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Modifying car stereo for MP3 input

Discussion in 'Audio' started by CitroenCrazy, Sep 23, 2012.

  1. CitroenCrazy

    CitroenCrazy

    2
    0
    Sep 23, 2012
    For some while, I've been messing around with the Philips car radio / cassette player in my car.
    The basic idea was to remove the entire cassette mechanism & use the audio input for an MP3 player. There are some pictures here:
    http://www.flickr.com/photos/citroencrazy/sets/72157602218493745/

    In brief, I traced the audio feed from the cassette to a Philips TEA 6320 chip.
    This is described in the spec sheet as a sound fader control circuit, so it handles switching between inputs and volume / balance control.
    Anyway, I lifted the pins associated with the cassette input and fed the audio output from an ipod into these pins.
    In addition, there's a small circuit board that mimics the control signals associated with a cassette, but this has no effect on audio.

    Getting to the question, I gave no thought whatsoever to impedance matching at the audio input. Although the quality of the sound is reasonable, there seems to be some background noise associated with the car, eg a faint whine that rises and falls with engine revs (it's a diesel car, so not ignition related). This seem to be more obvious when using the ipod input than using the FM radio, which is unmodified.

    Anyone any suggestions as to how I could improve the audio input please ?

    (For background, although I graduated in electronics, many years ago, I've never worked in the field, so I know what the words mean, but I'm short on practical experience)

    Many thanks
     
  2. CocaCola

    CocaCola

    3,635
    4
    Apr 7, 2012
    Alternator noise if it's changing with engine speed... There are 'big' caps you can get to bridge across your alternator that will reduce or eliminate the noise, most any car stereo place will sell them...

    http://www.ebay.com/itm/PAC-CSS-12-...394?pt=LH_DefaultDomain_0&hash=item5d35110cea

    Also the iPod might be introducing an ground loop that is aggravating this issue, there are little devices out to break this loop as well...

    http://www.ebay.com/itm/3-5mm-FILTE...er_MP3_Player_Accessories&hash=item5d273d8ff9

    And last but not least you are pumping 'speaker' level outputs into a likely line level input... You might want to attenuate that a little... This is a basic 1/10th speaker to line level attenuator... If you have volume issues you can adjust the ratios of the resistors up or down... Also note that the resistors in the attenuator 'load' the iPod output and simulate a speaker load to keep the amp inside that happy as well...

    [​IMG]
     

    Attached Files:

  3. GreenGiant

    GreenGiant

    830
    6
    Feb 9, 2012
    the whine is not usually an ignition issue its an alternator issue, you need to get a ground loop isolator like this and that should clear up the whine in the ipod connection, especially if you are charging it at the same time (this causes even more whine typically since the ground on the car is used for the ground pin on the headphone jack when plugged in)
     
  4. CitroenCrazy

    CitroenCrazy

    2
    0
    Sep 23, 2012
    Thanks guys, useful advice. Many thanks
     
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