AM Radio Interfence in my speakers

[johnnynine]

I have some AM radio antennas a mile or so from my home and during certain parts of the day I can hear the radio station in my subwoofer and in my computer speakers. I can also slightly hear it in my phone line which is not that big of a deal.

What is a big deal is that although I do not notice it in my home theater front or surround speakers, it is very obvious in my powered subwoofer. This makes watching a movie with the subwoofer on almost impossible.

And it is even worse in my pc speakers. I can have no sound coming from my pc and turn the volume on the pc speakers all the way up and hear the radio station as if I had actually tuned an am radio into it.

I have tried ferrite cores on some of the cables with no luck.

Any other ideas besides complaining to the radio station/fcc?

 

[WayneW]

It sounds like your line level cables are picking up the AM signal. You could try replacing the cables with ones with better shielding. You can also try asking the radio station for assistance. I seem to recall that they are obligated by the FCC to help reduce interference. But either way, if you ask nicely, they would probably help.

 

[johnnynine]

It sounds like your line level cables are picking up the AM signal. You could try replacing the cables with ones with better shielding.

The line level cables on my pc are cheap and could be replaced, but the subwoofer is using in-wall RG6 or RG59 which I would think would be pretty good (it's run is about 20ft).

 

[PaulH]

You could try ferrite supressors, those 'lumps' that you find on AC and Video cables. RadioShack used to sell a split one in a snap-close case. try putting one or more turns of the speaker cables thru the supressor as close to the amplifier as you can. What happens is the speaker leads act as an antenna, and the signal is feed back thru the amp, rectified and amplified. The ferrite will absorb the signal, kind of like a RF sponge.

Otherwise you may need a capacitor across the speaker leads to bypass the RF component of the signal.

Hope this helps,

Paul H

 

[kclark]

it is very obvious in my powered subwoofer

no sound coming from my pc and turn the volume on the pc speakers all the way up and hear the radio station

I think the common thread here is powered speakers. My bet is that your feeder lines and house wiring is a big antenna funneling the signal in through your speaker power. I'd try a filter on those power feeds.

One other dead giveaway - Do you ever hear the radio station through your washer or dryer? (sorry, couldn't resist)

Ken

 

[johnnynine]

Thanks guys, I have already tried ferrite cores on the wires with no luck. The wiring in the home is definitely acting as an antenna, but I'm looking for what I can use to filter out the RFI?

 

[mdesmarais]

Did you try the ferrites near the speakers or near the source?

I think Paul has it right.

To verify, you can try shortening the PC speaker wires- roll them up and put the speakers right near the PC. If they stop picking up, then that is the problem for sure.

Is the cable in the wall quad shield copper braid? make sure the contacts are good at both ends. Is the sub amp grounded?

 

[johnnynine]

Did you try the ferrites near the speakers or near the source?

I tried them in most every location (not at the same time though). I did try it at the speakers, but since the amp is located outside the pc speakers I didn't expect that to do anything. The RFI is already in the wire before it is amplified.

Is the cable in the wall quad shield copper braid? make sure the contacts are good at both ends. Is the sub amp grounded?

I'll have to check. It's the same wire they used for all the coax connection in the home... sat tv, cable to rooms, etc.

I got the ferrite cores at Radio Shack and I think they just don't or can't filter out AM interference.

 

[kclark]

So, when you tried ferrites on the power lines, did you get any reduction at all?

This is the kind of AC line RFI filter I was thinking might work. Radio Shack Surge Protector You don't need the surge protector part but this one has up to 40db noise reduction in the 100kHz to 100 MHz band. There might be better ones out there, but it's hard to get specs on the RF filtering ability of surge suppressors.

At $17 and a trip to RS it might be worth a try. If it doesn't work, it goes back.

Ken

 

[huggy59]

Radio stations are obligated to reduce/eliminate interference as much as practically possible. Ask nicely if they can help and tell them the nature of your problem. However, realize that you will probably not be able to eliminate the problem everywhere or completely and in the end, the radio station and FCC balances the "public good" done by the station vs. the few interference issues they can't resolve.

I'll bet the circuitry is the offending item in your amps. Try re-orienting them (turning or flipping) to find an orientation that reduces the interference as much as possible. Short of building a complete Faraday cage (a grounded metal shield all the way around the amp - or your home!), I doubt you'll get rid of all of the invasive signal. You are just too close to the transmitters.

If you can find the insertion point - where the signal is getting into your amps - you can usually make a tunable RF trap to further reduce the problem. Trouble is, if the circuitry is the problem, you may not be able to eliminate it without replacing the unit with a better-shielded system.

Good luck though, and let us know how it turns out!

 

[johnnynine]

A technician from the radio station came out to the house and took a look at the telephone, computer speakers and ht powered subwoofer.

The biggest culprit for the RFI to audio conversion in the telephone line was a Direct TV HD box. He put a filter (presumably ferrite) at it and it reduced the audible radio station in the lines some, but not completely.

The shielded coax line-level runs for the computer speakers and subwoofer are acting as antennas and picking up the RFI. The technician said he will make some new cables to run between the components and put unbalanced/balanced converters at the ends. He hopes that will fix it, I'll know later this week.

HOWEVER, the builder ran the subwoofer coax inside the walls and replacing it would require replacing drywall.

He said cat5 should have been run from the stereo to the subwoofer instead of coax and that the shielded coax cable provided a great AM loop antenna for the subwoofer.

edit: grammer

 

[Smarty]

He said cat5 should have been run from the stereo to the subwoofer instead of coax and that the shielded coax cable provided a great AM loop antenna for the subwoofer.

I am confused.... How would cat 5 hook up to your sub woofer?

This topic is timely, I just ran my sub woofer coax last night (new construction).

 

[johnnynine]

He said cat5 should have been run from the stereo to the subwoofer instead of coax and that the shielded coax cable provided a great AM loop antenna for the subwoofer.

I am confused.... How would cat 5 hook up to your sub woofer?

This topic is timely, I just ran my sub woofer coax last night (new construction).

A couple ways... balanced and unbalanced.

For unbalanced you would just split the wires into two sets (one ground and one line in). He claimed that it seems counter intuitive, but the tight winding of the wire actually resists RFI better than shielded wire (I have confirmed this elsewhere) however it is more prone to other forms of interference.

For balanced you would need a balun or something to convert from unbalanced to balanced and then back to unbalanced line level.

 

[johnnynine]

Resolution at last! :unsure:

The radio station gave me an Ebtech Hum Illiminator (plus we shorted the ground so it wasn't floating). That fixed the AM radio frequency interference in the sub woofer.

For my pc we simply replaced the cables. I found that the cheap cables I bought at MonoPrice.com did not have any shielding in them (I cut them open to verify this).

In any case the radio station tech made several trips to my home to work on the problem and I was not charged anything. Quite amazing!

 

[huggy59]

Congrats! And thanks for posting all the solutions! I'm sure they'll help others.