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Poor speaker connection blowing an amp?

Discussion in 'Electronic Repair' started by N Cook, Feb 20, 2007.

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  1. N Cook

    N Cook Guest

    Has anyone ever come across this ?
    Back emf induced by breaking the current to an inductor, in this case the
    speaker coil.
    Its a real effect but has it ever been known to blow an amp, as the induced
    voltage would be in the coil and not the amp, I would have thought.
    To demonstrate, if you're brave, connect a small 12Volt relay to a 12V
    source , holding your fingers over the terminals of the relay and then break
    the connection at the supply, you will get a belt of presumably 100Volts or
    more. I did it accidently once, testing a relay
  2. Guest

    It cant hurt the amp. At worst it could arc over the gap, but its
    current delivery, not v, and the amp's very much in control, not the
    LS. Think it through.

    & really dumb

  3. Ron(UK)

    Ron(UK) Guest

    What amp is it?

    Are we talking valve amp, transformer coupled solid state amp, or normal
    output stage SS amp?

  4. N Cook

    N Cook Guest

    I could see it as a problem with valve amps as the main "inductor" is on the
    amp side of a break. I should have limited the question to solid state amps
  5. Ron(UK)

    Ron(UK) Guest

    So, what amp is it? make & model number?

  6. N Cook

    N Cook Guest

    hypothetical, any amp + speaker + any sort of potential break/poor contact.
    I forget the maths now but in theory (no R etc) then for an instantaneous
    break can't the indiced V be near infinite for an infinitesimal time after
    the break. So a few hundred volts a nS or so after a break could be induced
    and easily arc across a practical, poor contact, rather than a theoretical
    clean break.
  7. That's why I hate impedance selectors... wired around
    the one on my Marshall. Terminal strip and jumper
    wire is the only way to do it right a la old PA's.
  8. RonSonic

    RonSonic Guest

    We've seen it in all kinds of gears. The more extreme cases are in dance club
    sound systems. I've seen Crown power amps with big craters in the cases of
    stainless steel TO-3 case transistors. When one of those subwoofers lets go
    there's a lot of energy looking for somewhere to go....

  9. Jim

    Jim Guest

    If it's a tube amp, you could get flyback voltages causing arcing at
    tube sockets, perhaps punched through insulation in the output
  10. JW

    JW Guest


    In 25 years in the repair industry, I've yet to see a failure where a TO-3
    ends it's life that violently.

    Got any pics? :)
  11. Doggone

    Doggone Guest

    Agreed. Everything else around it will melt and burn to a crisp
    before the TO-3 even discolours. On the other hand, the plastic
    package TO-3P (looks like an oversized TO-220) will flames
    out in a kaleidoscope of colours.
  12. Doggone

    Doggone Guest

    SS amps of the power MOSFET variety are suceptible to fail
    under those conditions. This is due to the low (20-30volt)
    gate-to-source breakdown.
  13. RonSonic

    RonSonic Guest

    Depends on how fast it goes, doesn't it. No, I don't have pic's but there's sure
    to be some out there. It really isn't that rare.



    Sweet Blues pedal demo up at
  14. Ron(UK)

    Ron(UK) Guest

    I`ve seen plenty of those big old Peavey poweramps with the T03s melted
    right through the can.

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