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ChAPTER 3: ChOOSING A POWER AmPLIfIER
The MM-4 is designed for high-quality distributed sound
systems. To realize the MM-4's full capabilities, a direct-
drive power amplifier capable of 49 volts rms (70 volts
peak) at the rated load impedance presented by the line
configuration is required. The MM-4 presents a nominal
load of 16 ohms, and each unit must be wired in parallel
with the line. As shown in Table 3.1, the number of MM-4
loudspeakers on a given signal branch can be used to
determine the required power rating for the corresponding
amplifier.
Table 3.1 Amplifier Power
Number of MM-4 Loudspeakers
1 (16 Ω load)
2 (8 Ω load)
3 (5.3 Ω load)
4 (4 Ω load)
8 (2 Ω load)
CAUTION:
The total load of the MM-4
!
loudspeakers connected in parallel should
not be lower than the minimum allowable load of the
amplifier. Otherwise, damage to the amplifier and
loudspeakers may result.
The following sections discuss important criteria for
choosing and configuring power amplifiers for use with an
MM-4 and MM-4CEU distributed system.
GAIN
The gain setting of the power amplifier is crucial to
system performance: if the gain is set too low, "upstream"
equipment may clip before the amplifier reaches full power.
Conversely, if the gain is set too high, not only will system
noise increase, the amplifier is capable of overcoming
the MM-4CEU's protection circuits and may damage the
loudspeakers.
The MM-4CEU is designed to work with amplifiers having
gain of between 10 dB and 30 dB.
NOTE:
The optimal amplifier gain setting for
the MM-4CEU is 20 dB.
The gain of an amplifier can be measured with an rms volt
meter by feeding the amplifier with a tone generator at
1 kHz and measuring the input voltage going in (V in) and
the voltage delivered at the output (V out) and applying the
following expression:
Gain (in dB) = 20 Log (V out / V in)
Continuous Power Rating
150 W
300 W
450 W
600 W
1200 W
NOTE:
Some manufacturers express the gain
of their amplifiers in voltage gain as a factor
instead of gain in dB. To covert the voltage gain in
dB, use the following equation:
Gain (in dB) = 20 Log (Voltage Gain)
For example, a voltage gain of "x20" equates to
26 dB of gain.
PROTECTION CIRCUITRy
Avoid amplifiers with extra "sensing" connections for load
protection; the MM-4CEU incorporates all the protection
circuitry necessary for reliable performance. A basic,
reliable, professional grade amplifier is all that is required.
The amplifier must be AC coupled to the load (i.e. protected
from producing DC at its output). The MM-4 is directly
coupled to the distribution line, and any significant DC
voltage on the line may damage the driver. A relay or
crowbar circuit to open or short the amplifier output in fault
conditions are preferred.
ELECTRICAL ChARACTERISTICS
Slew rate should be at least 10 volts/microsecond; 30 volts/
microsecond or better is preferred.
The amplifier must be stable under all load conditions
— shorted, open, inductive or capacitive. Hum and noise
should be such that when the amplifier input is shorted,
noise from the loudspeaker is indiscernible. Distortion (THD
or IM) should be equal to or better than 0.01 percent.
Balanced inputs are required. Input connectors should be
XLR-type (preferred) or TRS (tip, ring, sleeve) type 1/4-inch
phone female connectors.
The MM-4CEU is a balanced pass-through device
(balanced signal in - balanced signal out) and therefore has
no effect on the polarity of the signal it receives. Amplifier
XLR polarity, along with its output wiring, will determine the
polarity of the entire system. For example, an amplifier with
input as Pin 2 (+) and Pin 3 (-) on the XLR connector, with
the output terminal (+) of the amplifier connected to the
terminal (+) of the MM4 loudspeaker connector, will produce
a positive pressure pulse when a positive voltage pulse is
applied to Pin 2 of the MM-4CEU input XLR.
CHAPTER 3
9
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