Classé CT-M600 & CA-M600 monoblock power amplifiers

On the face of it, the power amplifier has the simplest conceptual task of any audio component. Fed an audio signal at its input, all it has to do to satisfy the demands for current made by the loudspeaker is to modulate a high-voltage voltage supply with that signal. Yet power amplifiers vary enormously in their ability to perform that task without editorializing. As a result, when I find an amplifier that appears to step out of the way of the music in the manner I desire, I make the commitment, I buy it, and I stick with it.

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 monoblock—which, 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 market—the "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 heat—their total surface area is said to be 31 square feet—and 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.)

Classé Audio
5070 François Cusson
Lachine, Quebec H8T 1B3
(514) 636-6384

tmsorosk's picture

I went out and auditioned a pair of CT-M600s after reading the review . It's hard to say much about the sound under those conditions , the system as a hole was impressive , I can say they sounded superior to the higher priced amps we heard at the same time , same everything . I won't mention what the other amps were , as I'm sure there large following would take issue .
Sure look pretty . Tim

300Binary's picture

I will keep reading your stuff - forever :)

In 1963, I heard a home system that was some big turntable, a tube preamp, a large mono tube amp and a huge VOT speaker.

I thought it was perfect (I was sixteen).

Perfect keeps getting better.

Life is not fair, thankfully.

PS I notice the picture is of the ugly one... nobody's perfect.

soulful.terrain's picture

Great review of the awesome Classe product. I have never listened to a Classe product without being impressed. Also, Classe products are amazingly reliable over the long term.

The only thing I dislike about the new CT series amps are the cosmetics. I feel Classe has taken a step backward in this area.

Mark Evans

Trickster's picture

I agree, the Delta series is way better looking than the Custom Theatre series. I don't mind the Sigma's look though.

Staxguy's picture

I had the occasion to listen to a pair of the Classe CT-M600's monoblocks with the CT-SSP pre-processor, driving a pair of B&W 803 Diamonds, in what was described as a great sounding room, and the sound was utterly horrendous - Bose would be a good description, but that doesn't give Bose enough credit - really! I was looking for a modern version of the old Aragon 8008BB that could drive dynamic B&W's to decent bass levels, and the Classe was recommended.

Was it the new 803 Diamonds that are "terrible"??? The Classe pre-dual-mono combo, the room, or the ensemble, I can not say, but I would derive much more pleasure for the old Matrix 801's, coupled with even similar vintage Rotel electronics, than I could with this combination. I have heard good things about the new 800/802 Diamond models, but it is a shame there was not a newer model similar to the older Matrix 800, to drive a decent amount of air for real-level listening.

The only hi-brand unknown piece of the combo was some random Blue Ray disc player, fed digitally into the Classe CT-SSP. Jitter, possible, in such a modern design, could it be?

Being a Canadian, I had hoped the Class would deliver the goods, but it would not stand even a cursory listening.

I like the low-power consumption in standby mode, which is mentioned in the review, a plus for a relatively larger-powered hi-fi amplifier.

Internally, the CT-M600 looks like a decent amplifier. Externally, the Classe Omega mono looks like a more fitting replacement for the ML 33H to my eye. Could the same designers have had a hand in the design?

Staxguy's picture

I had an occasion to listen to the -new- version of the CT-M600's, driving the new diamond B&W 802's, with a $5000 Classe CD Player, and a sub-$2000 Rotel pre-amp put in temporarily until the replacement $5500 Classe unit arrived, and, in a treated room, it was fantastic.

Great depth of stage for even solo voice, and less "CD player" sound than I had heard on a Sasha W/P / top Naim stack, at medium volumes. At medium-high levels, on decent modern tracks, the 802's driven by the CT-M600's really sounded as if they were augmented by a pair of nice subwoofers - much nicer than my old Velodyne F1500R in terms of fast and tight (and tunefull) bass response. That the 802's did this level of bass in a fairly small cabinet was fantastic, and totally unexpected, but the credit must go quite equally to the Classe CT-M600's.

At moderate levels, though, the meters read at about the 6W level, which really shows how far good, clean power goes.

The sound was still hi-fi, and cleary recorded, but the bass, dynamics and sound staging were a cut above the level. Voice was also quite natural, approaching the level of my understanding, with nice natural sibalance, etc. and tone. There was upsamling going on at the CD player, but this was clearly above-par 16-bit performance for the CT-M600's driving the 802's, even with a "substitute" Rotel pre-amp in the chain.

Listening to this setup, I could clearly hear what the blog entry of John Atkinson had to say.

Earlier in the session, I also listened to the competing McIntosh MC601 600W pair, which were much more imposing visually, and also sounded fine. Will definitely need to see what they each do, on the same material and gear, to make a decision, one over the other.

The new McIntosh 601's drove a pair of Sasha W/P's much more well than a all-NAIM stack with all the power supplies, and sounded much more musical on medium to high levels with them.

Anyone purchasing either amplifier pair should be quite happy, I imagine, unless their hearing is much more rarefied than mine.
The MC601 monoblock pair is the first McIntosh mono-amp than I can stand, driving Wilsons.

Funny that a pair (even older) of the CT-M600's would have sounded absolutely hideous with a B&W 803 diamond pair, while a newer pair of CT-M600's would sound really, really good on a pair of 802's (one step up speaker wise). I would imagine that the DAC design in the Classe SSP would be similar to that of their CDP, and top of the line Transparent Cable was in use on both systems.

I wonder how much better the new version CT-M600's would drive the latest 800's over the 802's, and if the bass would be as fast and sufficient as I heard today.

tmsorosk's picture

I was ready to pull the trigger on a pair of these till I found out Classe' had moved their manufacturing facility from a well regarded Canadian plant to a not so well regarded ( where audio is concerned ) country. I wonder if Stereophile will follow up there review to see if there's a sonic difference.


                                                              Regards  Tim

Trickster's picture

I recently had an in-home audition of the CA-2300 with the CP-800, driving my B&W 803Ds. The sound blew away my McIntosh MA7000. Yes... I know its unfair to compare an integrated with a 2-channel, but this was an incredible difference in every listening category. Unfortunately, my PSA-5 shut things down midway through the audition, but once we plugged straight into the wall, the power seemed limitless. I wonder how well the Classe's M600s would do against the McIntosh MC601s? M600's are almost half the price of the MC601s. Does that mean McIntosh is twice as good?

To all the outsourcing critics... You put a North American assembled Classe amp on the bench right next to one assembled in China and you would see no technical or visual difference. The only difference would be in the price tag.