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radiosmuck 27th November 2012 03:28 PM

FM Interference
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

Frank Berry 27th November 2012 03:31 PM

You might use an external FM antenna and locate it away from the interference zone.

DF96 27th November 2012 03:33 PM

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.

radiosmuck 27th November 2012 04:38 PM

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

DF96 27th November 2012 05:21 PM

A clamp-on ferrite choke on each speaker lead might help.

Speedskater 27th November 2012 10:53 PM

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.

For more of his papers:

Audio Systems Group, Inc. Publications
Audio Systems Group, Inc. Home Page

radiosmuck 30th November 2012 02:32 PM

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.

DF96 30th November 2012 03:16 PM

You didn't Google hard enough. There is information on the web about curing RF interference.

JustDoIt 30th November 2012 10:56 PM

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.

DF96 1st December 2012 11:22 AM

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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