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transformer question

Discussion in 'Electronic Basics' started by Fishy, Feb 17, 2004.

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  1. Fishy

    Fishy Guest

    Hello ...

    I have a couple of questions regarding the use of transformers ... (I am
    trying to power an audio amplifier)

    (1) is it possible to supply +V / 0 / -V by using just a single secondary?

    (2) considering it's an audio amp circuit (~ 70W), if the voltage output
    from the transformer secondary is too high (it's 35.90VAC, so roughly
    51VDC ... but I need around 40VDC), what's the most effective way of
    lowering the DC voltage?

    Thanks a lot!
  2. There are two terminals to the secondary, how do you propose to get
    three voltages? If you want the two sides to share a common ground
    then the answer is no. If the secondary circuit is isolated then the
    voltages are arbitrary and you can get +V/2 | -V/2 and build the
    circuit with ground between them.
    By building a power supply, either linear or switching, out of it.

  3. Bob Masta

    Bob Masta Guest

    Not easily.
    Many, if not most, audio power amps of the sort one is
    likely to build as a beginner project are pretty tolerant
    of different supply voltages. As long as the voltage
    doesn't exceed that of your transistors, this won't
    likely be a problem by itself.

    However, the thing you *do* need to watch out
    for is power handling of the output devices.
    If you plan on running this thing hard into a
    minimum load, you might exceed the device
    limits. Usually the supply voltages are what
    limits the output power. As a good approximation,
    you take 70% of the supply voltage, square it,
    and divide by the speaker impedance.

    You might need bigger devices, or more of
    them in parallel, or better heat sinks, or just
    make sure you never use 4 ohm speakers, etc.

    Bob Masta

    D A Q A R T A
    Data AcQuisition And Real-Time Analysis
  4. Yes. Even quite easily (2 diodes needed)

    <-------\|/-x-|>|------x-------> + DC out
    AC /|\ | D1 _|_ +
    in \|/ | C1 --- -
    <-------/|\------------x-------> DC common
    | _|_ +
    | D2 C2 --- -
    '-|<|------x-------> - DC out

    Select the capacitors depending on the load required.
    Note that the rectifiers are only half-wave each one.

    Note also that the diodes should be able to handle
    approx. 4 times the current requirement (each side)
    Try some workaround. A lot of audio amps just operate
    on a poorly regulated / varying voltage. Maybe power
    all electronics except the last stage from regulated
    voltage (linear is enough) and use some trick to make
    the last stage handle inexact PS voltages if possible.

    Amp circuit needed for details on what 'trick' to use, please
    post a small jpeg right here (I can't access ABSE at this time).

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