Meyer Sound CQ-2 Limiting and Protection Circuitry, TruPower Limiting System

Limiting and Protection Circuitry

TruPower Limiting™ System

Conventional limiters assume that the resistance of a speaker remains constant and set the limiting threshold by measuring voltage only. This method is inaccurate because the speaker’s resistance changes in response to the frequency content of the source material and thermal variations in the speaker’s voice coil and magnet. Conventional limiters begin limiting prematurely, which under-utilizes system headroom and deprives the speaker of its full dynamic range.

The TruPower Limiting (TPL) system accounts for varying speaker impedance by measuring current, in addition to voltage, to compute the power dissipation and voice coil temperature. TPL

•allows the speaker to deliver its highest SPL across its entire frequency range during limiting;

•eliminates long-term power compression when operated at high levels for extended periods;

•protects the drivers and extends the lifetime of amplifier and driver components.

Hi Limit and Lo Limit LEDs on the user panel indicate TPL activity for the high and low frequency drivers, respectively. The limiters for each driver function independently and do not affect the signal when the LEDs are inactive. Limiting begins when the driver temperature exceeds the maximum safe level and ceases when the temperature returns to normal.

The CQ performs within its acoustical specifications and operates at a normal temperature if the limit LEDs are on for no longer than two seconds, and off for at least one second. If the LEDs remain on for longer than three seconds, the CQ is hard limiting with the following negative consequences:

•Increasing the input level will not increase the volume.

•The system distorts due to clipping and nonlinear driver operation.

•Unequal limiting between the low and high frequency drivers alters the frequency response.

•Driver and amplifier components are subjected to maximum heat, which shortens their life span.

Troubleshooting with TPL

The TPL LEDs can indicate serious driver problems, if interpreted correctly. If one CQ in a system exhibits substantially more TPL activity than others receiving the same audio signal, then the driver corresponding to the excessively active LED may have a short circuit. This is a potentially dangerous condition for the electronics; shut the CQ down immediately.

The TPL circuit does not activate if there is no power dissipation in the driver, regardless of the input signal level. Therefore, if all CQs in a system receiving the same audio signal exhibit TPL activity except one, then that unit may have an open voice coil; disconnect it and contact Meyer Sound for repair information.

TPL can indicate an imbalance in a configuration of speakers by functioning like a spectrum analyzer. If the speakers in a subwoofer, mid-bass, or mid-hi subsystem begin to limit before reaching the required operating level for the entire system, then that subsystem needs to be supplemented with additional speakers.

NOTE: Although the TPL limiters exhibit smooth sonic characteristics, we do not recommend using them for intentional compression effects. Use an outboard compressor/limiter to compress a mixed signal.

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