conrad-johnson ART150 stereo power amplifier

By: Alan Sircom [hi-fi+ Issue 239]

It seems like yesterday, but we looked at the conrad-johnson ART300 mono power amplifiers in issue 173. We liked them so much that we awarded them a Highly Commended in our 2019 awards. A follow-up review of the conrad-johnson ART150 stereo power amplifier was planned in 2020, but... well, you know. Other products followed from the brand, but the stereo chassis slipped through the net, until now.

The ART150 and ART300 are very similar conceptually. Although the Virginian tube amp brand’s 50th anniversary is just two years away, the two power amps were designed to celebrate the company’s 40th anniversary. Although the first samples of these amplifiers hit the shelves six years ago, they show no signs of age. Maturity of concept and design, yes. But age? Not a chance.

A good break

Putting a few years between the mono and stereo amplifiers is a good thing because the two designs have so many similarities. To look at one after a few years effectively shines a light on the other. In the ‘what have you done for me lately?’ world, that’s extremely useful for a mono power amplifier launched six years ago. Those who don’t want to trawl through an issue from six years ago… virtually everything about the conrad-johnson ART150 stereo power amplifier applies to the conrad-johnson ART300 mono power amplifiers, and we’ll point out any difference along the way.

The ART power amp concept is one of abject simplicity. It’s like Occam’s razor of circuit design, with just a single 6922 double-triode tube as a voltage gain amplifier, a second 6922 per channel acting as a cathode-coupled phase inverter and then the comparatively new KT150 power tubes. There are two KT150s per channel in the stereo amplifier and four per chassis in the mono amp. Aside from the ‘ART150’ on the front panel and the second RCA input and loudspeaker terminal block on the rear, the only visible differentiation between the two is the ART150 has three 6922 double-triode tubes on the top plate and the ART300 has two. They have identical-sized chassis and the ART150 weighs a little over 3kg more than each 33kg ART300 mono amp.

Everything else; from the fit, finish and circuit are the same. Both have a low (about 12dB) amount of loop negative feedback to reduce distortion and increase damping factor. The two amplifiers bristle with CJD Teflon dielectric capacitors and laser-trimmed metal foil resistors. The design features c-j’s LED bias system because it thinks auto-bias circuits get in the way of the music. The larger power supply polypropylene capacitors are bypassed by CJD Teflons, and separate low-impedance discrete supply regulators isolate the 6922s in each stage. This is to keep the input and phase-inverter clear of the power demands of the output stage. The ART150 delivers 150W per channel and has an ideal 100kΩ input impedance. This means that although the amplifier is single-ended, it can be driven by long cable runs if the preamp has a suitably low output impedance and gain. While that naturally points the ART150 toward its ART88 stablemate, it does afford the power amplifier some flexibility in preamp choice.

There has been a recent change in the aesthetics of the ART150 in the intervening years; there has been a shift from an acrylic top plate and grille to a metal top plate with a slatted grille. The images provided are of the older design.