Stopping RFI. Mobile phone blips from speakers. - diyAudio
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Old 7th March 2007, 05:38 PM   #1
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Default Stopping RFI. Mobile phone blips from speakers.

Hi,

A long while ago I posted about mobile phone interference after completing my first iteration of a chip amp. I am now on my second 'revision' and I am still having trouble with RFI (cellphone blips coming from speakers before my phone rings are what's annoying me).

I remember Andrew recommended I try ferrite beads on the speaker cables, and I have finally tried this yesterday (take my time don't I!). Clip on ones. I clipped them on as close as I could to the amplifier outputs. This didn't seem to help. I also have a 470pF cap across the -ve and +ve inputs, and a Zobel.

Today I also tried a 100nF cap across the disconnecting network, as on Rod Elliots site, which didn't actually seem to do anything either.

I don't have winding wire (yet) to make an inductor for an L||R on the output, but do you think this has any chance of sorting it out? I know it's probably a good idea to have it there anyway, but aside from mobile phone interference I don't seem to be having any problems with it, even playing very very loud it still sounds clean to me. Might it probably be better to try an input RF filter first?

It is in a metal chassis which is connected to safety earth, so I wasn't really expecting any trouble with RFI anyway! I imagine it will only get worse when I get around to trying my hand at a DIY wooden chassis!

RFI is horrible!
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Old 7th March 2007, 06:38 PM   #2
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RFI can be a serious headache..
I once had a neighbor with a CB linear in his truck.
Every morning I would hear "Breaker breaker!!" blasting through my speakers at 5am.
I tried Ferrite,caps,you-name-it. Nothing helped my situation,until he finally moved. Talk about Irritating.

My suggestion: all of the above-ferrite,bypass caps,whatever. Try it all. On all the leads,input,power,and speaker.

Also,are you sure it's from the amp? Maybe the RF is getting in to something "upstream" like the preamp,or CD player?
Just a thought.
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Old 7th March 2007, 06:50 PM   #3
BWRX is offline BWRX  United States
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Default Re: Stopping RFI. Mobile phone blips from speakers.

Quote:
Originally posted by markiemrboo
I don't have winding wire (yet) to make an inductor for an L||R on the output, but do you think this has any chance of sorting it out?
No, it won't help. The problem is energy from your cell phone being transmitted into the input circuitry of your amp. The energy emitted by your cell phone is stronger than most other RF energy it receives because your phone is physically close to your amp. The simple solution is to not worry so much and move your phone further away from your source and amp.

Twisted and shileded input wiring will probably be better than ferrite based solutions.
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Old 7th March 2007, 08:43 PM   #4
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Default Re: Re: Stopping RFI. Mobile phone blips from speakers.

Quote:
Originally posted by DigitalJunkie
[B]RFI can be a serious headache..
I once had a neighbor with a CB linear in his truck.
Every morning I would hear "Breaker breaker!!" blasting through my speakers at 5am.
I tried Ferrite,caps,you-name-it. Nothing helped my situation,until he finally moved. Talk about Irritating.
Dear god! I feel for you there..!

Quote:
My suggestion: all of the above-ferrite,bypass caps,whatever. Try it all. On all the leads,input,power,and speaker.
Ok cool.

Quote:
Also,are you sure it's from the amp? Maybe the RF is getting in to something "upstream" like the preamp,or CD player?
Just a thought.
Well, I disconnected the preamp so it was just the power amp sitting on it's own. If I can figure out how to successfully prevent RFI on the power amp then I will move on to doing the preamp

Thanks for the reply!


Quote:
Originally posted by BWRX


No, it won't help. The problem is energy from your cell phone being transmitted into the input circuitry of your amp. The energy emitted by your cell phone is stronger than most other RF energy it receives because your phone is physically close to your amp.

The simple solution is to not worry so much and move your phone further away from your source and amp.


Darn! My little sister won a small pocket radio the other day and I put that next to it, and never heard radio.. I guess that might be a good start.

The thing is, my previous amp (Cambridge Audio A5) didn't pick up any mobile phone interference from the same phone, even sat right on top of it.

I figured if they can manage it, so can I.... well..... maybe

Quote:
Twisted and shileded input wiring will probably be better than ferrite based solutions.
I have twisted power wiring, and the input is shielded. The output is neither twister nor shielded (the ground return for the speakers doesn't run anywhere near the amp.....). I even twisted my external run of speaker cable

I guess I will have to try the input RF filter one day and hope for the best! Might be being picked up by the RCA barrels / connector or something.

Thanks for your reply also
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Old 9th March 2007, 09:38 AM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
Quote:
better to try an input RF filter first
I notice that many chipamp circuits omit the input filters.
Similarly many also omit the Thiele network on the output.

The Thiele network provides two functions.
1.) to give the amp a controlled high frequency load that aids stability.
2.) to attenuate Rf fed back from the speaker leads into the feedback input of the chipamp.

