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Old 18th January 2010, 07:38 PM   #1
kgb_m3 is offline kgb_m3  United States
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Default Radio frequency interference

What do you guys suggest I do to rid my amp of radio frequency interference. I followed this build: Mick Feuerbacher Audio Projects and without any source connected, I'm able to hear slight jazz coming though. Gain on my particular 3886 was set to 20 so it's not too loud but still extremely annoying.
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Old 18th January 2010, 07:42 PM   #2
Glowbug is offline Glowbug  United States
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Does it happen with any amp, or just that one?

Maybe ferrite beads over cable runs internally?
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Old 18th January 2010, 07:49 PM   #3
kgb_m3 is offline kgb_m3  United States
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Any amp, all my school projects did it, lm386 headphone amp does it, my old logitech setup does it. Don't have any experience with ferrite beads, thanks I'll look into those...
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Old 18th January 2010, 07:52 PM   #4
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Interferences mostly origin from AM radio stations. These become audible, i.e. are demodulated due to nonlinear behaviour of the semiconductors inside your amp combined with its small bandwith compared to radio frequencies.
And I experienced these effects are not restricted to input terminals of the amp but might take place at output terminals, too.
As you wrote that your input is shorted, try to remove the antenna connected to the outputs: shorten your speaker cable and / or insert
a choke of 10~22uH in series, placed close to the output pin of the IC.
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Old 18th January 2010, 08:22 PM   #5
kgb_m3 is offline kgb_m3  United States
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Thanks for the advice, so I would just run one inductor in series with the output from 10-22uH? or run 2 of them?
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Old 18th January 2010, 08:45 PM   #6
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For a single ended output, it takes only one choke on the "hot"
terminal. For a bridged output you need 2 chokes for both outputs.
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Old 19th January 2010, 04:18 AM   #7
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try some bypass caps to ground .1uf or .01uf maybe smaller on signal and power rails to chassis grnd it is been awhile but i stopped rf from getting into fostex 450 mixer using this tech niqe and was able to use with a hi power cb radio station i had to do the same with the effects rack before they would work also good luck i hope this info was of use
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Old 19th January 2010, 04:22 AM   #8
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f.y.i. the smaller caps will short the rf to grnd without effecting the audio
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Old 19th January 2010, 04:41 AM   #9
kgb_m3 is offline kgb_m3  United States
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Where do I put these caps guys? I have my signal and power ground stars separated so I'm a little confused as to what grounds bypass to ground.
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Old 19th January 2010, 09:47 AM   #10
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You might try using a 300pf cap from input to ground. To see an example of this look
at the A40 amp on Passdiy.com.
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