Is the Conrad-Johnson ART300 Mono-block Amplifier the Right KT150 Power Amplifier for Your System?
The Conrad-Johnson ART300 Mono-block Amplifier is a limited-edition valve power amplifier sold as a stereo pair, released for Conrad-Johnson’s 40th anniversary with production limited to 125 pairs. Conrad-Johnson designed the ART300 for its own reference use, as well as for industry professionals and dedicated audiophiles. Each ART300 mono-block drives one loudspeaker with 300 watts rms into 4 ohms. Four KT150 output valves develop that power, fed by a three-stage audio circuit. The Conrad-Johnson ART150 and ART300 amplifiers have received a 2019 hi-fi+ Highly Commended award and The Absolute Sound Editors’ Choice awards in 2019 and 2022.
Who Should Choose the Conrad-Johnson ART300 Mono-block Amplifier Over the ART150?
The Conrad-Johnson ART300 Mono-block Amplifier is the high-power, one-channel-per-chassis model in the Conrad-Johnson ART range. The ART150 is its stereo sibling. Both ART amplifiers share the same circuit design, chassis dimensions, input sensitivity, input impedance and 4-ohm or 16-ohm output options. Your choice rests on power, space and ownership.
The ART150 uses one pair of KT150 valves per channel to make 150 watts. The ART300 uses two pairs per channel to make 300 watts. Each ART300 chassis has its own power transformer, so the left and right channels don’t share a power supply. Choose the ART300 mono-blocks if your loudspeakers need more than 150 watts per channel, or if you want fully separate left and right amplifiers.
A stereo pair of ART300s means two chassis to house, ventilate and connect to mains power. It holds eight KT150 output valves. The ART150 holds four.
Will It Drive Your Loudspeakers?
The ART300 is rated at 300 watts rms from 30 Hz to 15 kHz at no more than 1.5% total harmonic distortion into 4 ohms. Conrad-Johnson specifies it for 4-ohm loads. A 16-ohm connection is available through an internal change, so confirm your loudspeakers’ impedance when you order.
Loudspeaker crossovers and drivers present reactive loads that change with frequency. The ART300 applies about 12 dB of loop negative feedback. That feedback lowers distortion and gives a damping factor high enough to control reactive loudspeaker systems. Small-signal distortion stays below 0.25% in the midband.
Which Preamplifiers Suit the Conrad-Johnson ART300 Mono-block Amplifier?
The Conrad-Johnson ART300 Mono-block Amplifier is a 1.0 V sensitivity design. That means any preamplifier delivering 1 V rms can drive the ART300 to its full 300 watts. Its 100 kΩ input impedance draws very little current from the preamplifier’s output stage.
The Conrad-Johnson ART300 Mono-block Amplifier is phase correct, so it doesn’t invert the signal. A system with an odd number of phase inversions, such as one using a phase-inverting preamplifier, needs one more inversion. Swap the positive and negative speaker leads on both channels to correct it.
What Connections Does Each Chassis Provide?
Each ART300 chassis is fitted with a single RCA input on the back panel, so a preamplifier with RCA outputs connects directly. Five-way binding posts take the speaker cable. Their spacing accepts a standard double banana plug, and they take spade lugs, bare wire and pin connectors.
How Much Space and Ventilation Does Each Chassis Need?
Each ART300 chassis is about 414 mm deep, 483 mm wide and 222 mm high, and it weighs about 33 kg. Valve power amplifiers give off a lot of heat, and the ART300 has ventilation holes in its base. Stand it on a flat, hard surface that keeps those holes clear. Leave at least 102 mm above the chassis and 76 mm on each side. Choose a shelf or cabinet that’s open at the front and back. Fit cooling fans for tighter installations.
Will the Conrad-Johnson ART300 Mono-block Amplifier Run on Australian Mains Power?
The Conrad-Johnson ART300 Mono-block Amplifier is supplied as a stereo pair, built in export versions configured for 220 V or 240 V operation. Each chassis in the pair shows its operating voltage and frequency on the back panel, beside the power cord, and each needs its own mains outlet. Line voltage at the wall should stay within +5% and −10% of the voltage marked on that panel. Contact us before you order and we’ll confirm the voltage and frequency of our ART300 pairs.
