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Stereo amp EMF protect

Discussion in 'Electronic Design' started by Klaus Jensen, Oct 5, 2006.

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  1. Klaus Jensen

    Klaus Jensen Guest

    I am using a 25W home stereo amp to experimentally drive various types
    of step up transformers, ie. auto ignition coils, rewound flybacks,
    etc. Of course, I will use an appropriate value load resistor, if
    required, but no external coupling cap.

    In this application, what kind of protection would be advised so that
    back EMF, etc. does not damage the amp's output stages.

    I am looking for something that will not affect the drive signal's

    Thank you for any advice.

    Klaus Jensen
  2. Joerg

    Joerg Guest

    Hello Klaus,

    Hard to say without any schematic. Large diodes to the rails could help
    some. However, if it is a pnp/npn pair that is driven from a hard source
    exceeding base current or reverse Vbe limits could already blow the

    Maybe you should find good spare transistors and keep a whole lot of
    them in a bin for replacement ;-)
  3. Phil Allison

    Phil Allison Guest

    "Klaus Jensen"

    ** That is mighty serious abuse for a domestic audio amplifier !!

    Only by having a series resistor of at least 4 ohms from the output AT ALL
    TIMES could a domestic audio amp be expected survive driving low such
    impedance and saturating magnetic loads.

    ** Back emfs are not your problem, the low output impedance of the amp will
    take care of them.

    If the amp has VI limiting protection built in for its output devices, then
    it will also have clamping diodes from the output to each supply rail. These
    are needed for use with ordinary loudspeakers.

    ** My god - what horrendous input signals is this clown using ??

    ........ Phil
  4. Tim Wescott

    Tim Wescott Guest

    Or a great big series resistor.

    I have seen 60Hz resonant electromechanical loads driven by a regular
    old Archer PA amplifier -- this is a load that consumes 60W at 12Vrms
    during 'normal' operation, and has an instantaneous impedance that's
    well below the apparent 2.4 ohms, and that will bite you if you ramp up
    the amplitude too fast. AFAIK the amp is still in working condition,
    buried in a lab someplace, ready to be dug out and used when it's needed.


    Tim Wescott
    Wescott Design Services

    Posting from Google? See

    "Applied Control Theory for Embedded Systems" came out in April.
    See details at
  5. Tim Shoppa

    Tim Shoppa Guest

    There is a long and deep tradition of using home audio equipment in
    scientific experiments.

    When I got started, tube based consumer amps were being used to drive
    everything from voice coil shake tables to piezo transducers.

    At the ultrasonic end, it didn't matter what you put in, what you got
    out didn't look like it :). And the shake tables were at the other

    When (not IF) they blew up in the lab, I got to fix 'em :).

    For the cost of a single output transformer back then, you could buy a
    dozen brand new solid-state amps of ten times the power capacity, any
    of which will do a much better job at driving the bizarre loads than
    the old tube/transformer amps (especially outside the AF range).

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