conrad-johnson ART300 mono power amplifiers

Tubed power amplifiers
Conrad Johnson ART300
conrad-johnson ART300 mono power amplifiers

Back in the late 1970s, when US high-end began to grow out of a burning desire to not put up with mediocrity, there were two main brands pushing valve amplification where all else was solid-state; Audio Research, and conrad-johnson. Others came and went, yet more joined the fray and continue to make outstanding audio devices that use ‘hollow-state’ technology, but it ultimately comes down to these two brands who put their respective flags in the ground. They are in part the reason we have a high-end industry, and in great part why that industry still uses valves as amplifier devices.

When conrad-johnson launches a new flagship valve-based mono power amplifier, the audio world should take note. This isn’t an everyday occurrence, and perhaps more than any other brand, c-j has stayed true to its original direction. Yes, there was a change a few years ago because that original direction sounded a little too laid-back for modern ears, but the performance of the ART300 monos define a new generation of c-j designs, as did the ACT and the Premier line of power amps before it, harking right back to the Premier One from 1981. What these flagship designs offered was valve performance mixed with the kind of power output normally associated with solid-state designs. So it is with the ART300; as the name suggests, these power amps deliver 300W per channel and do so thanks to two pairs of KT150 output tubes in each chassis. 

The KT150 is very much the ‘now’ valve, and since its launch and wider availability, it has become almost standard issue throughout the high-end audio world. For good reason, too; it has that great combination of linearity and power that every valve amp maker craves. Yes, there are curmudgeonly types who are convinced that any tube designed after the War is somehow compromised by the trappings of modernity. Some of the really out-there types are keen not to be drawn when asked, “Which war?” But three-watt power amplifiers capable of just about driving a 1920s cinema speaker are not really practical in the real world. Although the days of super-difficult loudspeaker loads are largely behind us, there is nevertheless a need to make a valve amplifier that behaves sonically like its hollow-state predecessors, but with modern power delivery and damping factor needed to drive more modern speaker designs. That’s something c-j has been doing right since before Thatcher got elected.

Paradoxically, alongside the launch of the ART300, conrad-johnson has also launched another limited-edition power amplifier called the ART27A, and where the ART300 is a 300W powerhouse, the other range-topper puts out just 36W per channel in Class A from its quartet of KT88s operating in triode mode. I’m sure some might think this sends mixed messages, but I think it’s c-j addressing two sides of the 2019 tube amp market; the demands for more power and more grace, That being said, the ART300 is no slouch in the grace department in its own right, as we shall see later on.

The conrad-johnson ethos is a smart one. The ART300 mono amplifier is essentially two ART150 amps, permanently switched to monophonic operation. Or put another way, an ART150 is one channel of the ART300, with two sets of input boards and stereo output. It’s an economy of motion thing. Once again the numbers are limited; 250 stereo amps and just 125 pairs of the monos. in essence all three power amps (the ART27A, ART150, and the ART300 tested here) are part of c-j’s 40th anniversary.

The audio circuit of these amplifiers has just three elements, each as simple as possible, and each essential to the operation of the amplifier. A voltage gain amplifier (a single triode) is direct-coupled to a cathode coupled phase inverter (each phase handled by a single triode), which in turn drives the output stage. This drives two pairs of the aforementioned KT150s to deliver 300 Watts into eight ohms. A small amount (about 12 dB) of loop negative feedback is used to reduce distortion and to produce a sufficiently high damping factor to control reactive modern loudspeaker systems. Separate, low impedance, discrete DC power supply regulators are used with the input and phase-inverter stages, isolating both stages from the power demands of the output stage.

The company has long been a proponent of high-performance parts even in its more affordable amplifiers. So, the ART300 is going to bristle with top-notch components as a matter of course. So, the ART300’s resistors are precision laser-trimmed metal foil types, and CJD Teflon dielectric capacitors are used for coupling the inverter stage to the output stage, and to bypass the large value polypropylene capacitors in the regulated power supplies. Proprietary wide-bandwidth output transformers are also used and are claimed to “contribute to excellent high frequency performance.” Even the machined gold plated OFC connectors and the internal wiring were chosen for their sonic benefits, and the brand’s long-standing relationship with Cardas pays dividends here.

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