Re: Loudspeakers for ur-quattro (car audio dribble)

[ti@amb.org (Ti Kan)]

Buchholz, Steven writes:
> ... no, the original reply was correct ... no contradiction I could see!
> Are you trying to say that someone can blow up 100W speakers if hooked up to
> a little 9v transistor radio?!  Let me try to explain it slightly
> differently to try to make things a bit more clear.  In the case of the
> "underpowered" situation what happens is that when you turn up the volume so
> high that the amplifier starts clipping, the output becomes a lot like a
> square wave.  A square wave signal has a lot more high frequency content
> than a sine wave at the same fundamental frequency.  These high frequency
> components are the things that kill the speakers.  The level of the high
> frequency components is much greater than you would get with normal audio
> signals even at a higher amplifier power without clipping.  In effect what
> is happening is that the amplifer isn't underpowered at all ... it doesn't
> have enough output to be able to reproduce the desired signal without
> clipping, but it can provide sufficent power to damage the speakers ...

Steve is absolute correct, but I would like to add that there are
actually several failure modes in speakers that have different causes.
The amplifier clipping scenario as described by Steve usually kills
tweeters.  This is because tweeters are designed to be small with
small voice coils (in order to reproduce high frequencies well the
diaphragm has to have a small moving mass, and to have good dispersion
it needs to have a small radiating diameter), and the voice coil
would quickly overheat and burn out in the presence of sustained
amplifier clipping and its associated high frequency harmonics.

A very low powered amplifier isn't likely to damage a tweeter that
is designed with reasonable power handling limits (and hooked up to
an appropriate crossover network).  So a transistor radio isn't
going to damage your hi-fi tweeters.  However, if the amplifier has
enough power *and* is overdriven into clipping it can certainly do
damage.  Given similar playback levels, it is possible that a higher
powered amplifier would avoid tweeter damage because it would still
be producing clean power rather than clip.  That said, however, 
you can get into a situation that a non-clipping amplifier can also
burn out speakers, when it is simply delivering more power than the
tweeter was designed to handle.

Woofers, on the other hand are less likely to be damaged by amplifier
clipping per se (unless the speaker has very small voice coils and
resultant low power handling).  They are more likely to be damaged by
simply overdriving them beyond the designed power handling limit.
Clipping or not.  This can cause a voice coil burnout just like the
tweeter, or some other failure in its diaphragm's suspension system
(because the woofer's cone must travel large displacements to move air).

-Ti
96 A4 2.8 quattro
84 5000S 2.1 turbo
80 4000 2.0
-- 
    ///  Ti Kan                Vorsprung durch Technik
   ///   AMB Research Laboratories, Sunnyvale, CA. USA
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