I remember that some of the early transistor amps with transformer output
had a similar problem - unloaded outputs could generate enough volts to
cause insulation breakdown in the transformer or high dissipation in the
output transistors.


Shouldn't be a problem these days I would hope!





Richard P



                    David Duffy
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                    ES.COM.AU>                       cc:
                    Sent by: pic                     Subject:     Re: [EE]: Unloaded outputs on audio amps.. will they burn out?
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                    18/08/00 14:21
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Damon Hopkins wrote:
>On rec.audio.car there is a recent discussion on whether or not unused
>speaker outputs on amplifiers should be loaded or not.
>so I figure there are far more knowledgeable people on this list when it
>comes to the actual how's and whys then on the car stereo newsgroup so I
>come to ya'll looking of answers :)
>
>                 Someone suggested that if left unloaded the output
> transistors could
>oscillate and burn themselves out it not protected (i.e. thermal
>disabling or some other method)
>I talked to my father and he explained the theory behind this and I
>believe it basically follows along the same thread of the grounding of
>unused PIC pins... hence my idea to post it here.
>someone also asked if the outputs on the headunit itself would also be
>prone to this or is it all a bunch of urban ledgend type stuff.
>
>         I guess I'm looking for yes, no or a maybe and sometype of
> description
>that I can relay back to the interested people.

I can't see any good reason for it in modern amplifier designs. The
amplifier
output stage should not rely on the speaker loading to determine its
stability.
I'd say it might be a left-over from the old days of valve (tube)
amplifiers which
were unstable if the output transformer was left unloaded. I remember
sparks
flying (internal to the amp) a long time ago when I accidently turned my
old
Sansui HiFi (valve) amplifier way up with the speakers not connected. As
you
say, what happens to the speaker outputs of a car audio head unit that is
only
feeding a bigger external amplifier via the line level outputs? (Hint:
nothing !)
As you've probably guessed, a lot of car audio people are badly informed.
:-)
Regards...

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