any noise. So it's either noise being generated on the line (ground loop or other issue) or it's RFI. If it is RFI is there a way I can somehow shield the dimmer box? Or would it require something crazy like lead to do that? I remember at home I used to have a CRT monitor that would generate a 60hz sound if the dimmer in the dining room was about half way, so think it's a similar situation here.

I also have a small set of speakers hooked up to the PC and there is noise on those too regardless of if the sound system is on. The real time speaker hooked up directly to the sound system has no noise, so I'm actually thinking the noise is at the PC and not the sound system itself. Would a better sound card maybe solve all of this? Think it currently has a sound blaster audigy circa 2004. Nothing fancy.

If I take two doorbell type transformers and hook them up together (step down + step up) to create a sort of 1:1 transformer, would that be safe for a few minutes? I could rig that up to at least see if the noise is RFI or not by plugging the pc into it.

 

[Modelworks]

Doorbell transformers are usually around 24VAC at very low amps so it would burn out before you could do much of anything.

The problem with shielding the dimmer is it is probably traveling into the power line back through the wiring. Once the noise gets far enough down the wire, the wire becomes an antenna that broadcast the noise to everything around it. You really need to filter our the noise as close to the dimmer wiring as possible.

Do you have any ferrite chokes like these ?

placing those on the cord of the dimmer as close to it as possible might just take out the noise, that is why a lot of pc gear like monitors have them on the cord.