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speaker isolation

Discussion in 'General Electronics' started by Ivan Sedneff, Jan 30, 2006.

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  1. Ivan Sedneff

    Ivan Sedneff Guest

    Hello,

    I made a portable enclosure for a car radio that has a negative ground
    but the left and right speaker outputs have isolated grounds from each
    other and the chassis.

    I needed to common out the (-) from the left and right speakers in
    order to attach them to the common connection on the headphone jack. I
    successfully connected each of the (-) outputs from the left and right
    speakers using non-polarized 47 uf electrolytic capacitors and joined
    the ends of the caps to the headphone jack.

    All is well when the jack is floating and un mounted, but when I mount
    the jack to the chassis, even though the audio output is great, I get
    annoying loud "clicks" and "pops" when I wiggle the headphone
    plug/jack, most likely because of the caps discharging/charging.

    Other than figuring a way to physically isolate the headphone jack
    from the chassis, is there a way to drain the charge using resistors,
    and if so, what values do you suggest?


    Thank you for your help,

    Ivan
     
  2. no_one

    no_one Guest

    are they really isolated? do you have enough circuit info to show that
    connecting the returns together will upset the stereo amp(s). many (older)
    car stereos would actually use the car chassis as a common return for the
    speakers. This was only a problem when you had a car with a positive ground
    such as the british used. I would expect that you can connect the returns
    together and not use the cap.
     
  3. SimonLW

    SimonLW Guest

    let me get this straight. You connected the - outputs together as a ground
    through the caps for the common of the head phone jack? If so, very bad! You
    are essentially shorting higher frequency signals not equal to each other
    through the amplifier's outputs. To derive more power from 12volt auto
    supply, many car stereo's use what is known a bridge amplifier. It is two
    single ended amplifiers outputs connected together through the load
    (speaker). One of the amps is fed the signal 180 Deg out of phase. This
    allows for double the voltage swing on the output, thus double the current
    which means a potential for quadruple the power output as compared to a
    single ended amplifier.

    So, connecting one negative terminal to the other channels negative terminal
    (not really a negative line, just used for proper phasing between the
    channels) means you are shorting high frequency signals through one side of
    the bridge amp to one side in the other channel. This could likely
    eventually damage your car stereo's output amplifier.

    The proper way is to take either the + or - lead (as long as the same for
    both channels), put a 220 ohm 1/2 watt resistor in series with a 100 uf cap
    from each channel. Connect to proper channel on the jack. The ground of the
    jack goes to the chassis ground of the car, near the ground of the stereo to
    prevent possible ground loop noise.

    The capacitor removes the DC component as each side of the bridge amp is
    biased a 1/2 the supply voltage. The resister limits the current so the
    voltage across the headphones is reduced so you don't blast your ears with
    full output from the amp.
    Simon
     
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