Meyer Sound Documentation

The 750-LFC is available with XLR 5-pin (Figure 14) or 3-pin connectors, shown in the figures below, for audio Input and audio Loop output. XLR 5-pin connectors accommodate both balanced audio and RMS signals.

XLR 5-Pin Audio Connectors, Input and Loop Output

XLR 3-Pin Audio Connectors, Input and Loop Output

Audio Input (XLR 3-Pin or 5-Pin Female)

The XLR 3-pin or 5-pin female Input connector accepts balanced audio signals with an input impedance of 10 kΩ. The connector uses the following wiring scheme:

• Pin 1 — 1 kΩ to chassis and earth ground (ESD clamped)

• Pin 2 — Signal (+)

• Pin 3 — Signal (–)

• Pin 4 — RMS (polarity insensitive)

• Pin 5 — RMS (polarity insensitive)

• Case — Earth (AC) ground and chassis

Note

Pins 4 and 5 (RMS) are included only with XLR 5-pin connectors.

Pins 2 and 3 carry the input as a differential signal. Pin 1 is connected to earth through a 1 kΩ, 1000 pF, 15 V clamped network. This circuitry provides virtual ground lift for audio frequencies while allowing unwanted signals to bleed to ground. Make sure to use balanced XLR audio cables with pins 1–3 connected on both ends. Telescopic grounding is not recommended and shorting an input connector pin to the case may cause a ground loop, resulting in hum.

Tip

If the loudspeaker produces unwanted noise or hiss, disconnect its input cable. If the noise stops, there is most likely nothing wrong with the loudspeaker. To locate the source of the noise, check the source audio, AC power, and electrical ground.

Audio Loop Output (XLR 3-Pin or 5-Pin Male)

The XLR 3-pin or 5-pin male Loop output connector allows multiple loudspeakers to be looped from a single audio source. The Loop output connector uses the same wiring scheme as the Input connector (see Audio Input (XLR 3-Pin or 5-Pin Female). For applications that require multiple 750-LFC loudspeakers, connect the Loop output of the first loudspeaker to the Input of the second loudspeaker, and so forth.

Note

The Loop output connector is wired in parallel to the Input connector and transmits the unbuffered source signal even when the loudspeaker is powered off.

Calculating Load Impedance for Looped Audio Signals

To avoid distortion when looping multiple loudspeakers, make sure the source device can drive the total load impedance of the looped loudspeakers. In addition, the source device must be capable of delivering approximately 20 dBV (10 V rms into 600 Ω) to yield the maximum SPL over the operating bandwidth of the loudspeakers.

To calculate the load impedance for the looped loudspeakers, divide 10 kΩ (the input impedance for a single loudspeaker) by the number of looped loudspeakers. For example, the load impedance for ten 750-LFC loudspeakers is 1000 Ω (10 kΩ /10). To drive this number of looped loudspeakers, the source device should have an output impedance of 100 Ω or less. This same rule applies when looping 750-LFC units with other Meyer Sound self-powered loudspeakers.

Note

Most source devices are capable of driving loads no less than 10 times their output impedance.

Tip

Audio outputs from Meyer Sound’s Galileo GALAXY Network Platform are rated at 50 Ω. This rating allows each output to drive up to 20 Meyer Sound (10 kΩ) loudspeakers without distortion.

Caution

Make sure that all cables for looped loudspeakers are wired correctly (Pin 1 to Pin 1, Pin 2 to Pin 2, and so forth) to prevent the polarity from being reversed. If one or more loudspeakers in a system have reversed polarity, frequency response and coverage will be significantly degraded

Two cable rings, shown in the figure below, are provided on the rear of the 750-LFC cabinet. Power and audio cables should be tied off to the rings to reduce strain on the cables and prevent damage to them during installation.

Cables Tied Off to Cable Ring

750-LFC loudspeakers employ Meyer Sound’s advanced TruPower® limiting. Conventional limiters assume a constant driver impedance and set the limiting threshold by measuring voltage alone. This method is inaccurate because driver impedances change as frequency content in the source material changes and as thermal values for the loudspeaker’s voice coil and magnet vary. Consequently, conventional limiters often begin limiting prematurely, which reduces system headroom and dynamic range.

In contrast, TruPower limiting anticipates varying driver impedances by measuring both current and voltage to compute the actual power dissipation in the voice coil. This approach improves performance, both before and during limiting, by allowing the driver to produce the maximum SPL across its entire frequency range, while also retaining signal peaks. TruPower limiting also eliminates power compression at high levels over lengthy periods, which helps regulate voice coil temperatures, thereby extending the life of the driver.

LF Limit LED

The 750-LFC low-frequency driver is powered by two amplifier channels, one for each voice coil, which are routed to a single limiter. When a safe power level is exceeded in either channel, limiting is engaged for both channels and the LF Limit LED ([→ _bookmark30]Figure 15) lights on the user panel (the HF Limit LED is disabled for the 750-LFC).

750-LFC Limit LEDs (HF LED Disabled)

When engaged, the limiter not only protects the driver but also prevents signal peaks from causing excessive distortion in the amplifier channels, thereby preserving headroom and maintaining smooth frequency response at high levels. When levels return to normal—below the limiter threshold—limiting ceases.

The 750-LFC loudspeaker performs within its acoustical specifications at normal temperatures when the LF Limit LED is unlit, or when the LED is lit for 2 seconds or less and then turns off for at least 1 second. If the LED remains lit for longer than 3 seconds, the loudspeaker enters hard limiting where:

• Increases to the input level have no effect

• Distortion increases due to clipping

• Drivers are subjected to excessive heat and excursion, thereby compromising their lifespan

Caution

The Limit LED indicates when a safe, optimum level is exceeded. If a 750-LFC loudspeaker system begins to limit before reaching the desired SPL, consider adding more units to the system.

The 750-LFC loudspeaker employs natural convection in its cooling system. The amplifier’s heat sink provides natural convection cooling from the air flowing near its fins. When exposed to high ambient temperatures or when driven continuously at high output levels, an ultra-low-noise, variable-speed fan circulates air internally to keep the 750-LFC cabinet at a safe operating temperature.

During normal operation, when the 750-LFC is powered on, the On/Status LED is solid green. If the loudspeaker encounters a hardware fault, or the unit begins to overheat, the LED flashes red. In some instances, the loudspeaker will continue to output audio while the LED flashes red, though with a reduction in the limiter thresholds (and gain) to protect the loudspeaker.

If a loudspeaker is overheating (for RMS-equipped loudspeakers, you can verify this situation in Compass RMS), a reduction in SPL may be necessary. If, after a reduction in SPL and an appropriate cooling period the On/Status LED continues to flash red (does not return to solid green), contact Meyer Sound Technical Support.

If the On/Status LED flashes red and the loudspeaker does not output audio, contact Meyer Sound Technical Support immediately.