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I have a budget (not cheap :P) pc-speaker set from Logitech (2.1) at home which are extremely sensitive to cellsignal interference. My iPhone is about 1m from any part of the set - including the wiring - but still i get the funky bzzzzz-bap-bap-bap-bzzzz-bap-bap every few minutes. (I guess it's when the iPhone "calls home" to the tower.)

I have another set at work, which were even cheaper, which don't have this problem. And here I have my phone docked just a few inches away from the controlling speaker. (2.0 system)

Is there anything I can do to the set to make it more shielded? I'm not afraid to open it up ;)

http://www.logitech.com/en-ca/support-downloads/downloads/speakers-audio/devices/232 <- this is the set I have. There is also a very audible ground-hum that I want to get rid of. Any tips there?

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  • \$\begingroup\$ Consumer electronics questions are off-topic here. \$\endgroup\$
    – Kellenjb
    Oct 18, 2011 at 18:35
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    \$\begingroup\$ @Kellenjb: This is a question about electronics design. Pay attention to the meaning of the rules and not just their wording. \$\endgroup\$
    – endolith
    Oct 19, 2011 at 0:42
  • \$\begingroup\$ @endolith It is more important to pay attention to the intent of the question. To me there is no intent to design here and it looks like at least 3 others agree. It could be turned into an on-topic question, but as it is now it is not. It would be nice to see some intent of wanting to know why this is happening and understand the theory behind a fix instead of just asking for the fix itself. \$\endgroup\$
    – Kellenjb
    Oct 19, 2011 at 12:51
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    \$\begingroup\$ @Kellenjb: The prohibition against "consumer electronics" is for questions like "which iPod should I buy". Designing circuits for consumer electronics is on-topic. Shielding and RFI are on-topic. Asking how to modify a circuit to eliminate interference is totally, 100% on-topic. \$\endgroup\$
    – endolith
    Oct 19, 2011 at 13:37

1 Answer 1

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It sounds like the price was more a function of the brand name than good design.

There are two possibilities: The noise is coming in via the audio cable or into the circuitry of the amplifier directly over the air.

The first is relatively easy to deal with. Try putting a 100pF or so ceramic cap accross the audio leads going into the speaker system as close to the case as possible. Check the cap datasheet and choose one with as high a useable frequency range as possible. If RF from the cellphone is getting in via the cable, then this can get rectified and otherwise partially demodulated by circuitry designed to handle five orders of magnitude lower frequencies. The 100pF cap should shunt most of the RF before it can cause trouble. You'll need one such cap on each audio feed, which is two of them if this is a stereo system.

If the capacitors doesn't help, then it gets more complicated since the circuitry in the box is apparently picking up the cellphone signal directly. To verify this, try wrapping the speaker box in alumninum foil that is connected to ground at the audio input jack. Only connect it at that one place. If that's what is required to fix the problem, then you'll have to add a shield somehow. You can keep the external aluminum foil if you don't care what it looks like. Otherwise you will have to open the box and add your own shield internally.

Or, you could sell the Logitech speakers and buy a cheaper pair that works better and even get some money back in the process.

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