User Guide
Sound System Design Reference Manual
Step Three:
For a simulated microphone input of 72 dB
SPL, adjust the HF and LF outputs of the DSC260
for nominal levels of 0.4 Vrms. Then, advance the
LF gain control on the MPX600 amplifier until a
reference level of 60 dB SPL has been reached at a
distance of 20 meters. Following this, increase the
level of the HF section to reach the same value.
Details here are shown in Figure 7-1.
Set up in this manner, there will be adequate
headroom, in the console, controller, and power
amplifier to handle nominal speech levels as well as
levels up to 25 dB higher, should this ever be
deemed necessary.
Amplifier and Loudspeaker Power
Ratings
A persistent question is: what amplifier power
rating do I choose for use with a loudspeaker of a
given power rating? The detailed answer is
addressed in JBL’s Technical Note Volume 1,
Number 16A; here, we will only summarize those
recommendations:
1. For systems that will be stressed with full
amplifier output for long periods of time, we
recommend that the amplifier’s continuous output
rating be chosen to be equal to the loudspeaker’s
input power rating. Situations of this sort occur
primarily in music reinforcement, where a constant,
wide-band signal predominates.
2. For applications, such as speech
reinforcement, where there is an operator who
controls levels carefully, we can confidently
recommend an amplifier with output capability that is
twice (3 dB greater) than the loudspeaker’s
continuous rating. The rational here is that peak
power requirements, often slightly in excess of the
loudspeaker’s continuous rating, can be handled
with no problem, and it makes sense to provide
amplification accordingly.
3. For certain critical monitoring applications,
as in recording studios or film postproduction
environments, amplifiers may be chosen that can
deliver four-times (6 dB greater) power than the
loudspeaker can withstand on a long-term
continuous basis. The rational here is that the
loudspeakers can ordinarily handle midrange and
high frequency peaks of short duration that are much
higher in instantaneous power than the long-term
continuous rating of the loudspeaker.
In most speech reinforcement applications,
condition 2 above will apply. Note however that there
is no absolute necessity to use the larger amplifier
unless high acoustical peak levels are anticipated.
Wire Gauges and Line Losses
In modern sound system engineering it is
standard practice to locate power amplifiers as close
to the loudspeaker loads as is possible so that line
losses become negligible. However, in some
applications this is not possible, and the designer
must consider line losses, choosing wire gauges that
will keep to an acceptable minimum.
Figure 7-3 shows the fundamental calculations.
Note that there are actually
two
sources of loss: the
loss in the wire itself and the loss due to the
impedance mismatch that the long wire run can
cause. For example, let us assume an input signal of
8 volts into a nominal load of 8 ohms. With no line
losses the power dissipated in the load would be 8
watts (E
2
/R
L
).
Let us assume that the wire run is 80 meters
and that AWG #10 wire is used. Using the table, we
can see that the wire resistance in one leg will be:
R = 80/300 = 2.6 ohms
and the total round trip resistance in the wire run will
be twice that value.
The voltage across the 8-ohm load will then be:
E
L
= 8/[8 + (2 x .26)] x 8 = 7.5 volts,
and the power dissipated in the load will be:
P
L
= (7.5)
2
/8 = 7 watts
The power loss is then:
Loss (dB) = 10 log (7/8) = 0.58 dB
The general equation for the loss in dB is:
Loss dB = 20 log
R
R + 2R
L
L1
where R
l
is the resistance in each of the two wire
legs, and R
L
is the resistance of the load.
As given here, the loss consists of
two
terms:
the actual loss generated in the wire run and the
added loss incurred due to the impedance mismatch
between the intended load and the actual load.
Good engineering practice dictates that losses
at the load be held to 0.5 dB or less.
7-5