For decades, the class AB output stage set the standard for high audio quality in power amplification. Although Class AB amplifiers are known for excellent sound, they do not use power efficiently. A great deal of heat is dissipated by the output stages, requiring large heat sinks and bulky fans to keep the output transistors inside safe temperatures. Larger power supplies are needed to produce power eventually dissipated as heat.
Two other familiar approaches to power amplification offer improved efficiency, though each comes with drawbacks. Class D switching amplifiers employ pulse width modulation (PWM) techniques to achieve very high efficiency, but most implementations of “pure” class D involve compromises in sound quality. Class H designs boost efficiency by modulating the power supply voltage with the input signal, “tracking” the input in order to provide the instantaneous voltage needed for optimum operation of the output devices. However, maximum efficiency with class H is achieved only within a relatively limited dynamic range.
The fundamental concepts behind both class D and class H amplifiers have been in the literature for decades, and are not covered by patents. However, specific circuits implementing these concepts can be patented, as is the case with Lab.gruppen’s breakthrough Class TD amplifier topology. The TD stands for “tracking class D,” which means that the power supply tracks the audio signal at all frequencies, supplying the required rail voltage while at the same time reserving additional headroom. The high-speed switching principles of class D are employed as well, though the final output stage remains a proven class AB component. The entire audio path remains analog, with the signal never converted to digital pulses and then filtered as in some class D designs. Very high efficiency is achieved without the ripple effect typical of a PWM output stage.
Lab.gruppen’s Class TD works perfectly under all load conditions. The output maintains its flat frequency response even into complex loads with nominal impedances as low as 2 ohms. Outputs may be bridged, reliability is very high, and there is no interference with nearby RF equipment. Superior efficiency allows greater power density while minimizing cooling requirements, yet sound quality matches that of the best class AB design.