Sidebar 3: Measurements
After installing the single GE 6BZ7 dual-triode tube, I measured the Schiit Vali 2+ with my Audio Precision SYS2722 system (see the January 2008 "As We See It"), repeating some tests with the magazine's sample of Audio Precision's current top-line APx555. I took a full set of measurements from both the front-panel headphone jack and the single-ended preamplifier outputs on the rear panel in both the High and Low gain settings.
The Schiit's input impedance is specified as 50k ohms. I measured 45k ohms at 20Hz and 1Hz, and an inconsequential drop to 40k ohms at 20kHz. The maximum gain from both the preamplifier and headphone outputs was very close to the specification at 14.1dB, High gain, and 0.9dB, Low gain. The Vali 2+ preserved absolute polarity (ie, was noninverting) from both outputs and with both gain settings. Its output impedance from the headphone output, with gain set to High, was extremely low, at 2.6 ohms at 20Hz and 1.7 ohms at 1kHz and 20kHz. Setting it to Low reduced the output impedance slightly. Though these values are a little higher than specified, they include the series resistance of an adaptor cable and 2m of interconnect. The preamplifier output impedance was 101 ohms from 20Hz to 20kHz.
The Schiit's frequency response was flat in the audioband and down by 1.3dB at 200kHz. With the very low output impedance, the response into 300 ohms (fig.1, cyan and magenta traces) was identical to that into the high 100k ohm load (blue, red traces). Fig.1 was taken from the headphone output with the Vali 2+'s volume control set to its maximum. The response from the preamplifier output was identical. However, a channel imbalance appeared at lower settings of the volume control. With the control set to 12:00, for example, the right channel was 0.6dB higher in level than the left.
Fig.3 plots the percentage of THD+noise in the Vali 2+'s headphone output as the level was increased into 300 ohms. Gain was set to Low. The THD+N gently rises above 200mV but remains well below 0.1% until the waveform starts to clip at 9V. Fig.4 repeats the test into 30 ohms, an impedance that is typical of many headphones. The THD+N is still low up to 5V, but now the Vali 2+ reaches 1% at 8.3V. This is well above the level necessary to drive low-impedance headphones to unbearably high sound levels. However, the picture changed dramatically when I switched the gain to High. Fig.5 shows that the THD+N is around 20 times higher at low output levels than it was with Low gain and that it rises in a linear fashion, reaching 1% at 3.1V. Repeating these tests from the preamplifier outputs gave identical results.
It is appropriate to note that Schiit's Jason Stoddard explains on the company's website that the Vali 2+ "is not a low-distortion design. It is designed to reflect the distortion of its tube gain stage, so total distortion is high, but it also falls off sharply for higher orders."
The distortion's harmonic signature was indeed almost entirely second harmonic with the gain set to both Low (fig.7) and High (fig.8), though the harmonic's level is very much lower with Low gain, –80dB (0.01%), than with High gain, –50dB (0.3%). Again, this performance was identical from the preamplifier outputs. I tested the Vali 2+ for intermodulation distortion with an equal mix of 19 and 20kHz tones at a peak level of 1V. With the gain set to Low (fig.9), the second-order difference product at 1kHz lay just below –90dB (0.003%), and the higher-order products all lay close to –120dB (0.0001%). While the intermodulation products were higher in level with the gain set to High (fig.10), they were not as high as I was expecting.
Even without taking its very affordable price into account, the Schiit Vali 2+ in its Low gain mode offers excellent measured performance. As the higher level of second-harmonic distortion in the High gain mode is not accompanied by high levels of intermodulation distortion, I would expect a "fatter" sound in this mode.—John Atkinson
Fig.1 Schiit Vali 2+, headphone output, frequency response with volume control set to maximum at 1V into: 100k ohms (left channel blue, right red), 300 ohms (left cyan, right magenta) (0.5dB/vertical div.).
Even though the tube is shared between the channels, channel separation was very good, at 80dB in both directions between 300Hz and 1kHz, though it decreased to 60dB at 20kHz (not shown). The wideband, unweighted signal/noise ratio, measured with the input shorted to ground but the volume control set to its maximum, was 86.75dB in the High gain condition and 99dB in the Low gain condition. (Both ratios are ref. 1V output and the average of both channels.) Restricting the measurement bandwidth to the audioband increased the S/N ratios to 95.6dB and 109dB, respectively, while switching an A-weighting filter into circuit further improved the ratios, to 102dB and 116.5dB. The drop in the Schiit's noise floor when the gain was switched from High to Low can be seen in fig.2, which shows the low-frequency spectrum as the Vali 2+ drove a 1kHz tone at 1V into 100k ohms. AC supply spuriae at 60Hz and 120Hz can be seen in this graph, but these are very low in level, even with High gain (blue and red traces).
Fig.2 Schiit Vali 2+, headphone output, spectrum of 1kHz sinewave, DC–1kHz, at 1V into 100k ohms with High gain (left channel blue, right red) and Low Gain (left cyan, right magenta) (linear frequency scale).
Fig.3 Schiit Vali 2+, headphone output, Low gain, distortion (%) vs 1kHz output voltage into 300 ohms.
Fig.4 Schiit Vali 2+, headphone output, Low gain, distortion (%) vs 1kHz output voltage into 30 ohms.
Fig.5 Schiit Vali 2+, headphone output, High gain, distortion (%) vs 1kHz output voltage into 300 ohms.
I measured how the Vali 2+'s distortion changed with frequency at 1V. The THD+N percentage was consistent across the audioband into both 100k ohms and 300 ohms and didn't increase into the lower impedance (fig.6). However, it was very much higher with High gain (blue, cyan, red, and magenta traces) than with Low gain (gray, yellow traces).
Fig.6 Schiit Vali 2+, headphone output, distortion (%) vs frequency at 1V: Low gain into 300 ohms (left yellow, right gray); High gain into 100k ohms (left blue, right red) and 300 ohms (left cyan, right magenta).
Fig.7 Schiit Vali 2+, headphone output, Low gain, spectrum of 50Hz sinewave, DC–1kHz, at 1V into 300 ohms (left channel blue, right red; linear frequency scale).
Fig.8 Schiit Vali 2+, headphone output, High gain, spectrum of 50Hz sinewave, DC–1kHz, at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.9 Schiit Vali 2+, headphone output, Low gain, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
Fig.10 Schiit Vali 2+, headphone output, High gain, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 1V into 100k ohms (left channel blue, right red; linear frequency scale).
