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How Amplifier Clipping can Damage
your Speakers
Amplifiers and speakers have a very
special relationship. They both need
each other to perform. But this
performance can be spoilt when the
two are not operated as intended.
The problem with all amplifiers
(excluding valve amplifiers which
behave slightly differently), is
that when they reach the limit of
their rated power, a rather drastic
thing happens. Before going any
further, it is important to remember
here that the limit to an
amplifier's power is almost ALWAYS
reached well before the volume
control (if it has one) is turned to
the maximum.
It is important to realise that the
level of a typical signal varies by
a staggering amount. This is called
the "Dynamic Range" of a signal and
it is no surprise that one part of a
signal can be 1000 times stronger
than the next. That is why signal
levels are expressed as logarithmic,
or decibel (dB) scale. The volume
control on an amplifier (or from a
mixer, etc) sets a level for an
AVERAGE signal level. It is quite
common for example, that a hi-fi
amplifier set on say "half volume",
will occasionally process peak
signals that go way beyond its power
capability when driven from a wide
dynamic range source such as a CD
player.
The Dynamic range of modern
recordings can be quite staggering,
when coupled with solid state
electronics driven by digital
sources such as CD players. This
combination can provide enormous
enjoyment, however it is critical to
understand that this combination has
the capability to destroy any
loudspeaker, regardless of cost,
either by overheating or by
exceeding the mechanical
capabilities of the drivers.
When a sine wave signal (for
clarity) reaches the limits of the
power supply rails, the tops of the
sine wavers are "flattened" because
the voltage limit of the power
supply rail has been reached. The
signal still goes through to the
speakers with the flattened tops.
This phenomenon is called
"clipping".
This presents a serious threat to
the life of the speaker.
The flattened "bit" at the top and
bottom of the signal is in fact raw
DC across the speaker voice coil,
positive one instant, negative the
next. The DC causes the voice coil
to heat up rapidly, cooking the
insulating enamel on the wire and
burning away the adhesive holding
the assembly in place. How long the
voice coil will survive under these
conditions depends on many factors.
The repetition rate and time
duration of the DC component of the
signal is the most important, but
voice coil construction magnet size
and ambient temperature also have a
bearing.
PA, hi-fi and car speakers are all
at risk. It does not matter whether
it is an expensive European,
American or lower cost Asian
product, they are all vulnerable.
You can generally hear quite easily
that an amplifier is clipping. The
sound from the speaker becomes very
harsh. The first thought is that the
speaker is overloaded, after all the
sound is emanating from them. On the
other hand, when a big amplifier is
in use with a small speaker, genuine
overload sounds far less harsh, and
is not nearly as dangerous to the
speaker.
In nine times out of ten, experience
has shown that what sounds like
speaker overload is in fact
amplifier clipping. Unlikely as it
sounds, it is easier to destroy a
200W speaker with a 35W amplifier,
than a 100W speaker with a 200W
amplifier!
Damage caused to a loudspeakers in
this manner is not a warranty item.
But by exercising sound engineering
practice and common sense, this does
not need to happen to you. But you
do need to buy an amplifier that is
designed and powerful enough for the
intended job - and that may mean
digging a bit deeper in the pocket
with the initial purchase. | |
