Conrad-Johnson ART27A Class A Triode Amplifier: A 36-Watt KT88 Stereo Power Amp for High-Efficiency Loudspeakers
Conrad-Johnson ART27A Class A Triode Amplifier is a limited-production stereo valve power amplifier for listeners who run high-efficiency loudspeakers and want an output stage built around triode-connected KT88s in Class A. It’s the stereo model in Conrad-Johnson’s ART amplifier line, and production is capped at 250 units. Each channel uses a single pair of KT88 output valves, and each amplifier stage draws on its own regulated power supply. Conrad-Johnson lists a Hi-Fi+ Power Amplifier of the Year award for the ART27A, marked Highly Commended in 2020.
This tube power amplifier suits owners who already have a separate preamplifier, have chosen sensitive speakers, and want to bias and re-valve the amp themselves without test equipment.
Why Does the Conrad-Johnson ART27A Run Its KT88 Valves in Class A Triode Mode?
The Conrad-Johnson ART27A is designed around one operating choice: KT88 output valves wired as triodes and biased for Class A. In Class A, both valves in each output pair conduct across the entire signal waveform. Neither valve switches off as the signal swings, so the amp never hands the waveform from one valve to the other. The KT88 is a beam tetrode, and connecting it as a triode gives up output power. That’s why one pair per channel delivers a modest rating.
The published measurements show the result: small-signal distortion below 0.25% at mid-band and frequency response within 0.25 dB from 20 Hz to 20 kHz.
The output transformers complete the design. An output transformer matches the valves’ high output impedance to a loudspeaker’s low impedance. Conrad-Johnson’s proprietary wide-bandwidth Translinear output transformers are optimised specifically for Class A operation of triode-connected KT88s, so the transformer and the output stage are engineered as one system.
Class A has a practical cost. The output valves draw full current whenever the amp is on, regardless of volume, and that heat shapes how you install the amplifier.
Will It Suit Your Loudspeakers?
This stereo amplifier is rated at 36 watts rms per channel into 4 ohms, from 30 Hz to 15 kHz, at no more than 1.5% total harmonic distortion. Conrad-Johnson positions it for highly efficient speaker systems. The manufacturer notes that well-regarded speakers range from 86 dB or less to 104 dB at one watt, one metre. Every 3 dB of extra sensitivity halves the power needed for the same volume, so a 104 dB speaker needs roughly one-sixtieth of the power an 86 dB speaker needs to play equally loud. Room size, speaker placement and listening distance change power demand too, so a large room with distant seating pushes the requirement up.
The ART27A is available connected for 4, 8 or 16 ohm loads. Confirm with your dealer which connection a unit carries, and match it to your speakers’ nominal impedance so the KT88s see their intended load.
Hum and noise measure below 200 microvolts. That figure matters with sensitive speakers, because they turn very small voltages into audible output. A low residual noise voltage keeps the amplifier’s noise floor low on the speakers it’s designed to drive.
Speaker cables connect through five-way binding posts spaced for standard double-banana plugs. They’ll also take spade lugs, bare wire or pin connectors.
How Does It Compare With the Conrad-Johnson ART108A Mono-Blocks?
The ART27A is the lower-power half of a two-model range, and the ART108A mono-block is its alternative. Both use the same basic circuits and are built to identical standards. They differ in the number of output valves, the size of the output and power transformers, and the scale of the power supply.
The ART108A uses four pairs of KT88s per chassis to produce 160 watts. Conrad-Johnson describes that as enough power for the vast majority of applications. It needs 1.0 V to reach full output against the ART27A’s 0.6 V, and its output valves are replaced as matched quads. A pair of ART108As holds 22 valves, while the ART27A holds seven. Each ART108A chassis measures 533 mm deep and weighs about 40 kg, and you need two of them. Production is limited to 125 pairs.
Choose the ART27A if your speakers are highly sensitive, you want a single chassis on one shelf, and you’d rather maintain four output valves than sixteen. Choose the ART108A if your speakers sit towards the lower end of the sensitivity range, your room is large, or you listen at levels that need more headroom than 36 watts provides.
Which Preamplifiers Work With the Conrad-Johnson ART27A?
The Conrad-Johnson ART27A is a power amplifier only, so it needs a separate preamplifier with a volume control. It connects through RCA single-ended inputs on the rear panel. Whether your sources are a turntable and phono stage or a digital front end, the ART27A sees only the preamplifier’s line-level output.
