Audio Research REF80S
Stereo valve amplifier from Audio Research, fitted with KT150 output tubes and a fully balanced differential design, for demanding listening in hi-fi systems.
Description
The REF80S is a stereo vacuum tube power amplifier, designed by Audio Research for high-end hi-fi systems. Positioned below the Reference 160S and 160M models in the manufacturer's range, it is aimed at listeners seeking the brand's sonic philosophy without requiring higher output power. The unit drives loudspeakers directly, without intermediate electronics, and delivers an output power of 2x75 watts RMS.
The architecture is built around two pairs of KT150 output tubes paired with two 6H30 tubes for the gain stage, all within a fully balanced differential design. This topology, combined with a four-layer circuit board offering a separate ground plane, aims to reduce noise induced by point-to-point connections and limit distortion. Bandwidth extends from 5 Hz to 70 kHz at -3dB, with a frequency response of 0.5 Hz to 110 kHz, and the signal-to-noise ratio reaches 110 dB for a noise level below 0.2 mV RMS. The damping factor is approximately 10, for output regulation close to 0.6 dB into a 16 ohm open-circuit load.
The amplifier incorporates a proprietary automatic bias circuit, which adjusts the tubes' operating point according to their ageing and mains voltage variations. Switchable operation between Triode and Pentode modes allows the amplifier's behaviour to be adapted to the loudspeakers used or to listening preferences. Output tube monitoring and a built-in hour counter allow their wear to be tracked over time, reducing the need for regular manual intervention.
Connectivity includes XLR and single-ended (SE) inputs, with an input sensitivity of 2.4V RMS and an input impedance of 200 kilohms via XLR. The output terminals accept 4, 8 and 16 ohm loads. The unit also features 12V input and output triggers, an RS232 input for integration into a control system, switchable fan speeds and an auto shut-off function that can be disabled as required.
This type of amplifier is aimed at high-end hi-fi installations, paired with loudspeakers of a sensitivity suited to 75 watts per channel, and at users attentive to the reproduction of dynamic and harmonic nuances. The presence of balanced and unbalanced inputs offers a degree of flexibility depending on the sources and preamplifiers used upstream.
Technical specifications
| Output power | 2x75 W RMS |
| Bandwidth ( 3dB) | 5 Hz - 70 kHz |
| Frequency response | 0.5 Hz - 110 kHz |
| Input sensitivity | 2.4V RMS |
| Input impedance | 200 kOhms (XLR) |
| Output terminals | 4, 8, 16 ohms |
| Output regulation | Approximately 0.6 dB into 16 ohms open-circuit (damping factor approximately 10) |
| Overall feedback | 8.8 dB |
| Slew rate | 20 volts/microsecond |
| Rise time | 1.5 microseconds |
| Signal to noise ratio | Less than 0.2 mV RMS - 110 dB |
| Tubes | 2 pairs KT150 (output stage V1-4); 2 6H30 (gain stage V5 and V6) |
| Dimensions (W x H x D) | 48.2 x 26 x 54.6 cm |
| Weight | 45.36 kg |
What to check before buying used
The KT150 tubes and the two 6H30 wear with operating hours: ask for the cumulative hours and check the bias balance between the two channels, as a tired set of tubes results in power loss or audible distortion. The four-layer board and internal wiring should show no signs of overheating or oxidation around the tube sockets. Also check the tightness and condition of the 4/8/16 ohm output terminals, which are stressed each time speakers are changed.
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