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Old 12th October 2004, 10:00 AM   #1
djdamix is offline djdamix  France
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Default Gsm and RF rejection ...


I'm looking for a good solution to prevent RF and GSM detection with my amplifier, I now it could be done by filtering the input.
The input is a basic differential pair as on the following file :

ps: there is no more DC offset ...

Any advice is welcome.

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Old 13th October 2004, 08:22 AM   #2
djdamix is offline djdamix  France
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was it a stupid question ? is case of the amplifier supposed to stop all the RF ?

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Old 13th October 2004, 08:54 AM   #3
boholm is offline boholm  Denmark
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The problem is that RF can get in anywhere - not just on the input. Also the wires to the speaker are excellent antennas for RF. So if you have a problem with RF you can try the old trick, which is to do one winding round a ferrite core ring or using one of these: . . .wel . . . please do a search on Farnell using this search word "535886". Then you will get the picture

Ohterwise you can make an input filter. I recommend using 1 MHz as cutoff, so you will not interfere with the audio signal too much - fase and amplitude wise.
Best regards
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Old 13th October 2004, 08:56 AM   #4
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Good RF immunity comes also with a good pcb with groundplane but as a start and good decoupling and a LP-filter at the input.
/Per-Anders (my first name) or P-A as my friends call me
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Old 13th October 2004, 09:01 AM   #5
djk is online now djk
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You need to add R2 and C1 to your input stage:

R2 - 2K

C1 - 390 pF mica (Arco DM15-391J or DigiKey 338-1057-ND

A ceramic cap may be used if nothing else is available.
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Old 13th October 2004, 09:11 AM   #6
djdamix is offline djdamix  France
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thanks, I'll try that, I hope my GSM phone won't "sing" in the loudspeaker anymore.
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