CHAPTER 2

M2D-SUB AMPLIFICATION

The M2D-Sub loudspeaker is powered by the Meyer Sound HP-2/M2D-Sub amplifier, a high-power two- channel amplifier. The amplifier utilizes complementary MOSFET output stages (class AB/H) capable of delivering 2250 watts total. All the specific functions for the M2D- Sub loudspeaker such as crossover points, frequency and phase response, and driver protection are determined by the control card installed inside the amplifier.

M2D-SUB LIMITING

The M2D-Sub loudspeaker uses Meyer Sound’s advanced TruPower limiting system.

Conventional limiters assume a constant loudspeaker impedance and therefore set the limiting threshold by measuring voltage only. However, this method is inaccurate, because the driver's impedance changes in response to the frequency content of the source material and thermal variations in the driver's voice coil and magnet. Consequently, conventional limiters begin limiting prematurely, which under-utilizes system headroom and lessens the driver's dynamic range.

In contrast, TruPower limiting accounts for varying driver impedance by measuring current, in addition to voltage, to compute the actual power dissipation in the voice coil. TruPower limiting improves performance before and during limiting by allowing each driver to produce maximum SPL across its entire frequency range. In addition, TruPower limiting eliminates power compression when the system is operated at high levels for extended periods, and also extends the driver life cycle by controlling voice coil temperatures.

The actual power is monitored for each of the two amplifier channels. When the safe continuous power level is exceeded in any channel, the TruPower limiter controlling both amplifier channels engages. Limiting activity is indicated by the Sub Limit LED on the user panel (Figure 2.5).

Figure 2.5. The M2D-Sub’s LED indicators

The M2D-Sub loudspeaker performs within its acoustical specifications and operates at a normal temperature if the Sub Limit LED is lit for no longer than two seconds, and

then go off for at least one second. If the LED remains on for longer than three seconds, this indicates that the amplifier is incurring hard limiting that can result in the following negative consequences:

Increasing the input level will not increase the volume.

The system distorts due to clipping and nonlinear driver operation.

The lifespan of the drivers is reduced because they are subjected to excessive heat

Each low-frequency driver is driven by a separate amplifier channel but is routed to one limiter; the Sub Limit LED on the user panel indicates TruPower limiting activity for the drivers. The Sub Limit LED indicates when the safe power level is exceeded (Figure 2.5).

CAUTION: While the limiters protect the system under overload conditions

and exhibit smooth sonic characteristics; we recommend that you do not drive the M2D-Sub loudspeaker into continuous limiting. If an entire system of M2D-Sub loudspeakers begins to limit before reaching the required sound pressure level, you should consider adding more M2D-Sub loudspeakers to the system.

M2D-Sub Excursion Clamp

The drivers in the M2D-Sub are protected by an excursion clamping circuit that provides instantaneous braking

for the drivers without the pumping effects commonly produced by compressor/limiters.

The circuit uses sophisticated filters to minimize the distortion normally caused by clamping and clipping. As the M2D-Sub’s input signal is increased past the clamping point at each frequency, the output signal remains at a fixed level for that frequency, protecting the drivers and minimizing negative sonic effects. The Exc.Clamp LED, shown in Figure 2.5, illuminates when the maximum allowed peak voltage at each frequency is reached.

This circuit works for all frequencies, not just very low frequencies where the drivers are more vulnerable to overexcursion.

The limiters cease operation when the power level in the channel returns to normal. Limiters have no effect on the signal when the LED is inactive

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