Solid State Power Amp Reviews

The Carver Lightstar Reference is a power amplifier whose ratings suggest that it is an ideal voltage source—meaning that its rated power doubles with each halving of load impedance (300W into 8 ohms, 600W into 4 ohms, 1200W into 2 ohms). That's difficult to accomplish with conventional power transistors, which operate safely only within a restricted range of voltage/current combinations. (The graph of this range is called the transistor's safe operating area.) Many amplifiers deal with this limitation by using several transistors wired in parallel to share the output current. This works, but multiple devices usually don't have exactly identical characteristics, and some designers feel that a single set of output devices per channel can sound slightly better.

Delivering high power into normal impedances requires high output voltages. To provide these without violating each MOSFET's 30V limit, the positive and negative power-supply voltages track up and down together with the audio signal, while the voltage difference across the MOSFETs is kept within a safe 10V range. In effect, all of the voltages in the output stage "float" up and down with the audio signal, while the high-current MOSFETs are free to provide virtually limitless current to drive a loudspeaker's voice-coil. While the incoming audio signal is fed to the MOSFETs to produce the output current, it also drives a pair of PWM (pulse-width modulation) amplifiers that modulate the power-supply voltages up and down. The result is a purist output stage that can provide both high current and a wide voltage range regardless of the speaker's impedance.

One potential difficulty remained: Music is full of transients, and speakers respond to transients by generating "back-EMF" pulses that must be absorbed by the amplifier without affecting its sound or its safety. Feedback diodes in the Lightstar provide a return path for back-EMF energy, which is fed back into the power supply.—Peter W. Mitchell