Use of Ferrite Cores for EMI Reduction???

I was getting taxi cab and cell phone calls through my speakers. I solved the problem by using rf stoppers on the interconnects from my pre amp to amp. The heavy duty ones 2 per interconnect.

be careful what you do with those ferrite clamps. I clamped 1ea. on a pair of Audioquest Lapis interconnects; they killed the highs & PRaT was all but gone. Sold the whole pack forthwith.

Bob, out of curiosity, where was this cable located at within the confines of your system ? I have found that ferrite works best on digital based products. In specific, i'm talking about the power cords of transports, dac's, cd players, digital tuners, digital amps, etc... Like anything else though, try things in moderation. You CAN get "too much of a good thing" if you try ganging up multiple ferrite beads or clamps.

As many of you know, digital components are VERY noisy devices. Since they can pump their digital "hash" back into the power lines and other components, placing ferrites on the power cords at the component chassis helps to minimize this. Besides reducing the potential for the "dirt" to re-enter the AC system, this also reduces the chance of the device using the power cord as an "antenna" to re-radiate the rf based digital signal. This also works in reverse, as rfi is less apt to enter the device via the "power cord antenna" due to the impedance bump that the ferrite creates.

One thing that we have to make sure of when using ferrite based filters is that the cable remains centered in the ferrite choke. If the ferrite actually rests against or is closer to one section of the cable than another, it can produce an erratic filtering action. Not only does this reduce the efficiency of the ferrite as a filter, it can produce various loading conditions on the different polarities of the signal due to proximity effect. I would "assume" that this could produce a slightly "disjointed" or "un-natural" effect to music, possibly resulting in the lack of PRAT and tonal balance that Bob mentioned. This is not to say that this was what caused the problems he noted in his specific situation, only that it could be a possibility.

Since many ferrite chokes or clamps fit loosely over some cables, a ( cheap and dirty ) solution is to fill in the gap between the cable and ferrite with something that is "non-offensive". While the use of a paper towel cut into strips and then wrapped around the cable comes to mind, anything that is non-metalic or prone to static build-up should work fine. With the paper towel, etc... you can make as many turns necessary to take up the needed space. The clamp is then applied over the "spacer".

Not only can this keep the clamp more secured and keep it from sliding around on the cable, it should keep the gap between the ferrite core and the cable pretty evenly spaced. This is not a "test proven method" by any means but something that came to mind. If others have found better solution to this problem, PLEASE contribute your suggestions as you see fit. Sean
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Bob, I already got the Highwire Wrap. It's very good, especially for the price. The improvements are just what Bluenose posted.

Those RF Stoppers were used on a much different rig than my present setup. I was having a lot of RFI problems from trucks on a nearby highway; I'm surprised that my amp or speakers didn't blow up when some of those clowns went by with their (non)linear amps broadcasting simultaneously on all 40 channels. Their $hi! was supersaturating an older preamp, then of course the hash was amplified by my PA. I put those RF stoppers on the analog interconnects driving the PA; I clamped them on the cables within a few inches of the amp. The RFI was indeed substantially reduced to the point of being tolerable, but they just ruined the sound, & these were just the 'junior model' RF Stoppers. They do have spacers on the inner diameters of the clamps in order to help avoid the aforementioned assymetrical positioning problem. I used some electrical tape for that purpose but I do like the wrapped-paper idea because then the clamp can be moved up or down the cable for positional optimizating.
I've seen ferrite cores installed on power supply cables & AC cords (the Audioquest AC12) which maybe makes a little more sense, but when used on analog interconnects they were awful.
Respected designer Charles Hansen (of Ayre Acoustics) actually told me that when Ayre was experimenting with ferrite AC filtering, they found that the initially realized improvements were degrading over the long term due to a "magnetic buildup" within the cores, which had to be degaussed periodically. He also mentioned that ferrite was causing some sort of 'grunge effect' on the sound, another reason that they didn't like it.
Francisco thx for the Powerwrap report; I need to order one to play with but I'm hoping to find an alternate source vs. the near-north retailer that sells them in our area. I still have a bad taste in my mouth leftover from my latest dealing there; it's not a BFD but next time I'm going elsewhere if possible. Does the package show the manufacturer's name & address? Please advise if you have this info? thx