Its 0.6 V sensitivity means a preamplifier needs to deliver just 0.6 V rms to drive the amp to full rated power. Its 100 kilohm input impedance draws very little current from the preamplifier’s output stage, whether that stage is valve or solid-state.
The ART27A is phase correct, meaning it doesn’t invert the signal. If your preamplifier inverts phase, your system has an odd number of inversions. Restore correct absolute phase by reversing the positive and negative leads at both speakers.
What Do Its Regulated Power Supplies and Parts Selection Contribute?
This amplifier is built with an independent discrete regulated power supply for the output stage and separate regulated supplies for the input and inverter stages. Each channel has its own regulators. Regulation holds supply voltage steady as current demand and mains voltage vary. Separate supplies stop one stage or channel from pulling down the voltage another relies on.
Conrad-Johnson specifies parts with long-term stability in mind. Precision laser-trimmed Vishay bulk metal foil resistors are used throughout the voltage gain stages and their power supplies. CJD Teflon dielectric capacitors couple the inverter stage to the output stage and bypass the large polypropylene capacitors in the regulated supplies. Machined OFC connectors and selected internal wiring complete the signal path. The manufacturer’s stated aim is for the amplifier to keep performing to its design potential for years.
Where Should You Install the Conrad-Johnson ART27A?
The Conrad-Johnson ART27A is a heat-producing component that needs open, ventilated space. Place it on a flat, hard surface so the ventilation holes in its base stay clear, and avoid soft surfaces that could cover them. Leave about 100 mm of clearance above the amp and about 75 mm at each side. The shelf or cabinet should be open at the front and back. Enclosed installations need cooling fans.
The chassis measures about 445 mm deep, 483 mm wide and 225 mm high, and weighs about 28.6 kg. Add the overhead clearance and you’ll need roughly 325 mm of vertical space, on a shelf rated for the weight.
Conrad-Johnson designed the amp to resist radio frequency and other electromagnetic interference through its materials, layout, grounding and power supply design. It doesn’t use heavy RFI filtering that would compromise audio performance. Careful installation can often avoid EMI problems. Keep interconnects to 3 m or less, and use shielded cable with two centre conductors and a shield connected at one end only. Position the preamplifier well away from the power amp. Route mains cords at least 100 mm from signal cables to minimise hum pickup.
Is This Amplifier Configured for Your Mains Supply?
The ART27A is a voltage-specific design built in different mains configurations. Units sold in the United States run on 108 to 126 volts at 60 Hz. Export versions show their operating voltage and frequency on the back panel near the power cord, and Conrad-Johnson’s fuse specifications cover 220 V and 240 V configurations. Actual line voltage must sit within +5% and -10% of the rated voltage. Australian mains is nominally 230 V at 50 Hz, so check that the back-panel marking matches before connecting the amp.
How Much Valve Maintenance Does the Conrad-Johnson ART27A Need?
The Conrad-Johnson ART27A is a seven-valve amplifier using three 6922 small-signal valves and four KT88 output valves. It was designed exclusively for KT88s. Replacement KT88s must be matched pairs selected for this amplifier, and Conrad-Johnson recommends buying them direct.
Valve life depends on how you use the amp. Switch it off when you’re not listening and the valves should last two to three years without degradation. Leave it on permanently and they can wear out within a few months. Conrad-Johnson recommends about fifteen minutes of warm-up before listening, so powering down between sessions costs little time.
Re-biasing is a job owners can do themselves. Built-in LED indicators replace a meter, a plastic bias tool comes with the amp, and no AC balance adjustment is needed. Connect the speakers, turn the preamp volume fully down, and switch on the amp. After three minutes, turn each of the four bias controls clockwise until its LED glows amber. Repeat the process after thirty minutes of warm-up. Bias the amp whenever you replace output valves and after every six months of average use. LEDs that flicker red during playback are normal.
What Happens if an Output Valve Fails?
This amplifier is fitted with plate supply fuses that protect its circuitry if an output valve fails. The fuse holders sit on the back panel near the speaker terminals and unscrew for access. Always switch the amp off before removing a fuse cap.
A blown plate fuse shows up as very low, heavily distorted sound from one channel. It usually points to a marginal output valve. To find it, fit a new fuse, switch on, and watch the output valves for a bright white or blue flash inside one of them. If the valves have run more than 1,500 hours, Conrad-Johnson suggests replacing the whole set.
Separate fuses protect the power transformer’s mains input and secondary circuits. A failure there signals a more serious fault and needs a qualified service technician. Replacement fuses are available from Conrad-Johnson dealers and from Conrad-Johnson directly.
