1230 Chapter 33
- The ability to mix at meter zero without the poten-
tial to clip a component farther down the
processing chain. - Optimized SNR in all components in the chain.
- The meter mixer can now serve as an accurate indi-
cator for all subsequent components, since all clip
simultaneously.
As with all things audio, the optimized method is not
without drawbacks. These include:
- It requires more time and expertise to set up.
- It requires a method to determine device clipping
(scope or spectrum analyzer). - It requires the purchase of or construction of pads.
- It makes component substitution more difficult,
since the replacement component may have a
different clipping level than the defective one.
A pad of 5–15 dB may be necessary for a profes-
sional mixer driving a consumer recorder.
Fig. 33-10 shows a system whose gain structure has
been optimized in this manner. The benefit of either
method is that a healthy drive voltage with a good SNR
is delivered to the power amplifier input. Both methods
also assure that any digital components in the
processing chain are being driven with a voltage high
enough to produce optimized A/D conversion.
33.7 Setting the Amplifier Sensitivity
Ideally the amplifier’s input stage should handle the full
output level of the preceding device without clipping. It
is possible for the amplifier input circuit to clip prior to
its output stage. This can be tested by setting the ampli-
fier attenuator at a very low level and observing the out-
put waveform of the amplifier when driven with the full
undistorted signal level of the preceding device. If clip-
ping is apparent at the amplifier output at a low attenua-
tor setting, the input stage is being overdriven. Insertion
of a pad or a reduction in drive voltage will be required.
If an active crossover is in the signal chain, its proper
settings should be established prior to switching on and
setting the amplifier input sensitivity. These settings are
best obtained from the loudspeaker manufacturer.
The amplifier could be calibrated in the same
manner as the other components—simply adjust its
input sensitivity (volume) control to produce an output
signal just short of clipping. Since this may be too loud,
it is better to use a broadband program source (pink
noise or music) and adjust the amplifier for the desired
LP at the listener position. The procedure is as follows.
With the program material being input to the mixer,
the mixer is set to produce a zero meter indication as
previously described. Note that this assumes a VI meter
and not a peak program meter. The input attenuator of
the power amplifier is then advanced until:- The desired acoustic level is reached in the audi-
ence, or - The amplifier begins to indicate clipping.
In either case, don’t turn it up any higher. The gain
structure is complete and the system is producing its
maximum undistorted LP. The technician can now
proceed to fine tuning of the crossover network and
equalizer to finish the system calibration.Figure 33-10. Optimized system gain structure.Microphone Mixer Processor Power amplifier Loudspeaker+26 dBV +20 dBV +29 dBV+20 dBW
at 8 730 dBVDynamic range Dynamic range Dynamic range+20 dBV0 dBV80 dBV6 dB
pad