RFI from the speaker cables (well, from everywhere)

[fogpeople]

I sometimes do work for a fringe theatre here in Seattle. Their current location is RF Central, with three giant radio towers a block away, a police station across the street, etc. It's one of 3 spaces in the building and all the sound systems have some kind of radio noise issues (in particular NPR, which comes in crystal clear even over a powered down system).

Anyhow, their current sound system has ridiculous amounts of radio interference (and again, I mean actual audible radio stations). I don't have make/model info but it's mixer/amp with banana clip outs to left/right that run the length of the room (which is long) to some cobbled together cabs (no subs). Even with the amp all the way down but powered we get radio signals.

I ran a similiar setup but from the mixer out to powered speakers and got significantly less RFI, but still some. (the cabling here was grounded XLR).

In conversations about possible ways to attack this RFI problem it was suggested the shielding on the cables might be insufficient (not sure of the gauge/size off-hand) and that running it through conduit might be a solution. I don't have any experience with this as a RFI solution so I wanted to ping the experts and see if there was any thoughts about RFI not as a power/ground loop issue but as a signal cable issue.

Any help appreciated!
-m

[inflatable]

Make sure you know where the RFI is being picked up. It might not be the speaker cables. Check every piece of gear from input to output and then go from there. It is possible that it is being picked up at multiple locations in your signal path and thus is "amplified" even more once it gets to your speakers.

After you find out where it is being introduced, follow the standard shielding/balanced recipe and it should be solvable.

[TapeOpLarry]

It's much harder for RFI to "enter" a speaker cable. That cable is carrying a rather high voltage signal compared to MANY other places in the signal chain. Use a very systematic process of elimination here. Check common grounds, inside of gear, XLR connectors, snakes, etc.

[inflatable]

I just want to reiterate what Larry said:

Systematic.

Start with only 1 mic plugged into the board, running to one power amp and 1 speaker. If that hums, then you have already eliminated most of the possibilities.

It really does sound like a bad ground to me.

[drumsound]

I'm gonna send this over to Brian's tech forum.

[ashcat_lt]

you are right next to a transmitter, but I'm not sure even that would induce enough voltage into the speaker cables to move the cones without being amplified first. More likely, the amp is the source of the interference. The volume knobs turn down the input, but leave the amp in full gain?

[arkestra]

Im having a similar problem with an AM station showing up in a TAC scorpion. The rf does not show up when using external pres. It is only present when using the TAC pres. Bad grounding in the TAC? any suggestions?

[brianroth]

As has been already mentioned, break down the system into small parts and test for the existence of RFI. Begin with only one power amp and one speaker and build up from there.

As for the TAC desk, check to see if pin 1 of the mic XLR goes DIRECTLY to chassis with a very short piece of wire.

Bri

[cafewalter]

I sometimes do work for a fringe theatre here in Seattle. Their current location is RF Central [...] Even with the amp all the way down but powered we get radio signals.

Yeah, I live on the other hill in Seattle with three towers, about a block away. I feel your pain. Used to get Dr. Laura on the phone all the time, without dialing. See KING-5 TV on my TV, even when I'm watching DVDs.

To be sure it's coming in through the speaker wires, take a couple male XLR plugs and short pins 2 and 3 together. (Just the plug: no cable attached.) Plug those into the inputs of the amp. Or if the amp takes some other connector, like 1/4" TRS, do the equivalent: short tip to ring for TRS, or tip to sleeve for 1/4" unbalanced.

If you're still getting interference with the amp inputs shorted that way, then it's coming into the amp either through the AC cord or the speaker wires, and getting demodulated by the circuitry inside the amp.

If it's the speaker wires, you might be able to fix it by giving the speaker wires a couple turns around a big ferrite toroid choke, as close as you can get it to the amp. (I think you can still get these at Radio Shack, or if not, try Car Toys.)

If it's the AC, you might be able to fix it by plugging the amp into a power strip that has an RF filter - this is not the same as a surge protector.

If neither of those help, then you may be able to solve it by getting your amp repaired or modified. This is something that can be done by a good electronics tech. (Commercial plug: or by me.)

- walter harley
http://www.cafewalter.com