[art]

Shielding at AM radio frequencies is only effective if you have lots of metal, preferably copper. Best way to achieve that is with double-shielded cable. Not something that you are likely to find at your favorite high-end shop.

Most ferrites, i.e. the ones that you are most likely to get hold of, have little effect at those frequencies. But, if it is big enough, and you get enough turns, it may help some.

Pat

[stereocilia]

Maybe someone has mentioned this, but you might also try reversing your interconnects, one at a time and see what happens. I changed ics a few weeks back, got a terrible hum and a.m. radio problem, reversed the ics (in this case, when against the manufacturer's suggested directionality)  and the problem disappeared.

Thanks.  Even with the long interconnects (10 ft.) RFI is not audible until I use the long speaker cable (35 ft).  My goal was to have the amp along a wall away from the speakers, but now it's in between.  It's working pretty well for now.

[stereocilia]

Shielding at AM radio frequencies is only effective if you have lots of metal, preferably copper. Best way to achieve that is with double-shielded cable. Not something that you are likely to find at your favorite high-end shop.

Most ferrites, i.e. the ones that you are most likely to get hold of, have little effect at those frequencies. But, if it is big enough, and you get enough turns, it may help some.

Pat

Thanks.  I might be able to find that at nearby "you-do-it electronics," but I'd probably have to by a 500 ft. spool or something.  I did nab a couple ferrites which made no difference on the speaker cables but I wrapped the lines from the wall wart to the squeezebox through it a few times.  Seemed like a good idea.  It almost seems like there was an improvement in dynamics.  Almost.

[art]

If you are just trying to make a simple, common-mode choke, then powdered iron might work better than ferrite, unless you have the right ferrite.

Ferrite materials act as a big resistor, without having to actually insert any resistance in the line. But, each ferrite "mix" only is effective over a given frequency range. Most ferrites do not exhibit any substantial resistance, at frequencies under 10 MHz or so. That is how ferrite beads or donuts work: they absorb RF energy and turn it into heat. Common-mode chokes use opposing magnetic fields to cancel out the crud. Two different approaches, both can be effective.

If you have time, the guys who make these should have a wealth of info on how they work, and how to employ them. Companies that I would look at are Steward, Fair-Rite, and Ferronics.

Pat

[Dan Banquer]

Here's link that should help:
http://www.audioholics.com/education/cables/bulletproofing-your-system-from-interference

Regards;
     Dan Banquer

[Quiet Earth]

RFI is not audible until I use the long speaker cable (35 ft).

Whoa! I didn't realize they were that long. Yeah, you need to shorten things up a bit for starters.

Lots of good info on this thread. 

[stereocilia]

Thanks for all the links and info.  Yeah, 35 ft. is about 15 ft. more than I actually need for now, but many installations must have longer cables than that without RFI.  Fortunately, 10 ft. speaker cables are short enough to essentially eliminate the problem.  I'll chop down my long cable within the next few days and see what happens.

[art]

Someone sent me this link about ferrites, and controlling RFI. I have not thoroughly read it, but I see some pictures that show the effects of winding a mic cable around a BIG toroid core. Examples of at least 2 different mixes, that show how number of turns moves the frequency they are effective at.

http://audiosystemsgroup.com/Ferrites-Ham.pdf

Pat

[stereocilia]

   I'm getting educated here, this is great information!  I'm unlikely to be able to devote the energy needed to use and understand Faraday's equations and so forth.  I do have a reasonable working knowledge of math, science, and "electromagnetical stuff," but I would have a long way to go to bring myself up to speed in order to understand the whole picture.

The odd thing is that in lieu of cutting a perfectly good long twisted pair of 10 ga. speaker cable to try to reduce RFI, I dug out an 18 ft. pair of naim NACA 5 speaker cable.  I used the hypotenuses instead of the corners to make it reach, but it works.  No audible RFI.  What's odd is that this cable has widely-spaced conductors to account for the need for naim amplifiers to see a low capacitance/high inductance load, which I would think has the opposite effect of twisting the wires together.  So, does this high-inductance cable somehow reduce the RFI, or is it simply just that it's shorter?  Based on what I've read it's probably the latter, but it sure is curious.

[KDus]

it's the length.
We don't use metal tape measures at AM radio sites because they'll burn you. As you spool them out, they go in and out of resonance.

[stereocilia]

yikes!  holy brain cancer and three-eyed babies, batman.

I notice that at night I can hear the am radio with my ear right up in the speaker still, but not in the day.  I guess they light it up when the sun goes down.

[Quiet Earth]

I think some stations change their radiation patterns after the sun goes down. They might even lower the power when they do this.

[srb]

I notice that at night I can hear the am radio with my ear right up in the speaker still, but not in the day.  I guess they light it up when the sun goes down.

AM radio reception is usually better at night.  Has something to do with nightime atmospheric conditions and solar winds (or something).  As a young kid, I built a few crystal radios, and the time to listen to them was at night.
 
Steve

[KDus]

The field strength of 50kW at 4km is somewhere between .3 and .4 Volts. In radio, that is a huge signal. 15 meters is about 1/20 of a wavelegnth in the AM band. So, you could be picking up RF in tenths of volts. If even milivolts get past the front end, you could hear it.
If the night pattern added 3dB,or more, in your direction, the 50kW would have the effect of 100kW, or more.
Because it is a 3 tower array, it is unlikely that they reduce power at night.

[Niteshade]

I almost forgot about something:

http://www.mfjenterprises.com/Product.php?productid=MFJ-931

This will allow your earth ground to be more effective. I used one once and it worked great. If a ground wire is too long or if there is something odd about its setup, this will make it conduct RF. I would call the company first and see if it would help with your situation since AM radio might be too low a frequency for it.

[stereocilia]

Thanks.  That is a cool device, even if it doesn't help my particular setup.  Fortunately, the talk radio signal is mitigated by using the shorter speaker cables for now.  From the website: "The MFJ-931 Artificial RF Ground covers 1.8 - 30 MHz," which I think is outside the a.m. band (unless it isn't.)