Meyer Sound MM-4CEU POWER SUPPLy, AbOUT bRIDGING, CONTROLLING ZONE LEVELS, EqUALIZATION

CHAPTER 3

POWER SUPPLy

Some ampliﬁers are designed so that the internal power

supply rails sag as the load current increases. While the

ampliﬁer’s rated continuous output at a given load may

appear high, its peak output voltage varies with the load

impedance. Because a loudspeaker’s impedance changes

with frequency, such an ampliﬁer will clip prematurely at

low points in the loudspeaker’s impedance curve, and may

deliver too high a voltage at high impedance points. Both

possibilities are potentially damaging to the loudspeaker.

A “stiff” power supply therefore assures the best

performance. One indicator of a stiff supply is that the

ampliﬁer’s wattage rating doubles each time the load

impedance is halved (as shown in Table 3.1). An ampliﬁer’s

supply is sufﬁciently stiff for the MM-4 if its no-load peak

output (at clipping) drops no more than 5 volts when the

ampliﬁer is connected to an 8-ohm load.

AbOUT bRIDGING

When a power ampliﬁer operates in bridged mode, the

polarity of one output is reversed with respect to the other

output, and both are fed the same input signal. The load

is connected between the channel 1 and channel 2 “hot”

terminals.

If you wish to use bridged ampliﬁers in your MM-4 system,

check your ampliﬁer’s documentation to make sure which

input is used for signal, and which is the + output terminal

(positive voltage out for positive input voltage applied).

CAUTION: In bridged mode, an ampliﬁer's

minimum allowable load is twice that of each

output in dual channel conﬁguration. For example,

if an ampliﬁer is rated for a 4-ohm load, then the

minimum in bridged mode should be 8 ohms.

Trying to drive a 4-ohm load with such an ampliﬁer

in bridged mode will cause ��current clipping” and

could damage the loudspeakers.

CONTROLLING ZONE LEVELS

In a typical MM-4 distributed installation, there are two

options for adjusting the level in a coverage zone:

1. The corresponding MM-4CEU “Attn dB” control

(input attenuation), which will adjust the level of all the

ampliﬁers fed by that channel of the processor

2.The corresponding ampliﬁer channel attenuators, which

will adjust the level of all the loudspeakers fed by that

ampliﬁer channel

With the ampliﬁer attenuators set to maximum (making sure

the gain does not exceed 30 dB), ﬁrst use the MM-4CEU

level controls to set zone levels. Where a single MM-4CEU

channel controls several zones (using MultiSense), use the

individual ampliﬁer channel attenuators to adjust individual

zones.

CAUTION: When using multiple ampliﬁers

and if adjustments are made on the ampliﬁer

channels, make sure that the two ampliﬁer channels

with the highest gains are the ones connected to

each of the MM-4CEU's two SpeakerSense inputs.

EqUALIZATION

For best results, Meyer Sound's Galileo 408 or Galileo 616

are recommended; for high-resolution measurement

(strongly recommended), the preferred tool is Meyer

Sound’s SIM®. Equalize zones individually, where adjacent

zones overlap signiﬁcantly (which is not normally preferred

in distributed systems), checking the overlap zone and

adjusting as necessary to provide the smoothest transition

between zones.

When the MM-4 is mounted against a wall surface or in

a ﬂush-mounting bezel, half-space loading may cause

low-frequency extension and boost. In voice-only paging

systems, low-frequency boost is to be avoided, since it

can harm intelligibility. Use the MM-4CEU Lo-Cut switch to

compensate for half-space loading effects.

The Lo-Cut switch may also be used to increase headroom

when the MM-4 is used with subwoofers for music

reinforcement. Consult your subwoofer’s documentation for

detailed information about crossover optimization.

TIP: Systems with subwoofers will almost

always require high-resolution measurement

and equalization to achieve the best results,

because low frequencies are most affected by

boundary conditions and localized acoustic

phenomena such as resonance.