Benchmark DAC1 Pre USB D/A headphone amplifier Measurements

Sidebar 2: Measurements

I mainly used Stereophile's loaner sample of the top-of-the-line Audio Precision SYS2722 system (see the January 2008 "As We See It" and www.ap.com) to examine the Benchmark DAC1 Pre's measured performance from its analog inputs.

The DAC1 Pre's performance from its digital inputs is identical to that of the DAC1 USB, which I reviewed in January 2008, other than the fact that the limited real estate on the rear panel has meant that the AES/EBU data input has had to be eliminated to make room for the unbalanced analog inputs. (You can find the DAC1 USB's digital-input measurements here, with further analysis here.)

The maximum gain for the analog input was a sensible 4.05dB from both the headphone jack and the unbalanced line outputs, and 9.8dB from the balanced XLRs (as set up for the review; these outputs are adjustable). All three sets of outputs preserved absolute polarity; ie, were non-inverting. (The XLRs are wired with pin 2 hot.) The input impedance was basically to specification, at a moderately high 19.6k ohms across the band, dropping inconsequentially to 18.5k ohms at 20kHz. The output impedance was way less than an ohm from the headphone jack and a still-low 30 ohms from the unbalanced RCA jacks, but a higher 408 ohms from the balanced XLRs, this last presumably due to the series resistance of the resistor network used to pad down the balanced output.

Fed an analaog input signal, The DAC1 Pre featured a flat response over a wide bandwidth from both its balanced and unbalanced jacks, with the output down just 0.25dB at 200kHz (fig.1), which was the same into a high load of 100k ohms (top pair of traces) or a low load of 600 ohms (bottom pair). This graph was taken with the volume control set to its maximum; a slight channel imbalance of 0.25dB is evident that favors the right channel. At lower settings of the volume control, the imbalance's favor shifted to the left channel by the same amount.

Fig.1 Benchmark DAC1 Pre, balanced frequency response at 1V into 100k and 600 ohms with volume control set to maximum gain (left channel blue, right red, 100k ohms; left channel green, right magenta, 600 ohms; 0.25dB/vertical div.).

Channel separation (not shown) was very good, at >110dB below 1kHz, L–R, and >90dB, R–L. At 20kHz, the separation decreased to a still-good 90 and 72dB, respectively. The wideband, unweighted signal/noise ratio with the input shorted but the volume control set to its maximum was 78.5dB ref. 1V output. This improved to an excellent 98dB when the measurement bandwidth was restricted to the audioband, and to a superb 105.1dB when the reading was A-weighted.

Even with its very low noise floor, the DAC1 Pre's distortion was still buried within that noise at all sensible levels, which is revealed by the downward tilt of the traces in the plot of the THD+noise percentage against balanced output voltage (fig.2). This graph reveals that the Benchmark can easily deliver enough output to drive power amplifiers to clipping: 7.6V RMS into 100k ohms, and still 4.5V into 600 ohms. The plot of the THD+N percentage against frequency at 1V output (fig.3) really shows only the effects of the noise floor.

Fig.2 Benchmark DAC1 Pre, distortion (%) vs 1kHz balanced output level into (from bottom to top): 100k, 600 ohms.

Fig.3 Benchmark DAC1 Pre, balanced THD+N (%) vs frequency at 1V into 600 ohms (left channel cyan, right magenta) and 100k ohms (left channel blue, right red).

It takes high-resolution spectral analysis to uncover the DAC1 Pre's distortion. Fig.4 shows the spectrum of the Benchmark's balanced output while it reproduces a 1kHz analog input signal at 2V into 100k ohms. The second harmonic is the highest in level at just –122dB (0.00008%), with the third and fifth harmonics just visible above the –130dB line. Reducing the load impedance to the demanding 600 ohms doesn't affect the level of the second harmonic (fig.5), and though the third harmonic has risen, it is still vanishingly low in level, at –117dB (0.00014%)! Finally, the Benchmark produced negligible levels of intermodulation distortion (fig.6).

Fig.4 Benchmark DAC1 Pre, spectrum of 1kHz sinewave, DC–10kHz, at 2V balanced into 100k ohms (linear frequency scale; left channel blue, right red).

Fig.5 Benchmark DAC1 Pre, spectrum of 1kHz sinewave, DC–10kHz, at 2V balanced into 600 ohms (linear frequency scale; left channel blue, right red).

Fig.6 Benchmark DAC1 Pre, HF intermodulation spectrum, DC–24kHz, 19+20kHz at 2V peak balanced into 100k ohms (linear frequency scale).

As an analog preamplifier, the DAC1 Pre is about as good as it gets, measurement-wise. And you also get a headphone amplifier and high-performance D/A section with USB connectivity.—John Atkinson

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