approximately -50dB (2.45mV) for a microphone,
+4dB (1.23 volts) for a line level signal. The exact volt-
age is not critical, and 1000Hz is a standard reference
frequency, but any other appropriate frequency can
be used.
2. Set the input channel level control on the mixer
at its rated "nominal" setting, and adjust the master
level control so that the output level is 20dB below the
rated maximum output level for the mixer. For the
Yamaha PM-180 Mixer used in the example, the
maximum rated output level is +24dB (12.3 volts), so
the output level should be adjusted to +4dB (1.23
volts), as indicated either on an external voltmeter, or
on the mixer's VU meter (0VU).
3. Assume that the rated maximum input level for
the graphic equalizer in the example is +14dB (3.88
volts). Subtr4dB from +14dB leaves only 10dB
of headroom, so a 10dB resistive pad must be inserted
between the mixer output and equalizer input. Now, the
signal level at the input to the equalizer should be -6dB
(388mV), which can be confirmed with a voltmeter.
4. Assume that the maximum rated output level of
the equalizer in this example is +18dB (6.16 volts).
Adjust the master level control on the equalizer so that
the output level is 20dB below this rated maximum, or
-2dB (616mV). Since the equalizer has no VU meter,
you need an external voltmeter to confirm this level.
5. Finally, starting with the attenuators on the
P-2200 at maximum attenuation (maximum counter
clockwise rotation), slowly rotate them clockwise,
watching the peak reading meters. When the peak
reading meters indicate 2.3 watts output from the
P-2200, there is 20dB headroom left before clipping.
To operate this system, use only the controls on the
mixer, and avoid levels that consistantly peak the mixer's
VU meter above the "zero" mark on its scale, or that
peak the P-2200's meters above a safe power level for
the speaker system. Any adjustments of the other devices
in the system will upset the headroom balance. However,
the P-2200's calibrated attenuators allow easy setups
and quick changes, if you decide to change the headroom
figure. They also allow you to momentarily fade the
entire program or a single channel and to later bring it
back up to exactly the same level.
To use this technique with any system, first design
the required speaker system, and calculate the number
of power amplifiers needed to safely operate the
speaker system with adequate headroom. Then, choose
the mixer, and other devices that feed the power
amplifiers, and set up the system according to the above
instructions.
In some cases, it may be useful to set up different
headroom figures in different parts of a complex
system. For example, background music and paging
should be severely compressed in a noisy lobby area,
but the same program material would sound more
natural in less noisy office and auditorium areas of the
same installation if the headroom figure were increased.
By placing a compressor/limiter in the circuit just before
the P-2200 that feeds the lobby areas, the headroom
figure can be lowered for that section only, without
affecting other parts of the system.
Cabling the System
Audio circuits may be divided into the following
classifications (by signal level):
1 . Low level circuits: any circuit carrying signals of
-80dB (77.5 microvolts) to -20dB (77.5 millivolts),
example: microphone lines.
2. Medium or line level circuits carrying signals of
-20dB (77.5mV) to +30dB (24.5 volts), example:
mixer outputs.
3. High level circuits carrying signals above +30dB
(24.5 volts), example: speaker lines.
4. AC power circuits, including lighting circuits.
5. DC control (or supply) cables to relays, from
batteries, etc.
Generally, each of these categories should be
physically separated from the others to avoid crosstalk,
oscillation, and noise spikes. One possible exception is
that DC control or supply cables and line level signal
cables can be routed together if the DC signal is
adequately filtered. Figure 45 shows the undesirable
results that can occur if line or speaker cables are placed
near microphone cables. This situation occurs in concert
sound when mixer outputs and mic inputs feed through
the same "snake" cable.
Fig. 45 - Example of Crosstalk
Figure 46 shows an equipment rack with a good
cable layout. Note that the different categories of cable
are carefully separated, and that where it is necessary
to cross two categories, they cross perpendicular to each
other. These suggestions apply to all types of systems,
portable as well as permanent.
Fig. 46 - Cable Routing in Equipment Rack. (Reprinted
from Sound System Engineering by Don & Carolyn Davis
published by H. W. Sams Co.)
Figure 47 shows the rear of a P-2200 amplifier with
its two inputs "chained" using a phone-to-phone cable.
In this mode, the signal fed to the first side is also fed to
the second side of the amplifier. This could also be
accomplished with an XLR-to-XLR cable.
For low and medium level balanced signal cables, use
good quality twisted pair shielded cable. For portable