How Should You Run Cables to Keep Hum Away?
The ART300 is rated for hum and noise 108 dB below rated power. It’s designed to resist electromagnetic interference through its materials, physical layout, earthing and power supply design. Conrad-Johnson chose not to fit heavy RFI filters or adopt earthing methods that reduce interference at the cost of audio performance. That choice makes careful cable layout part of the installation.
Keep interconnects as short as possible, at 3 m or less, and use shielded cable. The recommended type has two centre conductors and a separate outer shield connected at one end only. Position the preamplifier well away from the power amplifiers. Route mains cords at least 100 mm from signal cables. With two mono-blocks, plan the preamplifier’s position so both ART300 chassis sit within a short interconnect run.
How Easy Is It to Bias?
The ART300 is designed for owner biasing without a test meter. Each chassis has four bias controls beside the output valve sockets, and each control has its own LED indicator. Conrad-Johnson supplies a plastic adjustment screwdriver with the amplifier.
Connect the speakers so the amplifier has a load. Connect the preamplifier with its volume fully down so no signal reaches the amplifier. Switch on and wait about a minute. Turn each control anticlockwise until its LED just goes out. A control may need a clockwise turn to light its LED. Stop turning clockwise once the LED comes on. Repeat the procedure after a 30-minute warm-up.
Re-bias whenever you replace the output valves and after every six months of average use. The LEDs can flash or flicker during playback, and that’s normal. The ART300 needs no AC balance adjustment.
What Valve Replacement Will the Conrad-Johnson ART300 Mono-block Amplifier Need?
The Conrad-Johnson ART300 Mono-block Amplifier is a six-valve design, with two 6922 valves and four KT150 output valves in each chassis. Conrad-Johnson selected these valve brands for reliability and made its final choices by listening. A set should last two to three years in normal use if you switch the amplifier off when it’s not in use. A set left on permanently can wear out within a few months. Conrad-Johnson recommends about 15 minutes of warm-up before listening, so switching off between sessions costs little listening time.
Replace the KT150 output valves as matched quads selected for this amplifier.
What Protects the Amplifier When a Fault Occurs?
The ART300 is fitted with a plate supply fuse that protects its circuitry if an output valve fails. The fuse holder sits on the back panel near the speaker terminals. An LED beside it lights when the fuse blows. A blown plate fuse produces very low, heavily distorted sound from that channel.
A blown plate fuse usually points to a weak output valve. Switch the amplifier off, fit a new fuse and watch the output valves as you switch back on. A bright white or blue flash inside a valve identifies the faulty one. Replace the full set if the valves are more than two years old or have run past 1,500 hours.
A mains fuse and four internal fuses protect the power transformer. Failure of any of these points to a more serious fault, so have a qualified technician inspect the amplifier. Replace every fuse with the value and type Conrad-Johnson originally supplied.
Why Does This Mono-block Use a Three-Stage Circuit?
The ART300 is built on Conrad-Johnson’s view that simple circuits made with the finest available parts are the surest path to musicality and reliability. Its audio circuit has three stages, and each is essential to the amplifier’s operation. A single triode provides voltage gain. That triode is direct-coupled to a cathode-coupled phase inverter, where one triode handles each phase. The phase inverter drives the KT150 output stage. Fewer stages put fewer parts in the signal path.
Separate low-impedance, discrete DC regulators power the input and phase-inverter stages. These regulators isolate the small-signal stages from the power demands of the output valves. They’re designed to hold a stable supply at all audio frequencies.
Which Parts Set Its Construction Apart?
The ART300 is assembled with parts Conrad-Johnson selected to keep performance consistent over years of use. Its resistors are precision laser-trimmed metal foil types. CJD Teflon dielectric capacitors couple the phase inverter to the output stage and bypass the large polypropylene capacitors in the regulated power supplies.
The output transformer links the KT150 valves to the loudspeaker. Conrad-Johnson’s proprietary wide-bandwidth output transformers contribute to the amplifier’s high-frequency performance. Measured frequency response is 20 Hz to 20 kHz, +0/−0.25 dB, at 10 watts. Machined gold-plated OFC connectors and selected internal wiring complete the build.
