Homebrew mobile phone car-kit noise problems - diyAudio
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Old 29th April 2003, 08:09 PM   #1
diyAudio Member
Join Date: Apr 2003
Location: buckingham
Default Homebrew mobile phone car-kit noise problems

Due to the new laws in the uk about mobile phone usage while driving, including the use of a wired hands free, i decided to build my own car-kit.

I have cut the wired hands free kit open. the microphone is attached next to my sun visor on a lead down to my dashboard.

The earphone was chopped off and was wired into a 2.5W amplifier, based on the LM380, that I have built. I have tested it with a cd play and it works ok for vocal usage.

The power is supplied from the electrics and has been regulated and smoothed for the amplifier circuit.

The main problem is that i am hearing a drumming effect as an incoming call/message occurs (I can live with that). BUT i can not live with the noise during the call. The level of noise can be altered by moving the phone slightly.

This can be simulated by moving the phone, during a call, near powered speakers.

I have tried low-pass filters on the input and output of the amplifier but they didn't work.

Any ideas anyone?


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Old 29th April 2003, 08:55 PM   #2
diyAudio Member
Join Date: Jan 2002
Location: Puget Sound
Nice one! Personally, I'm holding out for a bluetooth enabled head unit.

I imagine there is some short of sheilding in the normal handsfree car kit amplifier. I have an old Nokia car kit somwhere. I'll pry it open and see what I can find.
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Old 29th April 2003, 09:18 PM   #3
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Join Date: Apr 2003
Location: buckingham
that would be a great help.

i did try a very simple can around the pcb but it didn't do anything. the noise must be leaking onto one of the cables.


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Old 30th April 2003, 05:16 PM   #4
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Matttcattt's Avatar
Join Date: Feb 2003
Location: Bristol
are you sure its the amplifier and not the phone thats being affected?
If it aint broke, don't fix it. If it is broke, fix it.
If you can't fix it, take it apart and see how it "worked".
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Old 1st May 2003, 06:57 PM   #5
diyAudio Member
Join Date: Apr 2003
Location: buckingham
this effect happens to other powered speakers with no link to the phone.

if the phone is used on the standard wired hands-free you can not hear the noise, etc.

my amp is only isolated from dc via a capacitor. i was thinking of isolating the phone and amp with a 1:1 audio transformer as i don't know if the output of the phone is balanced or unbalanced.

as i'm typing this i've just heard the drumming effect on the speaker even when the amp isn't powered or the phone being attached! it could just be the power of the phone.

does anyone know how to control a simple amp with balanced and unbalanced input?

or have any other idea?

ultrachrome - have had a chance to look inside you nokia kit?


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Old 1st May 2003, 08:52 PM   #6
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JOE DIRT®'s Avatar
Join Date: Apr 2003
Location: Brantford, ON
Andy I almost think your getting feedback between your phone and speaker????
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