Sound Quest Midi Quest 11 Pro V1
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Sidebar 3: MeasurementsI measured the AudioQuest DragonFly Red and Black with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). I performed a full set of tests on both converters with WAV and AIFF test-tone files sourced from an iPad 2 with a 30-pin-to-USB "camera connection kit" adapter. I then repeated some of the testing, playing the same files with Pure Music 3.0 on my MacBook Pro running on battery power, and on my iPhone 6 using a Lightning-to-USB adapter. AudioQuest's Stephen Mejias warned me that the DragonFlys' rejection of jitter would be optimized after they'd been powered up for 24 hours, so before I did any testing, I left both plugged into USB ports on one of the PCs in my lab for 36 hours.Apple's USB Prober utility identified the DragonFlys as "AudioQuest DragonFly Red v1.0" and "AudioQuest DragonFly Black v1.5," both from "AudioQuest." Both converters' USB ports operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that they accepted 24-bit integer data sampled at all rates from 44.1 to 96kHz. The DragonFlys' maximum output levels at 1kHz into a high 100k ohms load were 1.19V (Black) and 2.04V (Red). Both DragonFlys preserved absolute polarity (ie, were non-inverting), and both offered a very low output impedance of 110dB. This graph was taken with the Red, and the harmonics associated with the 19.1kHz tone all lie below –80dB. The Black's reconstruction filter offers similar rejection above the audioband (fig.3), but the harmonics of the 19.1kHz tone are all higher in level, at –57dB (0.14%).Fig.1 AudioQuest DragonFly Red, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).Fig.2 AudioQuest DragonFly Red, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).Fig.3 AudioQuest DragonFly Black, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) into 100k ohms with data sampled at 44.1kHz (20dB/vertical div.).Fig.4 shows the Red's frequency response with data sampled at 44.1 and 96kHz. The smooth rolloff above the audioband is broken by a sharper drop just below half of each rate. Channel separation at 1kHz was excellent, at 110dB R–L and 115dB L–R, but decreased to 75 and 87dB, respectively, at 20Hz. I measured the channel separation with the DragonFly Red connected to the iPad 2. This increase in crosstalk at low frequencies is usually due to increasing power-supply impedance, so I repeated the test with the MacBook Pro, only to get an identical result.Fig.4 AudioQuest DragonFly Red, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel cyan, right magenta), 96kHz (left blue, right red) (1dB/vertical div.).An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the Red's noise floor by almost 12dB (fig.5), suggesting a resolution of around 18 bits—good for a DAC powered by an iPhone or iPad, and a little better than that of the original DragonFly, particularly at low frequencies. Repeating this test with the Black (fig.6) suggested that the less expensive DragonFly offers 17 bits' worth of resolution, though this is still good enough to allow it to decode a 24-bit tone at –120dBFS (fig.7). When I played undithered data representing a tone at exactly –90.31dBFS, the waveform was symmetrical, with no DC offset, but the three DC voltage levels described by the data were obscured by high-frequency noise (fig.8).Fig.5 AudioQuest DragonFly Red, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel cyan, right magenta), 24-bit data (left blue, right red) (20dB/vertical div.).Fig.6 AudioQuest DragonFly Black, spectrum with noise and spuria
