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Fosgate3

Copper - Posts: 328

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Joined: January 29, 2004

Location: Louisiana, United States

Posted: July 23, 2007 at 9:08 PM / IP Logged

I think I know the answer to this but I'm going to ask anyway: Last night, while surfing ebay, I found someone selling "clamp on ferrite noise filters" as what you would find on the vga cable of a computer monitor or even clamped onto the power cable of some power adapters. I was wondering if these things have any effect on a mobile system on any type of noise. just a question of curiosity...

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haemphyst

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Joined: January 19, 2003

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Posted: July 23, 2007 at 9:46 PM / IP Logged

The purpose of a ferrite filter is to increase the inductance of a conductor. The desire to increase the inductance is to filter super-high frequencies (translation: digital) from the cable.

As far as I am concerned one would never want to increase the inductance of an audio cable, as this can only hurt the signal traveling through it. Most manufacturers of audio cables work their collective asses off to LOWER inductance in a cable, to improve hi-frequency performance...

Also, if you can hear the "noise" (which is what I am assuming these are sold for) it is too low to be affected by such a small increse in inductance, anyway. If it COULD be affected by the small addition of inductance, then any frequency ABOVE the noise frequency would ALSO be filtered, i.e. voices, snare drums, cymbals... you name it...

Kind of counterproductive AND counterintuitive, wouldn't you say?

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

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Fosgate3

Copper - Posts: 328

Joined: January 29, 2004

Location: Louisiana, United States

Posted: July 23, 2007 at 10:03 PM / IP Logged

great answer and after that explanation, I would agree with the comment on it being counterproductive and counterintuitive. Thanks for the info.

also out of curiosity, what makes up a typical noise filter such as the PAC SNI-1? I saw a few of these too but a different version than what I've been accustomed to for the last decade: instead of the typical SNI-1 that has RCA in and out, these were designed for the small 1/8" audio jack found on mp3 players. There were some that were generic but some allegedly produced by PAC. I havent torn an SNI apart... I dont have one lying around and when I do, I'm not thinking about how it works but rather in a troubleshooting mode or quick fix mode or something else other than being inquisitive. So how do they work?

thanks again for the other answer! that was a great explanation (I thought).

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haemphyst

Platinum - Posts: 5,054

Joined: January 19, 2003

Location: Michigan, Bouvet Island

Posted: July 23, 2007 at 10:10 PM / IP Logged

Several types of noise filters available. Power supply or signal.

If it's an active type, and for a power supply, I honestly don't know how they work, but passives are simple RLC networks, with a capacitor or two in parallel to/with the load, and a choke in series with the load.

A signal type noise filter, or a ground loop isolator, is just a 1:1 audio transformer, wound on a ferrite core, or I have seen air-core versions as well. The function of these is to simply break the signal's shield, so no current can flow over it, and induce noise. The signal then references the RCA gounds at both ends of the car, without a common point of reference.

It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."

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ferrite filters?

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