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Interfacing split-supply preamp to single-ended amp

Discussion in 'Electronic Basics' started by Doug Laidlaw, Dec 29, 2003.

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  1. Doug Laidlaw

    Doug Laidlaw Guest

    I have a stereo preamp built around the LM338 dual amp, which requires
    + and - 15 volts split supply. I want to interface it to a transistor
    amp with about 25 volts single supply. The kit manual merely asures
    me that it can be done.

    I was thinking of putting a voltage divider across the 25 V supply and
    returning the tap to the ground on the preamp board. To avoid
    shorting out the negative half of the supply, I can have no continuous
    earth return to the main board, i.e. cable shields must be earthed at
    only one end. Will this work?

  2. The cable shields are not the problem. The problem is that the 25
    volt supply must not be connected to any other circuit that grounds it
    differently. I think I would connect the amp outputs directly to the
    speakers, but return the speakers back to the center node of a pair of
    capacitors that are in series across the supply. You can share one
    pair with both speaker outputs, or you can make two series pairs, one
    for each channel. This will produce a lot less pop at power up than
    connecting the speakers to one side of the supply, with a single
    series capacitor between output and speaker.
  3. Ian Bell

    Ian Bell Guest

    Basically you need to modify the biasing arrangements of the LM338 to work
    from a single supply. Assuming the input goes to the non inverting input
    then I would suggest you connect a pair of 10K resistors across the 25V
    supply each decoupled with 25uF and use this to bias the opamp something
    like this (the junction of the two 10Ks is labelled 0V(op) ).

    10uF |\
    in -- C ----------|+\
    | | \____
    | --|- / |
    in-- R | | / |
    | | | |/ |
    | | | |
    | 0V(op)|--- R --|
    | |
    0V(25) |
    0V (25)


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