Power Amplifiers

Classe CA-M600 Monoblock Power Amplifier

ARTICLE INDEX

Introduction

Classé updated their Delta power amplifier series in late 2010, including the CA-M400 which is now the CA-M600. The entire amplification signal is conducted on a single circuit board, and they have introduced a new cooling system called ICTunnel™, which utilizes a fan in the front at the air intake where the cover is flush with the front panel. The air flows around an offset cover plate and past specially spaced fins which are inside the chassis and which provide about 31 square feet of surface area, which makes the cooling system very efficient.

The air does not flow over the circuit boards, because this would result in dust from being pulled onto them, which would degrade performance over time. The circuit boards attach to the outside of the cooling tunnel and air passing through the tunnel draws heat away from them into the fins. In use, I found the fan to be whisper quiet, with just a gentle breeze coming from the front amplifier port. This kept the entire chassis cool, but also, the temperature of the circuit board is electronically controlled by adjusting the fan speed, to optimize the performance, which turned out to be stellar.

Specifications

  • Design: Monoblock Power Amplifier; Class AB
  • Power: 600 Watts RMS into 8 Ohms, 1,200 Watts RMS into 4 Ohms
  • MFR: 1 Hz - 100 kHz, -3 dB
  • THD+N: 0.002% Using XLR Input; 0.004% Using Unbalanced RCA Input
  • Input Impedance: 50 kOhms
  • S/N: - 120 dB Peak Output into 8 Ohms
  • Inputs: XLR Balanced, RCA Unbalanced, Triggers, USB, RS-232, BUS
  • Dimensions: 8.8" H x 17.5" W x 17.5" D
  • Weight: 88 Pounds/each
  • MSRP: $14,000/pair USA (CT-M600 is the Rack Mountable Version and is $13,000/pair USA)
  • Classé Audio

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.


In Use

For the listening tests, I used a McIntosh MCD-500 SACD player, OPPO BDP-93NE universal player, OPPO BDP-95 universal player, VPI HR-X turntable with Sumiko Blackbird MC cartridge, Manley Steelhead phono preamplifier, Lamm LL1 preamplifier, Balanced Audio Technology preamplifier, Carver Mark IV ribbon speakers, and Thiel CS3.7 speakers. Cables were Emotiva, Marc Audio, and Legenburg.

Wow, this amp is a smooth talker. All the detail was there, but it wasn't in my face like some amplifiers are. I cranked it up with some albums, and I turned it down low with others (room lighting adjusted accordingly), and at one point, as Stan Laurel said in one of their Laurel and Hardy short films, "I woke up and found myself asleep."

The CA-M600 is as comfortable with classical jazz, thunderous symphonies, or candlelight and chocolate covered strawberries easy listening music.

This power amplifier was a great combination with my vinyl setup, and I noticed that the ride cymbals, which can often sound gritty with amplifiers that have significant IM distortion, sounded perfect with the Classé.

Of course, I tried out my amp-buster, Copland's "Fanfare for the Common Man" on Telarc, but the Classé didn't flinch with room filling volume and teeth rattling bass.

Copland Fanfare

Although this Buddy Rich recording is old (Pacific Jazz/Blue Note), it was still perfectly clear that he knew how to shred. Impeccable music should be played on equipment built to that same standard. I heard Buddy Rich live in Seattle the year I graduated from the University of Washington. I just chanced on a sign downtown on the door of a bar saying that his orchestra would be there that evening. So, my fianceè Susan and I got a table 10 feet away from his drums, and I will never forget what I heard. The CA-M600 sure brought back some fond memories in a terrific way.

I am not only a big fan of classic jazz drummers like Rich, Louis Bellson, and others, but have a passion for cymbals. I collect them, and rotate them through my drum setup every few months or so. Cymbals are difficult to reproduce. The attack of the ping on a ride cymbal, or the complex sound of a cymbal crash, will not sound clean if the amplifier has much harmonic distortion. Rich's cymbals on this album, and all the others that I have in my music collection, were reproduced as clearly as I have ever heard.

classe-ca-m600-amplifier-music-buddy-rich

Ravel's Bolero is an excellent piece to judge how an amplifier responds to slowly increasing demands as the music builds. The finale, of course, is every instrument playing full throttle, but the CA-M600 handled it with ease. I really never encountered the harsh reality of audible distortion with this amplifier.

Comparing the BAT VK-5i preamp with the Lamm LL1 preamp, connected to the Classé CA-M600's, the Lamm provided a bit more lushness to the overall sound that was quite nice. The Lamm has significant amounts of second order harmonics. However, because the Lamm is single-ended, there was a bit more background noise (balanced circuits have common mode rejection of noise inside the preamplifier and also in the cable connecting the preamplifier to a balanced power amplifier).

classe-ca-m600-amplifier-music-bolero

If you are wondering which album was playing when I fell asleep, this is not the one. Telarc does not compress its music, and Erich Kunzel doesn't hold back the orchestra when ffff is required. Neither does the CA-M600. Pairing the CA-M600's with the Carver speakers resulted in a slightly more laid back sound than with the Thiel CS3.7's. If you like your sound up front, the CA-M600 - Thiel CS3.7 combination would be a killer.

classe-ca-m600-amplifier-music-scary-music

I really have nothing negative to say about the Classé CA-M600 monoblock. The sound was always satiny smooth, detailed without being too up front, never harsh, no midrange mushiness. Nothing but marvelous sound. It's one of the best amplifiers I have ever heard in any system.


On the Bench

Distortion measurements were made within an 80 kHz bandwidth. Some tests were performed with the amplifier driving one of the Carver speakers, while others used an 8 ohm power resistor.

At 1 kHz and 10 volts into the Carver speaker, THD+N was a miniscule 0.001%.

classe-ca-m600-amplifier-1-khz

IMD, using the SMPTE standard of 60 Hz and 7 kHz sine waves at a 4:1 ratio, was 0.002%. This is superb performance.

classe-ca-m600-amplifier-imd

Using 19 kHz and 20 kHz sine wave test signals, the B-A peak at 1 kHz was 122 dB below the fundamentals.

classe-ca-m600-amplifier-19-khz-20-khz

THD+N vs. Frequency into the Carver speaker was flat out to about 5 kHz and then rose steadily up to 50 kHz.

classe-ca-m600-amplifier-thd-plus-n-vs-fr

Here is an Impedance - Phase plot of the Carver speaker. The region around 5 kHz, where the impedance is low and the phase is - 650, is very difficult for amplifiers to drive, and that is part of the reason the distortion goes up in that frequency range in the graph shown above.

carver-amazing-mark-4-speaker-impedance-phase

The measured frequency response was down 0.3 dB at 20 kHz, and down 3 dB at 100 kHz, for both 8 ohms and 4 ohms, which is right on the specification.

classe-ca-m600-amplifier-fr

THD+N vs. Power Output resulted in the sharp knee at 650 watts into 8 ohms, and clipping (1% THD+N) at 700 watts. With a 4 ohm load, the sharp knee was at 900 watts, with clipping at 1,200 watts.

classe-ca-m600-amplifier-thd-plus-n-vs-power-output-8-ohms

classe-ca-m600-amplifier-thd-plus-n-vs-power-output-4-ohms


Conclusions

The Classé CA-M600 monoblock is one of the finest power amplifiers I have ever had in our lab. It sounded wonderful, and the bench test results were spectacular. Now that's what I call an upgraded design!