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I apologize if this is not the right place to post this.

I am working on a small PA system for a weekly gathering. It catches the signal from wireless mics and output it to the speakers. I am pretty sure that the machine itself is not damaged.

Recently, I've found that there are very sharp digital noise coming out from the speakers and it is so annoying that it masks the voices and songs. I've done some investigation and found out that it could be caused by 4G cell phone signal. So I tried some experiments and the observations seem proved the theory.

I've learned from my online research that I can either ask everyone to turn off the 4G signal (which is unlikely since the population is around 100), or replace my PA system.

Is there another way out? Such as a 4G signal filter or jammer? If so, my concern is, will these filters and jammers also get rid of the signal from the wireless mics?

Thanks.

Update 1:

Thank you all for your advice. Unfortunately, I cannot get access to the equipment until this weekend. I'll mark the models of the gears and post them here then. I'll also try to upload a sample noise for your analysis.

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  • \$\begingroup\$ A sample recording of the noise would be interesting. \$\endgroup\$ Jun 23, 2014 at 8:57
  • \$\begingroup\$ @JamesCameron I'll upload it this weekend if we experience the interference again. Thanks. :-D \$\endgroup\$ Jun 23, 2014 at 9:14
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    \$\begingroup\$ Jammers are (1) almost certainly horribly illegal and (2) going to cause more interference and hence PA noise than the phones. Just don't. \$\endgroup\$
    – markt
    Jun 23, 2014 at 14:25

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I've worked on small PA systems for weekly gatherings for about 20 years. The problem you describe is common, but usually fixable, and usually indicates a design issue with the wiring of the PA system. I've never had any success with filtering, every success was from changing the wiring.

Describe the wiring totally; the type of connectors, the number of wires in cables (balanced, or unbalanced), where the cables come from and go to, and the type of amplifier. Photographs of the amplifier would help.

You may be able to shorten this effort by testing one channel of the system at a time, with other channels disconnected.

If a particular channel is sensitive to the problem, change to balanced cabling and test again.

Check the earth of the system exists in only one place.

If the wireless receiver unit is separate from the PA, disconnect it and use another device to listen for the noise. If you truly have noise arriving with the wireless signal, you'll probably have to replace the wireless receiver.

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  • \$\begingroup\$ Hi, James. Thanks for the tips. The equipments are now stored in another place that I can only get access to at weekends. I'll upload the wiring diagram as soon as I have it. I will try out your suggestions as well. Thank you very much! \$\endgroup\$ Jun 23, 2014 at 9:02
  • \$\begingroup\$ @James you said "I've never had any success with filtering " . Why? modern day filters are very advanced. You cannot fix everything with wiring alone. \$\endgroup\$
    – Adi
    Jun 23, 2014 at 9:29
  • \$\begingroup\$ @Adi, 'cause I never had to try filtering once the wiring was fixed. \$\endgroup\$ Jun 23, 2014 at 9:36
  • \$\begingroup\$ @James, "'cause I never had to try filtering once the wiring was fixed". So you have not used filtering my confusion has gone now. But this will work with some EMC/EMI compliant systems which are getting marginally succeptible. \$\endgroup\$
    – Adi
    Jun 23, 2014 at 9:50
  • \$\begingroup\$ @JamesCameron Hi, James. I've asked the guy who's in charge of the equipment, and got the manufacturer name: Shure. \$\endgroup\$ Jun 23, 2014 at 9:55
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I would begin by working out for sure whether the interference is being picked up:

  • the wireless microphone receiver (does it go away if you turn the receiver off but leave it plugged it?)
  • the leads between the receiver / other microphones / instruments and the mixer (i.e. does it go away when you unplug all the mixer inputs?)
  • somewhere else in the system (i.e. occurs simply by having the mixer plugged in to the speakers)

If it's the wireless receiver picking up 4G phone signals, the receiver may not be 'faulty' so much as 'not designed to cope with that much interference'. You might be able to move the antennae closer to the transmitter; if that's not possible you either have to buy a 'better' wireless mic system (or one operating on a different frequency band), or persuade everyone to turn their phones off.

