TruPower™ Limiting

Background and Development

Introduction

TruPower™ Limiting technology (patent applied for) is the latest development in Meyer Sound's ongoing improvement in self powered speaker systems. It adds to Meyer Sound's previous innovations of SpeakerSense™ and Ferro-oil heat sinking for the transducers.

The TruPower sense system means no power compression even when the system is run for hours. Meyer Sound's self powered speaker systems have full power at the end of a show, when you need it most. They provide greater maximum output capability than any other speaker in their class on the market today.

Technology Review

When Meyer Sound developed SpeakerSense in the late 1970's, it was a significant and major advance in speaker protection. This technology incorporated a controller circuit in the speaker system that measured the amplifier output voltage and controlled the maximum level that supplied the speaker. Meyer Sound was the first manufacturer to incorporate SpeakerSense technology into its speaker systems. (The Meyer Sound M1 Controller for the UM stage monitor was introduced in 1980). This new technology allowed the use of high power amplifiers for ample headroom and offered protection if the system was overloaded.

The system computed the power by sensing the voltage at the amplifier output and fed this signal to the controller via a sense line. When output power was increased to the point where it would overheat the transducer, the sense system lowered the input to the amplifier to a safe level. The Meyer SpeakerSense was self-calibrating and worked with amplifiers with gain ranges of 10 to 30 dB. This was a large improvement over the non-sensing predictive limiters because the power level was minimally affected by the amplifier gain. Predictive limiters required re-calibration every time the gain of the amplifier was changed. Furthermore, calculating power by sensing the amplifier's output voltage made it possible to parallel speakers and still protect the transducers.

In order to calculate power from the output voltage, a fixed impedance was assumed. However, speakers do not have a constant load value over their useful frequency range. The power limiter was set for the average speaker impedance, which meant that there was a possibility that the speaker could be damaged if a continuous tone was applied at the minimum impedance point. To overcome this, an auto-safe circuit was incorporated, which, after a period of time, attenuated the drive level to the amplifier by a few dB to help protect the speakers from long-term overdriving. This circuit would automatically reset when the power was reduced.

With the advent of Meyer Sound's revolutionary introduction of its self-powered products in the fall of 1995, it became possible to achieve greater reliability and yet allow maximum dynamic range because amplifier power and amplifier voltage gain could be optimized as a whole.

TruPower Limiting Technology

TruPower technology is a significant innovation as applied to Meyer Sound's self powered products. TruPower increases the output level and extends the life of the driver under heavy continuous usage by calculating the true power (in watts) delivered to the transducer. With the dedicated amplifier/speaker systems of the self-powered line of products, it becomes practical to calculate the true complex impedance of the transducer on a real-time basis by continuously monitoring the complex voltage and current of the power amplifier.

Performance Advantages of TruPower Technology

Typically, high power systems show 3 to 6 dB of dynamic compression when run at extended high power levels. Have you ever noticed at the beginning of a concert how a traditional, non-powered system will have tremendous dynamic impact, and yet later on, seems to lack punch? This is not necessarily due to your ears compressing. As the speaker coils warm up their impedance rises. As this happens, the current flow is reduced. To achieve the same acoustic power, the voltage must increase proportionately. Conventional limiter topologies are voltage based. If the voltage threshold remains fixed the system compresses.

The TruPower technology is immune to increased impedance, allowing the driver to receive constant power. Because of this more effective power sensing, there is no dynamic compression even when operated at full power for long periods of time. The new Meyer Sound self powered speaker system with TruPower has almost twice the output as a standard Meyer Sound speaker system with SpeakerSense.

No Assumptions

There is very little dispute on the role of a sound reinforcement limiter. It must provide total reliability and yet allow maximum dynamic range. Finding the exact balance point between these two goals is the key parameter of the limiter. To find this point, we must know the exact peak and long term power capabilities of the speaker and we must continually monitor it in order to verify that these limits are not overdriven. Assumption of amplifier or speaker parameters can lead to errors that cause excess compression or component destruction. The evolution of system limiters is one of gradually reducing the assumptions made in the calculation of system power. Meyer Sound's TruPower limiting has removed assumption by continuously monitoring the voltage and current to the speaker and thereby seeing the actual power dissipation.

Summary

Meyer Sound's TruPower offers an extended life over the standard systems when subjected to long term, continuous, high power situations. The TruPower limiter circuit resides on the controller board and can be simply exchanged on most of the Meyer self-powered systems if you purchase a TruPower upgrade kit from the factory.