Conrad-Johnson Classic 120 and 120SE Vacuum-Tube Stereo Power Amplifier: Which Version Suits Your System?
Conrad-Johnson Classic 120 is a 125-watt-per-channel vacuum-tube stereo power amplifier from Conrad-Johnson’s Classic range. That range adapts technology from the company’s flagship ART amplifier for a wider audience. Conrad-Johnson aims the Classic amplifiers at listeners new to modern tube amplification and at owners moving on from entry-level valve gear, with ease of use, low maintenance costs and reliability as the stated priorities. The Classic 120 comes in two versions: the standard model with EL34 output tubes, and the Classic 120SE with KT120 output tubes. Owners handle routine bias adjustment themselves using built-in indicators, with no test equipment required.
How does the Conrad-Johnson Classic 120SE differ from the Classic 120?
Conrad-Johnson Classic 120SE is the special-edition version of the amplifier. It differs in its output tubes and in selected passive parts. Its eight output tubes are KT120s rather than EL34s. It incorporates CJD Teflon capacitors and metal-foil resistors at strategic points in the circuit. Both versions share the same power rating, the same three 6922 input tubes, and the same dimensions and weight. The choice comes down to output tube type and component grade, not power.
Decide on the version at purchase. Conrad-Johnson states that the KT120 isn’t a plug-in replacement for the EL34, because the bias circuit has to be revised to make that substitution. Fitting KT120s to a standard Classic 120 isn’t a simple tube swap, and replacement output tubes must suit the version you own.
Should you choose it over the Classic 62?
Classic 120 is the higher-powered of Conrad-Johnson’s two Classic power amplifiers. The Classic 62 uses a single pair of EL34 output tubes per channel and is rated at 60 watts per channel. The Classic 120 adds a second pair of output tubes to each channel to achieve twice the output power. The Classic 62 is the more space-saving option at about 340 mm deep, 441 mm wide, 168 mm high and 20 kg. Both models are available in standard and SE form, so the EL34 or KT120 choice applies at either power level.
Which loudspeaker loads can it drive?
This valve power amplifier is rated at 125 watts RMS per channel, with both channels driven into 4 ohms, from 30 Hz to 15 kHz, at no more than 1.5% total harmonic or intermodulation distortion. The output can be reconnected internally for 8- or 16-ohm loads. The Classic 62 offers a 16-ohm option only.
The published power rating applies to the 4-ohm connection. Reconnection takes place inside the chassis, which operates at high voltages, and Conrad-Johnson says only qualified, experienced people should open it. Confirm your loudspeakers’ nominal impedance, and the amplifier’s configuration, before you buy.
What preamplifier does the Conrad-Johnson Classic 120 need?
Conrad-Johnson Classic 120 is a power amplifier designed to be driven by a preamplifier, which connects to the RCA inputs on the back panel. A preamp output of 0.9 V rms drives it to rated power. The Classic 62 reaches rated power with 0.6 V, so a preamp moving across from a Classic 62 needs to supply more voltage to drive the Classic 120 fully. Input impedance is 100 kilohms.
The amplifier is phase correct (non-inverting). If your system has an odd number of phase inversions, such as a phase-inverting preamplifier, Conrad-Johnson recommends reversing the positive and negative speaker connections on both channels to restore correct absolute phase.
How much room and ventilation does it need?
This stereo tube amplifier is about 480 mm deep, 483 mm wide and 194 mm high, and it weighs 34 kg net. Check your rack shelf’s depth and load rating before buying.
Conrad-Johnson notes that the Classic 120 dissipates a significant amount of heat. Mount it on a flat, hard surface with the ventilation holes in its base unobstructed. Leave at least 100 mm of clearance above it and 75 mm at each side. The shelf or cabinet must be open at both front and back. In more restricted installations, Conrad-Johnson says cooling fans are essential.
Is the Conrad-Johnson Classic 120 set up for Australian mains?
Conrad-Johnson Classic 120 is configured at the factory for a specific mains voltage and frequency. Australia’s supply is 230 V at 50 Hz. US-market units are configured for 108–126 V at 60 Hz. Export versions show their operating voltage and frequency on the back panel near the AC power cord, and Conrad-Johnson lists fuse values for 220 V and 240 V configurations. Your actual line voltage should sit within +5% and −10% of the rated figure. Check that the back-panel marking matches the Australian supply before you buy.
How should the cables be connected and routed?
Classic 120 is fitted with five-way binding posts spaced for standard double-banana plugs. The posts accept spade lugs, bare wire and pin connectors. Connect each amplifier “+” terminal to the red or “+” terminal on each speaker, and each “−” terminal to the black or “−” terminal, so both speakers move in phase.
Conrad-Johnson chose not to fit heavy-handed RFI filters. It says these would compromise audio performance in the 99.9% of installations where electromagnetic interference isn’t a problem. Its installation guidance covers the rest:
- Use shielded interconnects of 3 metres or less.
- Position the preamplifier well away from the power amplifier.
- Keep mains cords at least 100 mm from input and output cables.
What does bias adjustment involve?
Classic 120 is designed for owner bias adjustment without instruments. Eight screwdriver-adjustable controls sit near the output tube sockets, each with an associated indicator LED, and a plastic screwdriver comes with the amplifier.
- Connect the speakers, turn the preamplifier volume fully down, and play no signal.
- Switch the amplifier on and wait about one minute.
- Turn each control anticlockwise until its LED just goes off. Some controls may need a clockwise turn first to light the LED. Don’t turn any further clockwise once it’s lit.
- Repeat the procedure after a 30-minute warm-up.
LEDs that flash or flicker during use are normal. Re-bias whenever you replace the output tubes and after every six months of average use.
How long do the valves last?
Classic 120 is expected to run two to three years without tube degradation in normal use, provided you switch it off when you’re not using it. Left on permanently, it can exhaust its tubes in a few months. Because each output tube is individually biased, replacement tubes needn’t be matched pairs, and no AC balance adjustment is required. Conrad-Johnson chooses its supplied tube brands from those known to be most reliable, with final selection made by listening. It recommends buying replacement sets from Conrad-Johnson.
What happens if an output tube fails?
Classic 120 is protected by plate supply fuses in series with the output tubes, which guard the amplifier’s circuitry if an output tube fails. A FUSE OUT LED beside each speaker terminal lights when that channel’s plate fuse blows. The symptom is very low, very distorted sound from the speaker on that channel. The plate fuses are inside the chassis, which the manual says only qualified people should open, and replacement fuses are available from Conrad-Johnson dealers. Failure of any power-transformer fuse indicates a more serious problem that needs a competent service technician.
Why does the Conrad-Johnson Classic 120 use such a simple circuit?
Conrad-Johnson Classic 120 is built on the company’s position that the best results come from simple circuits executed with quality parts. Single-ended triode voltage-gain and inverter stages, using three 6922 tubes, are coupled to an output stage of four ultra-linear operated output tubes per channel. Discrete voltage regulators supply stable DC voltage to the critical voltage-gain and inverter stages. The output transformers are Conrad-Johnson’s proprietary trans-spectral wide-bandwidth design.
Which components go into the circuit?
Classic 120 is built with precision metal-film resistors, which Conrad-Johnson specifies for low noise. Polypropylene and polystyrene capacitors, chosen for temporal accuracy, are used in the audio circuits and in the regulated plate voltage supplies for the input and inverter stages.
How is each amplifier built and checked?
Classic 120 is hand assembled. An experienced audio technician individually inspects each unit and verifies its performance.