Pickup on the mic leads may be solved by a filter such as the circuit given. I found googling "microphone radio interference filter" turned up a number of ready-made commercial products and DIY projects.

If the interference is somewhere else in the system, it's possible something (the mixer or the amps) is faulty, or there is bad earthing, or faulty wiring (e.g. screen disconnected).

I take it you have balanced connections on all interconnections between receivers, mixers, amps, etc?

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I am an Audio Technician and spend a lot of time working with PAs. The digital noises that are caused by cell phones typically only appear on unbalanced or unshielded items. For example my computer speakers are very susceptible to the noise.

I have used a lot of Shure product from the SLX (low end line) to the UR (high end line) and have not had issues with the receivers being the pickup point for the cell phone noise. (I'm not saying it's impossible). The only time I have had a phone cause problems at the RF end is when the phone was directly between the mic and the receiver and very close to the receiver. And even this only caused dropouts not the noise. This is most likely due to Shure's use of a pilot tone. I have had unbalanced cables pick up the cell phone noise. I have also had balanced mic cables with a damaged shield pick up AM Radio, so make sure your cables are all balanced and good quality. The other pickup point for cell phone noise was the podium mic. There are a lot of them that are not adequately shielded, so if there's one in your system, that may be the problem.

I concur with James that 95% of the time you can solve your problem by ensuring your cabling is correct. Filters are great, but you don't normally need to go to that extent. I will look for your post of how the system is setup and hopefully that will point us in the direction of your problem. I understand your frustration, and if I can help, I will. Nothing is more frustrating than having your sound destroyed by an uncontrollable outside force! :-)

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The service manual for 3G/4G mobile has audio sections , there you may find how the cell phone ckt design uses filters is preventing its own noise from being coupled to its microphone and speaker. You can also low pass filter the microphone and speakers cables in the GSM band.

Update:

enter image description hereThe above image shows a tentative filter to remove noise from the electret mic. It should be low pass filtered in 800MHz and above or at your cell phone operating frequency. Similarly speaker side also should be covered for interference. It is important to make carefull filter placement and routing in your PCB layout .

JamesCameron said: We don't know yet, we haven't heard it, and we don't know where it is entering the system.

@JamesCameron,I am posting my followup here as it didnot fit below, what is that you haven’t heard about it ?. If you are an audio system design engineer you wont put forth these statements. Start reading system designs against RF Immunities. I would recommend you buy some good books and do some reading ;-) .The RF energy makes its ways into audio equipments because of non linearities , any semiconductor junction is a potential candidate and would rectify this energy in audible domain. It is not only limited to junctions but also to substandard components, which are potential culprits. This is why symmetrical wiring is so important , but is not the sole factor as perfect symmetries don’t exist and everything cannot be done with wiring if in the first stage the amp design is poor. If one can eliminate coupling in the high frequency domain then it will likely cancel in the audio domain , I have seen coupled noise as high as 50dBuV in GSM band . Designing audio or RF equipments should be done with great care and adhering to EMC/EMI guidelines, proper shielding, etc, etc. there are so may things to be taken care off . An SMT package filter as used in cell phones are more efficient than discrete owing to parasitic components.

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  • \$\begingroup\$ Hi, Adi. Thanks for your advice. What kind of filter you think may do the job? \$\endgroup\$ Jun 23, 2014 at 9:39
  • \$\begingroup\$ @ChangeMyName, I have updated my reply pls check. \$\endgroup\$
    – Adi
    Jun 23, 2014 at 10:34
  • \$\begingroup\$ Hi, Adi. Thanks for the update. May I know where I can find the picture please? Thanks a lot. \$\endgroup\$ Jun 23, 2014 at 10:59
  • \$\begingroup\$ I guess from some GSM datasheet, I had this pic available on my PC. \$\endgroup\$
    – Adi
    Jun 23, 2014 at 11:23
  • \$\begingroup\$ An 800 MHZ filter won't do a thing to remove audible noise. \$\endgroup\$
    – Matt Young
    Jun 23, 2014 at 18:07

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