Classé CT-M600 & CA-M600 monoblock power amplifiers
Though I have occasionally been seduced by tube amps, only to fall out of love after a while, the task of providing the necessary grunt to my speakers has been almost entirely assigned to solid-state designs. A quarter-century ago, the original Krell KSA-50 was a constant in my system. This was replaced by a pair of Mark Levinson No.20 monoblocks, through their No.20.5 and No.20.6 incarnations; then, in 1998, I purchased the pair of Mark Levinson No.33H monoblocks that had been reviewed by Wes Phillips in January 1998.
The No.33Hes were in constant use for the next decade, until fall 2008, when one failed. Getting it fixed has been on my to-do list ever since; meanwhile, this terminal procrastinator has been experimenting with a number of possible replacements, the most promising of which was Ayre Acoustics' MX-R monoblock, which Wes reviewed in April 2007.
Then, at the 2010 Consumer Electronics Show, I was introduced to the first of a new series of amplifiers from Montreal-based Classé Audio, the CT-M600 monoblockwhich, at least on paper, looked like a contender.
A Classé Act
Costing $13,000/pair and offering 600W into 8 ohms, the CT-M600 is aimed at the home-theater marketthe "CT" stands for "Custom Theater"and is a mundane-looking black box with a detachable rack-mount front panel that matches the styling of Classé's CT-SSP preamplifier-processor, which recently impressed Kal Rubinson. The beauty resides inside.
One of the reasons I committed to the Krell and Levinson designs was their use of class-A operation for the output stages. 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 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 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. And environmentally it is a disaster, at least in summer, as you waste even more electricity by having to run your air-conditioning to counteract the amplifier's waste heat.
With the CT-M600, Classé took a different approach to ensure that the output transistors operate in a thermally stable environment. Instead of conventional, massive external heatsinks, the CT-M600'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 heattheir total surface area is said to be 31 square feetand a fan draws in air from a concealed slot in the front panel and exhausts it from a rear-panel vent. In itself this is not new, but the key to what Classé calls the Intelligent Cooling Tunnel, or ICTunnel ("icy tunnel"; get it?), 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 output devices thus continually run at their optimal temperature regardless of the signal's voltage and current conditions.
In standby mode, the amplifier consumes only 0.5W. When you first switch on the amp from standby with the front-panel switch, the fan briefly operates at full speed, then turns off to allow the amplifier to reach thermal equilibrium in 11 to 12 minutes. (An infrared thermometer indicated that the output devices remained at a constant 91.4°F, or 33°C.) In the months I used the CT-M600s, I never heard any noise from the fans, even when playing music at party levels. And an advantage of the internal heatsinking is that the amps can be directly stacked, one on the other. This is obviously a major benefit for home-theater systems, but the saving in real estate was very useful even in my music-only room. The only downside to the vertical stacking was that even though the bottom amp was raised from the floor on a short wooden stand, its fan pulled in more carpet fluff and cat hair than the upper amp; after several weeks, the front-panel LED flashed red and blue to let me know the filter needed cleaning. (The CT-M600's protection circuitry also monitors the AC mains and signal input and output parameters and signals problems to the user with the LED.)