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Impedance and pre amp

Discussion in 'Electronic Design' started by OBones, Mar 2, 2006.

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  1. OBones

    OBones Guest

    Hi all,

    I just got hold on a "box" to plug at the back of my in car radio, in
    order to be able to plug an external source. The radio has an inout
    impedance of 50kOhms while the external source (mp3 player) has 32 Ohms
    on its output.
    The "box" I have features a preamp, with a gain of 5 according to the
    INA2126 datasheet (RGA and RGB are disconnected).
    Is that gain enough?
    And mostly, what is(are) the formula(s) to use for that kind of
    calculations. Having those would allow me to better understand that kind
    of signal chains.
    Thanks in advance.

    Olivier
     
  2. Ian Bell

    Ian Bell Guest

    As a rule you aim to drive from a low impedance to a higher impedance. The
    source 32ohms impedance and radio 50K input impedance meets this
    requirement. However, you also need to match signal levels. You do not say
    what signal level is output from your source, or what input level the radio
    requires. Only when these are known can you calculate the gain required.

    Ian
     
  3. OBones

    OBones Guest

    Ah, I get it then. The volume difference does not really come from
    impedance mismatch but more from level difference.
    I know that the output is around 0.7v but I have no idea what the input
    requires. I'll try to dig that up, but the car manufacturer is not too
    keen on getting this out...
     
  4. Ian Bell

    Ian Bell Guest

    That output level is probably more than adequate. Try hooking it up direct
    and see how load it is. You may even need to attenuate it.

    Ian
     
  5. OBones

    OBones Guest

    Thanks for the infos.
     
  6. Ian Bell

    Ian Bell Guest

    That should have been 'see how LOUD it is' not load ;-)


    Ian
     
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