• The Class-A mode does not (unlike some designs) clip abruptly if faced with an unusually low impedance load. It instead moves seamlessly and without audible disturbance into Class-AB operation.
• One of the disincentives to building a Class-A amplifier is that a mistake in the quiescent-control circuitry can let through a huge fault current that destroys all the output devices in an instant. The same applies to bias-generators in Class-B, but these are normally much simpler and the chances of getting something disastrously wrong is much less.
  With this in mind the Trimodal amplifier includes a safety-circuit to limit the current flowing even if the Class-A controller is completely non-functional. The safety-circuit consists of very few components so it can be meticulously checked in very little time.
• The radical change in internal conditions from B to A is accomplished completely silently, while the amplifier is operating. Direct comparison between the two modes is extremely simple.
• The Trimodal principle allows you to experiment with Class-A with confidence. Many constructors of Class-A amplifiers find that the resulting unit gets worryingly hot, even if the heatsinks are enormous compared with a Class-B design of the same power. The Trimodal allows you to experience the highest grade of Class-A reproduction without committing yourself to it. The alternative of superb Blameless Class-B operation is always available at the touch of a switch.
• Optional input bootstrapping to raise input impedance.
• Full overload and short-circuit protection.
• Fixing holes and TO3 positions provided for direct mounting to a heatsink.
• High-quality single-sided PCB, roller-tinned with solder mask and full screen print. Board size 135 by 162 mm.