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| Date: |
March 01, 2007 at 11:20:24 |
| From: |
m.x., [213.164.91.2] |
| Subject: |
Re: Sub power range |
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The problem with amps is power
boasting (not boosting!).
I.e.: the numbers the manufacturer
publishes are often very optimistic,
to say the least.
The power supply or the cooling does
often simply not permit the rated output
power. Also clipping can pose problems.
For a low frequency channel we often need
to play the driver BELOW resonance.
In resonance the current has a minimum.
Below and above it will run to itīs
maximum. Since radiation resistance rises
with frequency, this means we need added power
in the low end.
Between the very lowest
frequency and resonance current and voltage
will be out of phase, also above resonance,
where series resonance comes up.
Out of phase means that the voice coil will not
dissipate P=Ieff**2*R, but less than that:
P=Ieff**2*R*cos(phi).
Unfortunately this will also cause more power
dissipation in the power stage transistors than
a pure resistive load would make. smoke ...
Depending on enclosure and chassis you
get the speaker impedance curve.
Depending on frequency range and
excursion vs. frequency you get the maximum
current you need (compliance tolerance!)
and the constant drive voltage amplitude
(voltage source).
Depending from current and voltage you
can compute the power dissipation in the
voice coil and in the amplifier.
It depends on frequency.
I think the worst case situation
will be a pure sine wave.
The power rating for the voice coil
is always optimistic, unless it is
AES constant RMS power. Still than
this means rapid aging. 50% or 66%
should be the target.
From this we can derive:
-it makes no sense to have a heavily overdimensioned
amp, you can never use this current at all
frequencies, this will always hit excursion limit
or voice coil dissipation
-this depends on your lowest frequency target
-the amp must be able to dissipate the out-of-phase
excess power losses
-the amp should be able to deliver this power
for a long time (power supply and cooling),
longer than the voice coil can take it
-for fool safe operation you need a excursion
monitor (electronic 2nd order lowpass) and
a power monitor. This will prevent overexcursion
and overheating
The thermal design can rely on the fact that constant
sine waves are not likely. In reality it
will be sine bursts. This gives considerable
relieve on the thermal side.
The design of the amp power robustness must
not consider this, because transistors die
in micro-seconds.
The design of the power supply must also
be able to deliver for infinitely long
time under worst case conditions
(mains voltage tolerance, line drop).
m.c.
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Posted with TalkShop version
2.76 BETA |
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