Solid State Power Amp Reviews

Sidebar 1: A Class-A Icy Tunnel

Up to their continuous-current limit, a class-A amplifier's output transistors are fully turned on all the time. As well as the devices thus being in thermal equilibrium and therefore relatively immune from having their transfer function thermally modulated by the signal current, the nonlinearities that result from switching the devices on and off during every cycle of the signal—generally referred to as crossover distortion—are sidestepped. Electronically, this is an elegant solution; practically, it is the opposite, in that two-thirds of the power drawn by the amplifier from the wall is wasted in the form of heat.

Classé has take a different approach to ensure that the output transistors operate in a thermally stable environment. Instead of conventional, massive external heatsinks, the Delta Mono's devices are attached to an aluminum tunnel that runs the entire depth of the chassis. The inside surface of the tunnel is stuffed with multiple aluminum fins of relatively low mass to dissipate heat—and a fan draws in air from a concealed slot in the front panel and exhausts it from a rear-panel vent. The key to what Classé calls the Intelligent Cooling Tunnel, or ICTunnel ("icy tunnel"), is to take advantage of the low thermal mass of the heatsink array inside the tunnel by allowing a microcontroller, fed by pressure and temperature sensors, to actively control the operating temperature. The Delta Mono's fan mechanism is 35mm thicker than that used in earlier amplifiers and can spin at the lowest speed necessary. The output devices therefore continually run at their optimal temperature regardless of the signal's voltage and current conditions.—John Atkinson