Locating the Thiele network at the speaker terminals rather than on the PCB avoids the risk of RF being re-radiated from the wires connecting the chip to the terminals.

Dr Cherry did some work on the Thiele network and one of his conclusions was that improved RF susceptibility is achieved if the NFB tapping point is moved from the top of the Zobel to between the R & C (the R must be first and the C connected to ground).

If the Zobel is connected to the speaker terminals then the NFB will have a long wire to connect back the the PCB. This may impact on the sound quality, but which is more important? RF attenuation or better quality?
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Old 16th March 2007, 11:32 AM   #6
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Hi Andrew,

Quote:
Originally posted by AndrewT
Hi, I notice that many chipamp circuits omit the input filters.
Similarly many also omit the Thiele network on the output.

The Thiele network provides two functions.
1.) to give the amp a controlled high frequency load that aids stability.
2.) to attenuate Rf fed back from the speaker leads into the feedback input of the chipamp.

Locating the Thiele network at the speaker terminals rather than on the PCB avoids the risk of RF being re-radiated from the wires connecting the chip to the terminals.


The only reason I like to do it on the PCB is that I am a bit rubbish at 'free hand' / P2P soldering But this does make sense.

Quote:
Dr Cherry did some work on the Thiele network and one of his conclusions was that improved RF susceptibility is achieved if the NFB tapping point is moved from the top of the Zobel to between the R & C (the R must be first and the C connected to ground).

If the Zobel is connected to the speaker terminals then the NFB will have a long wire to connect back the the PCB. This may impact on the sound quality, but which is more important? RF attenuation or better quality?
Definately RF attenuation for me I don't like the thought of RF polluting the audio signal!



I know it has been a long while, but I ordered some tiny little SMD caps in various values, Ceramic NPO's (they're supposed to be very linear / good from what I hear, even though most dismiss them just because they're Ceramic?). I wanted them partly because I think I am going to one day (hopefully soon) try an 99% SMD version PCB, but I figured I would strategically place some on my current design.

I was reading some datasheet for an opamp used in mobile phones for audio, where RF is likely a major problem I guess. One of it's suggestions was 10pF bypassing the power rails. So, as crazy as it sounds I tried this. I'm not sure if it made a difference. It immediately struck me that it seemed to make the highs / treble quite a bit nicer (I wasn't listening for any audible differences either, just tested mobile phone interference and started listening to music as it was still there), but I am sure this is just my imagination.

This morning I tried 300pF across the +ve and -ve opamp inputs (RIGHT at the pins, not on the PCB) on my preamp. Why the preamp now, instead of the LM3886 power amp? Well, it's currently easier to get at, and also the preamp is in a plastic case so.. This seems to have reduced the mobile phone noises ALOT! If I sit my mobile phone right over where the preamp PCB is, the blips are now really faint. Before they were quite loud.

It could of course just be something else has changed. Maybe for some reason the RF my phone is emitting today isn't as strong (is that possible? I don't know...). I'll have to keep testing before I am convinced, but I just thought I would let you know Should it work I will try removing the 470pF on the LM3886's (not SMD) and try the 300pF SMD's there too, and hope for the best!
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Old 13th May 2007, 09:35 AM   #7
skidude is offline skidude  United Kingdom
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Try using decoupling capacitors in the range of 10pF, 27pF or 33pF. The value depends on the frequency that your phone network is on. I think we use 10pF for 900MHz (GSM) or 33pF for 1800MHz/1900MHz (DCS/PCS).

The problem is caused by demodulation of the RF signal and can be seen on an oscilloscope as 217Hz pulses, very audible! There'll be harmonics of this frequency too, 434Hz etc...

Pre-amps are very prone to picking up this kind of interference.

Good luck!
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Old 13th May 2007, 10:29 AM   #8
Nordic is offline Nordic  South Africa
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My logitech X530s is sensitive to this interference too... but it doesn't bother me... Have you tried a simple 10 to 20pf cap over the inputs?, you will notice a pair on the reference LM4780 PCB form National.
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Old 13th May 2007, 11:03 AM   #9
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This is what the CE testing rules are all about - to prevent electronics kit from interfering and being interfered with.

There is plenty of good information available to help product designers available on the Web - and although DIY designs don't have to be CE tested - we are all striving for the highest standards aren't we?

Have a look at:

http://www.compliance-club.com/

You have to register on this site - but it is worth it. Find the EMC articles in the archives written by Tony Wauldron (of CADAC - UK designed and made mixing consoles - used for Musical Theatre productions around the world).

If you can imagine a Music Festival situation (or perhaps the Eurovision Song Contest) - how many radio microphones are in use by performers? Then add all the channels in use by the technical and production staff......

Finally, almost every member of the audience will have a mobile phone in their pocket!

RF immunity is very important everywhere!

JG
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Old 13th May 2007, 11:26 AM   #10
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"If you can imagine a Music Festival situation (or perhaps the Eurovision Song Contest)"


I would rather listen to the RF.
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