Conrad-Johnson ART108A Class A Triode Amplifiers: 160-Watt KT88 Mono-Blocks for Reference Systems
Conrad-Johnson ART108A Class A Triode Amplifiers are mono-block valve power amplifiers that deliver 160 watts rms into 4 ohms from triode-connected KT88 output valves running in Class A. Conrad-Johnson designed them for reference-quality systems owned by a limited number of listeners, and production is capped at 125 pairs. The ART108A brings triode Class A operation to systems whose loudspeakers and rooms demand substantial power. Hi-Fi+ named it Tube Power Amplifier of the Year.
Should you choose the Conrad-Johnson ART108A Class A Triode Amplifiers or the stereo ART27A?
Conrad-Johnson ART108A Class A Triode Amplifiers are the higher-power option of two Conrad-Johnson amplifiers that share the same basic circuit and identical build standards. The stereo ART27A produces 36 watts per channel and is intended for highly efficient speaker systems. The ART108A produces 160 watts per mono-block, which Conrad-Johnson considers enough for most installations. Only three things separate the models: the number of output valves, the size of the output and power transformers, and the scale of the power supply.
Power demand settles the choice, not circuit preference. The ART27A fits a system built around high-efficiency loudspeakers where one stereo chassis is preferred. The ART108A fits a system where speaker sensitivity, room size or listening distance call for more than 36 watts. A pair of ART108A mono-blocks means two chassis to place and power and sixteen KT88s to maintain rather than four. Because both models share one circuit, an ART27A owner moving to less efficient loudspeakers can step up to the ART108A and keep the same amplifier design.
How do eight KT88s produce 160 watts in Class A triode operation?
Conrad-Johnson ART108A Class A Triode Amplifiers are built around KT88 output valves connected as triodes and running in Class A. Each mono-block uses four pairs of KT88s, with transformers and a power supply scaled to match, to reach 160 watts. Rated output holds from 30 Hz to 15 kHz at no more than 1.5% total harmonic distortion. Small-signal distortion measures under 0.25% at mid-band.
Conrad-Johnson designed the ART108A exclusively for the KT88. The company knows of no other output valve that improves its performance, so the amplifier isn’t intended for other output valve types.
What do the Translinear output transformers do?
Conrad-Johnson ART108A Class A Triode Amplifiers are coupled to loudspeakers through proprietary wide-bandwidth Translinear output transformers. Conrad-Johnson optimises these transformers specifically for Class A operation of triode-connected KT88s. Frequency response measures within +0/−0.25 dB from 20 Hz to 20 kHz.
Which loudspeakers suit the Conrad-Johnson ART108A Class A Triode Amplifiers?
Conrad-Johnson ART108A Class A Triode Amplifiers are rated at 160 watts into 4 ohms per mono-block. Conrad-Johnson sized that output for the wide range of loudspeaker sensitivity in high-end systems. Some highly regarded speakers measure 86 dB or less from one watt at one metre, while high-efficiency designs reach 104 dB. Room dimensions, speaker placement and listening position add further to the power a system needs. The ART108A covers installations where those factors call for more power than the ART27A supplies.
The published specifications rate the ART108A into 4 ohms only. Contact us to discuss matching it with your loudspeakers.
What does a preamplifier need to drive these mono-blocks?
Conrad-Johnson ART108A Class A Triode Amplifiers are fed through an RCA input on each rear panel, so your preamplifier needs an RCA output for each channel. The preamplifier must deliver 1.0 V rms to drive the ART108A to full output. That’s more than the ART27A’s 0.6 V, so the ART108A relies a little more on the preamplifier’s gain. The 100 kΩ input impedance places a light load on the preamplifier’s output stage.
How does absolute phase affect system matching?
Conrad-Johnson ART108A Class A Triode Amplifiers are phase correct (non-inverting). A system plays in correct absolute phase when it contains an even number of phase inversions, or none. If your system has an odd number, for example because of a phase-inverting preamplifier, add one inversion by reversing the positive and negative speaker connections on both channels.
Where should the amplifiers be placed?
Conrad-Johnson ART108A Class A Triode Amplifiers are valve amplifiers that give off significant heat. Each one needs a dedicated amp stand with clear space on all four sides and above it for ventilation. Each chassis measures about 483 mm wide, 533 mm deep and 219 mm high and weighs roughly 40 kg. Choose stands that can take that depth and load for each mono-block.
