Handbook for Sound Engineers

1228 Chapter 33

cating meter describes the waveform in a way that
correlates with its loudness as perceived by human
listeners. The indication of such a meter is in VU, or
volume units. When sound systems used the impedance
matched interface, this voltage was developed across
the input impedance (usually 600:) of the next input
stage. Since voltage and impedance were known, the
power equation could be used to calculate the output
power of the mixer, which became

The power transfer was relevant since the matched
interface was used, optimizing the circuit for power
transfer. One milliwatt provided a useful reference, as it
falls in the middle of the range of power levels found in
the sound system. A voltmeter calibrated to read zero at
0.775 V could directly indicate the circuit power level
in dBm (assuming a 600: matched impedance inter-
face). This calibration would make the voltmeter a dBm
meter when placed across a 600: circuit. When dBm
meters are used at other impedances, a correction factor
is required. As the sound reinforcement industry
migrated to the constant voltage interface, the 0.775 V
reference lived on due to the proliferation of voltmeters
so calibrated, and signals were then described in dB ref.
0.775 V or dBu. In modern systems, the term level is
used to describe the field quantity of interest at a
component interface, which is the signal voltage for the
constant-voltage interface. This a good place to note
that many modern mixers do not use the +4 dBu refer-
ence level for meter zero, so the reader is advised to
consult the literature or perform a measurement. A more
common meter zero level today is 0 dBV, or 1 Vrms.
Let us now advance the trim control (or the drive
voltage) until the waveform becomes distorted when
viewed on the scope. Some mixers have a clipping indi-
cator to warn of this condition. When the waveform
flattens on top, reduce the trim control until the wave-
form appears undistorted. Since mixers are made up of
several stages, it is usually informative to move each the
main fader, channel fader, and trim control until clip-
ping is observed to assure that each stage is clipping
simultaneously. This produces the maximum output
voltage of the mixer at the mixer’s output terminal.
Using the scope or a voltmeter, measure the voltage of
the waveform. Note that the clipping occurs on the peak

of the waveform, yet it is standard practice to measure

the rms value of the waveform and include it on the

specification sheet. Ideally, this maximum output

voltage is at least ten times the voltage measured at the

meter zero indication, providing 20 dB of peak room

above meter zero. The drive level (or trim control

setting) should now be reduced to produce the meter

zero operating level of the mixer.

We now have knowledge of the operating and clip-

ping level of the mixer (e.g., +4 dBu and +24 dBu

respectively). These values should be recorded in the

system documentation. The noise floor of the mixer can

be measured by muting the input signal and measuring

the mixer’s no signal output voltage, but this is of little

interest in practice.

33.5 The Unity Method

Our mixer is now at an optimum operating level with

good SNR and 20 dB of peak room. The signal from the

mixer is fed to the next component in the chain. If the

component has input and/or output level controls, they

are adjusted to produce the same level of the mixer at

that component’s output terminal. In like manner the sig-

nal is fed through subsequent signal processors, and the

mixer’s level eventually ends up at the input of the

power amplifier, whose input sensitivity control is set for

the desired output voltage (i.e., the target playback level

of the system). As the amplifier’s voltage is impressed

across the loudspeaker load, the amplifier supplies cur-

rent flow as determined by the impedance of the loud-

speaker. Power will flow, but the signal level is a linear

function of the applied voltage over the useful operating

range of the amplifier. So, the output voltage is the

parameter of interest in a properly configured ampli-

fier-to-loudspeaker interface under normal operating

conditions. Fig. 33-9 shows such a processing chain for a

mixer with a 0 dBV meter zero. The unity amplification

method has a number of advantages, which include:

1. Ease of calibration.


2. Fast implementation.


3. Easy substitution of components.

Unfortunately, there are some drawbacks to this

approach, mostly due to the nonstandardization of clip-

ping levels between product lines and manufacturers. A

mixer operating at 0 dBV that clips at +20 dBV will

have 20 dB of operating peak room for transient peaks. If

the component after the mixer clips at +18 dBV, that

component will only have 18 dB of operating peak room.

In this case an undistorted full-scale waveform from the

mixer would cause clipping in the next component. The

W E

2

R

----- -=

Lout 10 1.23

2

600

= log------------

=4 dBm