Tube Power Amp Reviews

This Audio Research amplifier presented a measuring challenge. It has balanced and single-ended inputs, two different modes of operation, and three output-transformer taps. It is therefore 12 different amplifiers. I performed full sets of tests using its balanced input in both Triode and Ultralinear modes from the 4 ohm and 8 ohm output taps, then repeated some of the tests using its single-ended input and the 16 ohm tap. After I installed the tubes, I used the Tube Monitor button on the front panel to make sure all was well with the KT150 output tubes—all four LEDs illuminated green, indicating no problems. One point on the output stage mode: the central LED on the front panel glows green for Ultralinear operation, blue/white for Triode. However, unless you're looking directly at the LED, these colors look very similar. I wish ARC had chosen colors more easily distinguished. The beautiful meter appeared to be incorrectly calibrated: 30W into 8 ohms in Triode mode with a 1kHz tone was indicated as 0.75W.

I tested the REF 160M with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). The voltage gain at 1kHz into 8 ohms depended on the input, mode, and output tap. Using the balanced input and Triode mode, I measured 26.3dB from the 16 ohm tap, 23.7dB from the 8 ohm tap, and 20.95dB from the 4 ohm tap. In Ultralinear mode these gains were all 0.4dB higher and, unusually, the unbalanced input offered almost 6dB more gain than the expected reduction of 6dB. The amplifier was non-inverting (ie, it preserved absolute polarity) with both inputs, both modes, and from all output taps. The balanced input impedance was a very high 186k ohms at 20Hz and 1kHz, dropping to a still-high 153k ohms at 20kHz. The unbalanced input impedance was half these figures, as expected.

The REF 160M's output impedance depended on mode and output tap. In Triode mode from the 8 ohm tap the impedance ranged from 0.7 ohm at 20Hz to 0.8 ohm at 1kHz, and to 1.35 ohms at 20kHz. The impedance was higher from the 16 ohm tap, ranging from 1.1 to 2 ohms, and lower from the 4 ohm tap, ranging from 0.44 to 1.15 ohms. The output impedances were all slightly higher in Ultralinear mode, but even so, the REF 160M has a lower output impedance in all of its modes than many of the tube amplifiers that have passed through my test lab—a tribute to its output transformer.) As a result, the modulation of the amplifier's frequency response, due to the Ohm's law interaction between this source impedance and the impedance of our standard simulated loudspeaker, was relatively small, at ±0.7dB (fig.1, gray trace).

Fig.1 was taken in Triode mode from the 8 ohm tap; the response in Ultralinear mode was almost identical (fig.2). In both modes, the response into 8 ohms (fig.1, blue trace) was flat to 10kHz, with then rolloffs of –0.6dB at 20kHz and –3dB at 50kHz, which correlate with the slightly lengthened risetimes with the amplifier's reproduction of a 10kHz squarewave into 8 ohms (fig.3). However, the squarewave is commendably free from overshoot and ringing. As the load impedance drops, the top octave starts to shelve down, reaching –1.4dB at 20kHz from the 8 ohm tap into 4 ohms (fig.1, magenta trace), and –2.1dB at 20kHz into 3 ohms (red). The rolloff in the top octaves was greater from the 4 ohm tap, with the output into 2 ohms down by 2.8dB at 20kHz (fig.4). This might be why people prefer to use 8 ohm taps to drive Wilson's Alexia 2 loudspeakers.

The REF 160M is a very quiet amplifier: its unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms, taken with the input shorted to ground, was an extraordinary 113dB when the measurement was A-weighted. Spuriae were present in the amplifier's noise floor at the 60Hz power-supply frequency and its harmonics, but these all lay at or below –100dB ref. 1W into 8 ohms (fig.5).

In Ultralinear mode the REF 160M is specified as delivering up to 140W (21.5dBW into 8 ohms). Using our definition of clipping—ie, when the output's percentage of THD+noise reaches 1%—the amplifier exceeded that output, clipping with a 1kHz signal at 150W from the 8 ohm tap into 8 ohms (21.75dBW, fig.6). As expected, the clipping power into 8 ohms from the 8 ohm tap in Triode mode was lower, at 76W (18.8dB, fig.7). Less power was available when the load was not matched to the nominal transformer tap. In Triode mode from the 8 ohm tap, the REF 160M clipped at 30W (11.75dBW, fig.8), which might be why JVS preferred the Ultralinear mode with the Alexia 2s. In the worst case, the 16 ohm tap driving 4 ohms in both Triode and Ultralinear modes, the REF 160M clipped at just 14W (8.45dBW).

The upward slope of the traces in figs. 6–8 suggest that the ARC uses only a modest amount of loop negative feedback. I examined how the percentage of THD+noise changed with frequency at 12.65V, which is equivalent to 10W into 16 ohms, 20W into 8 ohms, and 40W into 4 ohms. From the 8 ohm tap in Triode mode the THD+N is very low in the bass and midrange into 16 ohms (fig.9, blue trace), but rose both at higher frequencies and into lower impedances. Into 4 ohms the THD+N was very high across the audioband (red trace), the amplifier being driven into clipping at this level. The distortion from the 8 ohm tap was generally lower in Ultralinear mode (fig.10), but the amplifier is still clipping above the midrange at this level. The 4 ohm tap offered lower distortion in both modes.

Fortunately, the REF 160M's distortion was predominantly the subjectively innocuous second and third harmonics (fig.11), and higher harmonics are all low in level (fig.12). When the amplifier drove an equal mix of 19 and 20kHz tones at 20W into 8 ohms from the 8 ohm tap in Triode mode (fig.13), the second-order difference product at 1kHz lay just below –60dB (0.1%) and the higher-order intermodulation products were lower in level.

The Audio Research Reference 160M's measured performance appears to be affected by the design team's decision not to use a lot of loop negative feedback. It will offer relatively high levels of low-order harmonic distortion when the load impedance is less than the nominal output transformer tap. However, I suspect that the reason JVS found the REF 160M worked well with the low-impedance Wilson Alexia speakers is that they are very sensitive—I measured 91.2dB/2.83V/m—so he probably wasn't driving the amplifier anywhere near clipping, especially in Ultralinear mode.—John Atkinson