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Old 27th November 2012, 03:28 PM   #1
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Default FM Interference

Hi,
I have replaced my SS amp with an off the shelf T2020 amp which I love.
I listen mainly to internet radio but would like to have the option of FM.
It appears that the D amp causes interference to the FM signal, I at first, thought it was my antenna, I shifted it around but no luck.
I have a portable all wave radio with stereo FM that works well with the telescopic antenna. With the radio placed next to the unpowered amp the signal is clean but when the amp is powered up the radio is un-listenable. There was no connection between amp and portable.
Moving the portable around while listening to headphones, I needed to be about 5 ft away from the the amp for a clear reception.
Is there a remedy besides moving the tuner out of my cabinet?
Thanks, Terry
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Old 27th November 2012, 03:31 PM   #2
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You might use an external FM antenna and locate it away from the interference zone.
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Old 27th November 2012, 03:33 PM   #3
DF96 is online now DF96  England
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Fix the Class D output filter. It may be that it was designed/built by someone who doesn't understand RF so what he thinks is a ground connection is actually a VHF loop antenna.
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Old 27th November 2012, 04:38 PM   #4
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Thanks,
I have 2 amps, a Lepai 2020A+ and a 2020 SMSL and they are both problematic.
The antenna I'm using is just a 300 Ohm pin to the wall job, I guess I'll need a 75 Ohm down lead?
I would be interested to know if any of the Lepai modified amps suffer from interference after the changes?
Thanks again
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Old 27th November 2012, 05:21 PM   #5
DF96 is online now DF96  England
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A clamp-on ferrite choke on each speaker lead might help.
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Old 27th November 2012, 10:53 PM   #6
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Jim Brown has several papers and Power Points on interference.

Start with:

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

For more of his papers:

Audio Systems Group, Inc. Publications
Audio Systems Group, Inc. Home Page
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Old 30th November 2012, 02:32 PM   #7
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I did a little Googling and did not find a definative fix for FM interference.
This might prove interesting Switching Amplifier (Class D) Basics — Reviews and News from Audioholics
I think I will stick with internet radio.
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Old 30th November 2012, 03:16 PM   #8
DF96 is online now DF96  England
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You didn't Google hard enough. There is information on the web about curing RF interference.
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Old 30th November 2012, 10:56 PM   #9
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Curing RF interference from legally transmitted signals is one thing, radiating RF from an audio amplifier or other device not licensed as an RF emitter is something else again. The manufacturers have a duty to comply with Part 15 of the FCC rules regarding these emissions. If people are experiencing RFI on the FM band, I have to wonder what the situation is with the lower frequencies - HF instead of VHF. The emissions should be fixed at the source.
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Old 1st December 2012, 11:22 AM   #10
DF96 is online now DF96  England
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EMC rules are poorly enforced by many administrations. I think they regard them as an inhibition to trade and enterprise, instead of protection from electromagnetic vandalism.

HF may be better or worse than VHF. It depends on signal rise times, and the exact details of filter implementation.
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