Audio Research REF160S
140 W RMS stereo valve power amplifier from Audio Research, fitted with eight KT150s and switchable triode or ultralinear modes, for a high-end hi-fi system.
Description
The REF 160 S is the stereo tube power amplifier from Audio Research's Reference line. It brings the architecture of the 160M monoblock into a single chassis and sits behind a preamplifier to handle the entire power amplification for both channels. Two GhostMeters on the front panel reveal the eight KT150 power tubes lined up behind them, while an output tube monitoring mode allows their condition to be checked without dismantling the unit.
The power stage delivers 140 watts RMS and relies on four matched pairs of KT150s, with the gain stage entrusted to four 6H30s. An automatic bias circuit adjusts the tubes' operating point on its own, removing the need for periodic manual adjustment. Operation can be switched between triode and ultralinear, giving two distinct sonic characters from the same unit. The stated bandwidth spans 5 Hz to 70 kHz, with frequency response from 0.5 Hz to 110 kHz, a slew rate of 13 volts per microsecond and a rise time of 2.0 microseconds: indicators of a stage able to follow fast transients without lagging.
Connection can be made in balanced or unbalanced form, with an input impedance of 300,000 ohms balanced and 100 kilohms unbalanced, high values that place no constraint on the preamplifier upstream. Input sensitivity is 2.4 V RMS balanced for rated output, giving a gain of 25.5 dB into 8 ohms. On the speaker side, three transformer taps are provided, at 4, 8 and 16 ohms, allowing the amplifier to be matched to the actual speaker load rather than working around it. A four-layer printed circuit board and large low-profile 120 mm fans, mounted horizontally in the bottom plate, complete the design.
This model is aimed at listeners building a separates system around a preamplifier of equivalent standing, seeking the character of valves without giving up power. Its dimensions of 48.26 × 26 × 54.61 cm, extending to 60.96 cm deep with the handles, and its 45 kg weight call for a sturdy, clear and well-ventilated stand; the two rear handles have specifically been provided to ease handling.
Technical specifications
| Power | 140 watts RMS |
| Tubes | 4 matched pairs KT150 (output V1-4); 4 × 6H30 (gain stage V5 and V6) |
| Bandwidth | 5 Hz to 70 kHz |
| Frequency response | 0.5 Hz to 110 kHz |
| Input sensitivity | 2.4 V RMS balanced for rated output (gain of 25.5 dB into 8 ohms) |
| Input impedance | 300,000 ohms balanced; 100 kilohms unbalanced |
| Output impedances | 16 ohms, 8 ohms, 4 ohms |
| Slew rate | 13 volts/microsecond |
| Rise time | 2.0 microseconds |
| Output polarity | Non-inverting, balanced input pin 2+ (IEC-268) |
| Dimensions (W × H × D) | 48.26 × 26 × 54.61 cm (60.96 cm with handles) |
| Weight | 45 kg |
What to check before buying used
On an amplifier of this class, the first point to clarify is the history of the KT150s: number of hours run, date of last replacement and matching of the four pairs, as an unbalanced set will result in an off-centre soundstage. The built-in monitoring mode and automatic bias make checking easier, but it remains worth verifying that every tube position responds normally and that the 6H30s in the gain stage have not been neglected. Listen in both triode and ultralinear modes in succession to confirm the switching functions correctly, and check that the fans on the bottom plate turn freely, without mechanical noise or a build-up of dust. Finally, check the condition of the speaker terminals and the three impedance taps, and plan for two-person transport given the 45 kg weight and the depth of the chassis.
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