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Old 17th August 2013, 04:13 PM   #1
M Gregg is offline M Gregg  United Kingdom
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Default RFI and speaker cables?

Simple question,

Has anyone tried RFI suppression on speaker cables or speaker output from the Amp?

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M. Gregg
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Old 17th August 2013, 04:17 PM   #2
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I can't think of an amplifier with RFI in the loudspeaker connections. What would the purpose be?
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Old 17th August 2013, 04:19 PM   #3
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I have tried typical clip-on type ferrite filters on speaker, and/or signal, and/or AC power cables.
IME adds an odd harshness that don't belong.

Dan.
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Old 17th August 2013, 04:24 PM   #4
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by JonSnell Electronic View Post
I can't think of an amplifier with RFI in the loudspeaker connections. What would the purpose be?
The feedback circuit.

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Old 17th August 2013, 04:28 PM   #5
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Surely the speaker cable, typically 100pF per metre will stop any RF before it gets in.
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Old 17th August 2013, 05:05 PM   #6
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Well no, speaker cables along with power cables, control cables and interconnects make good RFI antennas. Once inside the chassis, the circuit looks very different to radio frequency RFI than to the audio frequency schematic printed in the manual.

Member Bob Cordell covers this in his big book 'Designing Audio Power Amplifiers'.

Jim Brown covers this in his paper:

A Ham's Guide to RFI, Ferrites, Baluns, and Audio Interfacing
Revision 5a 5 Jun 2010
by Jim Brown K9YC
Audio Systems Group, Inc.
http://www.audiosystemsgroup.com/RFI-Ham.pdf

Don't let the title fool you, this is just a new edition of a paper with aditional information for Ham's

***************************************
All unshielded multi-conductor cables should be twisted to reduce RFI pick-up.
***************************************
Shielded interconnects (balanced & unbalanced) depend on corrected mounted.connected chassis jacks to reduce RFI.
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Last edited by Speedskater; 17th August 2013 at 05:08 PM.
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Old 17th August 2013, 05:09 PM   #7
jcx is offline jcx  United States
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I have read of AM radio from nearby commercial transmitters being heard in speakers - with the amp power off

as often the longer wires (after the mains power wiring - in conduit?) in the stereo setup speaker cables may be of concern

EMI could couple differential and common mode

differential mode coupling may be reduced by twisting - but you have no control over what happens in the speaker - what are the XO air core inductor SRF?

clamp-on ferrites over speaker cable both strands together only affect common mode couping - and often only by single digit dB

http://www.elmac.co.uk/pdfs/ferrite.pdf
http://www.arvelo.net/N2MZZ/graphics...acing-k9yc.pdf - yes the same as above - it gets around...
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Old 17th August 2013, 05:50 PM   #8
raul_77 is offline raul_77  Europe
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Some about the topic:

Capacitor Sounds, Speaker Cables and Crossover Inductors.

Regards
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Old 17th August 2013, 06:40 PM   #9
brig001 is offline brig001  United Kingdom
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I always thought that the Zobel and output choke were more about RFI than capacitive loads etc. or maybe it was just a side benefit. My new build will have both as I have an interference problem with my current system and want to get rid of it - it seems to be coming from a neighbouring apartment.

Brian
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Old 17th August 2013, 09:48 PM   #10
DF96 is offline DF96  England
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The first time I saw this issue mentioned was back in the early/mid 1970's when an update to the PW Texan amp was described. It included an RF rolloff filter in the feedback network, so RF picked up on the speaker leads would not cause the 741 opamps to get confused. It crops up again from time to time, but then seems to be ignored for a few years before reappearing. Maybe each new generation has to 'discover' it?
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