FM Interference

[radiosmuck]

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

 

[Frank Berry]

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

 

[DF96]

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]

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

 

[DF96]

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

 

[Speedskater]

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

 

[radiosmuck]

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]

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

 

[JustDoIt]

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]

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.

 

[JustDoIt]

I'm afraid the EMC rules aren't enforced by anyone, ever. Those of us who are involved in HF radio know all about the noise generated by switching power supplies in wall-warts, low-voltage halogen supplies, variable speed furnace fan controllers, etc. But, I don't expect to have to deal with the problem in high-end audio devices.

 

[analog_sa]

All class D amps leak the free-running frequency at disturbing levels - hundreds of millivolts seems to be the norm. Actively driven speakers with ultrashort speaker connections are probably a good solution but 5m cables are bound to radiate quite a bit. I have noticed that my phono setup sounds significantly worse with a class D amp operating near by.

 

[radiosmuck]

All class D amps leak the free-running frequency at disturbing levels - hundreds of millivolts seems to be the norm. Actively driven speakers with ultrashort speaker connections are probably a good solution but 5m cables are bound to radiate quite a bit. I have noticed that my phono setup sounds significantly worse with a class D amp operating near by.

Thanks,
cd, minidisc and internet radio are absolutely quiet, it's just an analog fm signal that is unlistenable.
After reading your reply, I connected the fm radio to a pair of self powered D-Box speakers, these look to be Minimus 7's with a D amp in one box. The tuner sounded great, even though there is a speaker cable running from one box to the unpowered box.
It must be either the chip or the design, the chip in the D-Box is not a 2020

 

[dmills]

Its mostly the design of the output filter, together with managing the switching current in the input supply wiring appropriately (Something that is all too often missed).

Really the common single stage L/C lowpass does not cut it, especially as the cap is usually so large as to have unfortunate series resonances at RF. 4th or 6th order is better, and in all cases you really need an additional filter designed to catch the VHF harmonics (The main reconstruction filter inductor(s) always looks capacitive up there).

Current and voltage probes hooked up to a halfway decent SA can be very revealing.

Regards, Dan.

 

[radiosmuck]

I have just connected a small CV transformer before my dc power supply and the RF noise has cleared up. There is a slight hiss but this is just a reception problem with the telescopic antenna.
After listening to 128k internet radio for a few months, FM does not sound that good anymore.
The cvt is only .127 amp which gives me only about 1 amp @ 14v dc yet it drives my big old speakers, no problem!

 

[UltraNutZ]

I have just connected a small CV transformer before my dc power supply and the RF noise has cleared up. There is a slight hiss but this is just a reception problem with the telescopic antenna.
After listening to 128k internet radio for a few months, FM does not sound that good anymore.
The cvt is only .127 amp which gives me only about 1 amp @ 14v dc yet it drives my big old speakers, no problem!

I know this is an old thread and pertains to something other than car audio but man we're having fits with EMI and FM interference with these Class D car amps.. Radiosmuck, can you elaborate a little more sir on the CV transformer you're talking about here (a possible link to one) and do you think it will help with the interference we're experiencing with these car audio Class D amps?

Thanks,
UN

 

[radiosmuck]

I know this is an old thread and pertains to something other than car audio but man we're having fits with EMI and FM interference with these Class D car amps.. Radiosmuck, can you elaborate a little more sir on the CV transformer you're talking about here (a possible link to one) and do you think it will help with the interference we're experiencing with these car audio Class D amps?

Thanks,
UN

Sorry, it did not really work for me, the enthusiasm was premature.
I'm now using a very inexpensive TDA7297, it sounds, for me, better than the 2020 and has no FM issues.
Being an enthusiastic embiber of the sparkling beverage I just felt the need. Very hypnotic, I find.

 

[nigelwright7557]

SMPS designers shield RF by putting an earthed screen around the PCB.

A class D amp is similar with high power inductors.