Power Amplifiers

Classe CA-M600 Monoblock Power Amplifier


The Design

I have always enjoyed the Classé sound, having used their CA-5200 in our home theater lab for years. The CA-5200 was updated last year to become the CA-5300, a member of the new Delta series, adding 100 watts of output to each of the five channels. The CA-M300, which output 300 watts, was also introduced in the Delta series, and the product under review here, the CA-M600, contains two CA-M300 circuits to give the combined output of 600 watts RMS. The CA-M600 is fully balanced from input to output.

The output stage uses 18 devices (bipolar) per side (push-pull) for a total of 36. The 2.35 kVA (2,300 watts) toroidal power supply transformer and 134,400 µF of power supply capacitance give the amplifier plenty of room for transient demands. The amplifier is biased about 3 watts into Class A. The amount of Vbias applied is designed to fit the point where conduction is close to 180° but not at the expense of increasing crossover distortion due to rapidly increasing transconductance (gm).

There are only two gain stages, the voltage (input) and current (output), with a driver (no gain) interfacing the two gain stages. The output signal is in phase (non-inverted) with respect to the input. In general, the lower the number of gain stages, the better, because it means less components (transistors or tubes) that the signal has to pass through.

If you look at the photo of the front of the CA-M600 on Page 1, you can see a square black plate near the left side. The plate is actually about 0.25" in front of the main panel, and is open on all sides, being attached to the main panel at the corners. This is where the air from the fan is drawn into the chassis. As I mentioned, the temperature is maintained at an optimum point by feedback of the circuit board temperature to the fan control. Even with the room dead quiet, I could not hear the fan, and could only feel a soft breeze at the sides of the front plate. It is a very good idea, as heat is not only one of the biggest enemies of electronic circuits, but some warmth is desirable for optimum performance, and the Classé innovation achieves this.

A close-up of the fan port is shown below.

The rear panel has two pairs of five-way speaker binding posts, a grounded AC receptacle, a fan, one each XLR and RCA input (the unit comes with pins 1 and 3 connected with a U pin, which you remove if you want to use the XLR input), and on the left side, connections for triggers, USB, RS-232, and a BUS for connecting other Classé products together.

The inside of the chassis, shown below, illustrates the tunnel through which air passes from the fan on the front (left side) to the exit (right side) in the direction of the red arrows. The tunnel is the main gateway for heat dissipation, and therefore, the rest of the amplifier, including all the circuitry, is open only to small vents on the top and bottom of the enclosure. Because these vents don't have to be large in order to allow heat to be dissipated, very little dust can get onto the surface of the circuit boards. The output devices are mounted on the surface of the tunnel for cooling, as they get especially warm during use.