Sidebar 3: Measurements
I measured the Audio Research Reference 6's electrical performance with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). In general, this well-engineered preamplifier's measured performance is similar to that of ARC's earlier Reference 5 SE, which Brian Damkroger reviewed in November 2012, although the Ref 6's volume control operates in 103 steps of approximately 0.3dB rather than the Ref 5 SE's 103 steps of 0.95dB. The unity-gain setting was "61" on the front-panel display. The gain with the volume control set to its maximum, "103," was 12.5dB, balanced input to balanced output, and 6.5dB, unbalanced input to unbalanced output. Both sets of inputs preserved absolute polarity (ie, were non-inverting), and the unbalanced inputs offered an input impedance of 37k ohms at low and middle frequencies, dropping inconsequentially to 31.5k ohms at 20kHz. The balanced input impedances were twice the unbalanced values, as expected.
The output impedances at 1kHz and 20kHz were 305 ohms unbalanced and 612 ohms balanced. At 20Hz, the impedances respectively rose to 600 and 1378 ohms, leading to a reduction of low-bass output into the demanding 600 ohm load, and reaching –3dB at 25Hz (fig.1, cyan and magenta traces). Into a more typical load, the output was flat down to the 10Hz limit of this graph (blue and red traces), and was down by just 1.5dB at 200kHz at the other end of the spectrum. This graph was taken with the volume control set to its maximum; note the superb matching between the channels in this graph (the match was equally good at lower volume-control settings). However, as with the Reference 5 SE, the Reference 6's ultrasonic bandwidth decreased at lower settings. Fig.2, for example, was taken at unity gain; the output is now down by 1.5dB at 40kHz, and –0.5dB at the top of the audioband.
Fig.4 plots the THD+noise percentage against the left channel's balanced output voltage into 100k ohms. Below 2V output the distortion is buried beneath the noise floor, indicated by the upward slope of the trace with decreasing voltage. The distortion slowly rises above 2V, but is still just a low 0.1% at 62V RMS, which was when the Audio Precision was outputting its maximum signal of 15V. The Ref 6's balanced output clips into 100k ohms at more than twice the Ref 5's 32V. The single-ended output clips at 12V into 100k ohms (fig.5), which is still well above any level needed to drive a power amplifier into overload, though the distortion at lower levels is higher than it is with the balanced output. The balanced output clipped at 2.7V into 600 ohms; like the Ref 5, the Ref 6 should not be used with power amplifiers having an input impedance of less than 10k ohms.
Fig.1 Audio Research Reference 6, balanced frequency response with volume control set to maximum gain at 1V, into: 100k ohms (left channel blue, right red), 600 ohms (left cyan, right magenta) (0.5dB/vertical div.).
Fig.2 Audio Research Reference 6, balanced frequency response with volume control set to unity gain into 100k ohms (left channel blue, right red) (0.5dB/vertical div.).
Like the Ref 5, the Ref 6's channel separation was superb, at >100dB below 3kHz, and still 83dB in both directions at 20kHz. The wideband, unweighted signal/noise ratio, ref. 1V output with the input shorted but the volume control set to its maximum of "103," was good, at 80dB, and these figures improved to a superb 100.3dB right and 99.5dB left when I switched an A-weighting filter into circuit. The spectrum of the Ref 5's balanced noise floor while it reproduced a 1kHz tone at 1V into 100k ohms is shown in fig.3. Some very low-level, odd-order harmonics of the AC supply frequency can be seen in both channels, perhaps due to magnetic interference from the AC transformers being picked up by the tubes' steel pins, while a very slight amount of even-order harmonics is present in the right channel (red trace). But all of these spuriae lie below –100dB (0.001%) and are thus inconsequential.
Fig.3 Audio Research Reference 6, spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms (left channel blue, right red) (linear frequency scale).
Fig.4 Audio Research Reference 6, balanced distortion (%) vs 1kHz output voltage into 100k ohms.
Fig.5 Audio Research Reference 6, unbalanced distortion (%) vs 1kHz output voltage into 100k ohms.
I examined how the THD+N percentage varied against frequency at a balanced output level of 2V into 100k ohms, which is both sufficiently high to ensure that I'm measuring THD rather than noise, and close to the maximum that will be required of the preamplifier in practical use. The left channel's distortion (fig.6, blue trace) is slightly higher than the right's (red), but both are extremely low in absolute terms. Fig.7 reveals more second harmonic present in the left channel than in the right, but the distortion is still low. Intermodulation distortion was also extremely low (fig.8).
Fig.6 Audio Research Reference 6, THD+N (%) vs frequency at 2V into 100k ohms (left channel blue, right red) (linear frequency scale).
Fig.7 Audio Research Reference 6, balanced spectrum of 50Hz sinewave, DC–1kHz, at 2V into 10k ohms (left channel red) (linear frequency scale).
Fig.8 Audio Research Reference 6, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 2V into 100k ohms (left channel blue, right red) (linear frequency scale).
The Reference 6 continues Audio Research's tradition of producing well-engineered preamplifiers whose measured performance reveals little hint of the presence of tubes in the circuit. The only criticism I have is that unlike the Ref 5, the Ref 6 dispenses with Audio Research's traditional control knobs.—John Atkinson