What mains supply do the Conrad-Johnson ART108A Class A Triode Amplifiers need?
Conrad-Johnson ART108A Class A Triode Amplifiers are configured for a specific mains voltage, which is marked on the back panel next to the power cord. In 240 V/50 Hz form, each mono-block draws 420 watts (1.75 amps), so a pair draws 840 watts. Line voltage must stay within +5% to −10% of the rated figure. Contact us to confirm the voltage configuration of the units you’re buying before purchase.
How should cables be routed to avoid hum?
Conrad-Johnson ART108A Class A Triode Amplifiers are designed without heavy-handed RFI filters or grounding practices that would reduce audio performance. That makes cable layout part of the installation. Use shielded interconnects of 3 metres or less. Place the preamplifier well away from the power amplifiers, and keep mains cords at least 100 mm from input and output cables.
How do speaker cables connect?
Conrad-Johnson ART108A Class A Triode Amplifiers are fitted with five-way binding posts that accept double banana plugs, spade lugs, bare wire and pin connectors.
Both channels need correct relative phase so the left and right speaker diaphragms move in and out together. Connect the amplifier’s “+” terminal to the red or “+” speaker terminal, and its “−” terminal to the black or “−” speaker terminal. You can check by ear with a solo vocal recording, ideally in mono. With the speakers in phase, the voice sits in a focused image between them. Out of phase, it sounds diffuse. Swapping the leads at one speaker only corrects relative phase.
How often do the output valves need replacing?
Conrad-Johnson ART108A Class A Triode Amplifiers are equipped with eleven valves per chassis: three 6922s and eight KT88s. Conrad-Johnson expects two to three years of normal use before the valves degrade, assuming the amplifiers are switched off when not in use. To maintain performance and reliability, replacement KT88s must be matched quads selected for this amplifier.
Can owners bias the Conrad-Johnson ART108A Class A Triode Amplifiers themselves?
Conrad-Johnson ART108A Class A Triode Amplifiers are equipped with built-in bias indicators, so owners can set output valve bias without test instruments. The only tool needed is the plastic bias tool supplied with each amplifier, and no AC balance adjustment is required. Connect the amplifier to your speakers, turn the preamplifier’s volume fully down and apply no signal. After three minutes, turn each of the eight bias controls beside the output valve sockets clockwise until its LED glows amber. If an LED turns red, turn its control anticlockwise until it shows amber again. Repeat the process after thirty minutes of warm-up.
Re-bias whenever you replace the output valves and after every six months of average use. LEDs that flash or flicker red during playback are normal.
What protects the circuit if an output valve fails?
Conrad-Johnson ART108A Class A Triode Amplifiers are protected by a plate supply fuse in each chassis, which guards the circuit if an output valve fails. The fuse holder sits on the back panel beside the speaker binding posts. Always switch the amplifier off before removing the fuse holder cap.
A blown plate fuse leaves the affected speaker playing very low, heavily distorted sound, and it usually means a marginal KT88. Fit a new fuse, power up and watch the output valves for a bright white or blue flash to find the faulty one. If the valves have run more than 1,500 hours, Conrad-Johnson suggests replacing the whole set. A separate mains fuse and four internal secondary fuses protect the power transformer. A failure of any of these points to a more serious fault that requires service. Never fit a fuse of a different value or type.
Why do the Conrad-Johnson ART108A Class A Triode Amplifiers use separate regulated power supplies?
Conrad-Johnson ART108A Class A Triode Amplifiers are powered by independent discrete regulated supplies for the output stage. Separate regulated supplies feed the input and inverter stages, so the low-level stages don’t draw from the output stage’s supply.
Which parts are used in the signal path?
Conrad-Johnson ART108A Class A Triode Amplifiers are built with precision laser-trimmed bulk metal foil Vishay resistors throughout the voltage gain stages and their power supplies. CJD Teflon dielectric capacitors couple the inverter stage to the output stage. The same capacitor type bypasses the large polypropylene capacitors in the regulated supplies for the gain and inverter stages. Machined OFC connectors and selected internal wiring complete the path. Conrad-Johnson chooses these parts so the amplifiers keep their designed performance for years.
