System Gain Structure 1229mixer could be turned down a bit if the overload is not
severe. If the level mismatch is more than a few dB then
a different solution may be required. It should be pointed
out that this condition is not as prevalent as it once was,
as many postmixer product manufacturers have modified
their products to handle the higher output voltages
produced by modern mixing consoles.
33.6 An Optimized Method
The drawbacks of the unity amplification method can be
overcome with an optimized method of establishing the
gain structure of the system. While the unity method
establishes a consistent operating voltage from compo-
nent to component, the optimized method sets each
device to clip simultaneously, regardless of the actual
signal level. A mixer outputting +24 dBV and an equal-
izer outputting +20 dBV are both set to reach their clip-
ping point simultaneously. This method often requires
the insertion of a resistive attenuator between the mixer
and equalizer, allowing the mixer to output its maxi-
mum voltage and still not over-drive the equalizer.
To optimize the system gain structure, feed a sine
wave to the mixer in the same manner previously
described, but this time advance the trim control until
clipping occurs at the mixer output. All power ampli-
fiers should be off or fully attenuated. The clipping can
be determined with the aid of an oscilloscope or a spec-
trum analyzer capable of handling at least +30 dB ref.
1 V (+30 dBV). With the mixer set just short of clip-
ping, connect the output of the mixer to the input of the
next component (i.e., an equalizer). Set all controls on
the equalizer at their unity setting. Move the clipping
indicator (scope) to the output of the equalizer and note
whether the waveform is clipped. If it is not, the equal-
izer is capable of passing the full output voltage of the
mixer. If the equalizer is clipping, first try reducing the
setting of its input level control. This often doesn’t work
since the stage being overdriven likely precedes the
level control stage. Some manufacturers design their
equipment to handle higher input levels than they can
output, and the input level control may indeed elimi-
nate the overdrive condition. If not, an attenuator is
placed between the mixer and equalizer and adjusted to
produce an undistorted waveform from the equalizer.
The same procedure is repeated for each subsequent
piece of equipment in the processing chain.
Compressor/limiters should be set at their highest
threshold setting and lowest compression ratio. Cross-
over networks require either:- Selection of a sine wave within the pass band of the
output being tested, or - Readjust the crossover frequency control to allow
the output being tested to pass the test signal.
Remember to restore the setting before turning on
the amplifier!
Resistive pads for the “too hot” source are available
commercially. If a component is being underdriven by
the previous one (i.e., the full output voltage of the
mixer is insufficient to clip the equalizer), it may be
advantageous to increase the underdriven component’s
input level until just short of clipping. This will provide
a stronger drive voltage to the next component (and
possibly an improved signal to noise).
The advantages of the optimized method include:Figure 33-9. The processing chain.Microphone Mixer Processor Power amplifier Loudspeaker0 dBV 0 dBV +29 dBV+20 dBW
at 8 750 dBVDynamic range Dynamic range Dynamic range+20 dBV0 dBV80 dBV