Sidebar 3: Measurements
I performed a full set of measurements on the Wattson Madison LE using my Audio Precision SYS2722 system, repeating some of the testing with the magazine's higher-performance APx555 analyzer. I used network data streamed with Roon and optical S/PDIF data. (The TosLink input accepted data sampled at rates up to 192kHz.) I adjusted the processor's settings with the Wattson Music app, installed on my iPad mini.
The Madison preserved absolute polarity from the balanced and unbalanced line outputs, and from the headphone output. The volume control operated in steps that varied between 0.6dB and 1dB. With the volume control set to the maximum, the output levels with a full-scale 1kHz signal were 4.42V balanced, 2.25V unbalanced, and 2.82V headphone. The output impedances were usefully low from the balanced and the single-ended outputs: 147.3 ohms and 74.6 ohms, respectively, both consistent from 20Hz to 20kHz. The headphone output impedance was very low, at 2 ohms 20Hz–20kHz.
The measured performance of the Wattson Audio Madison LE is state of the digital art, at a relatively affordable price.—John Atkinson
I performed a full set of measurements on the Wattson Madison LE using my Audio Precision SYS2722 system, repeating some of the testing with the magazine's higher-performance APx555 analyzer. I used network data streamed with Roon and optical S/PDIF data. (The TosLink input accepted data sampled at rates up to 192kHz.) I adjusted the processor's settings with the Wattson Music app, installed on my iPad mini.
The Madison preserved absolute polarity from the balanced and unbalanced line outputs, and from the headphone output. The volume control operated in steps that varied between 0.6dB and 1dB. With the volume control set to the maximum, the output levels with a full-scale 1kHz signal were 4.42V balanced, 2.25V unbalanced, and 2.82V headphone. The output impedances were usefully low from the balanced and the single-ended outputs: 147.3 ohms and 74.6 ohms, respectively, both consistent from 20Hz to 20kHz. The headphone output impedance was very low, at 2 ohms 20Hz–20kHz.
Fig.1 Wattson Madison LE, impulse response (one sample at 0dBFS, 44.1kHz data, 4ms time window).
Fig.2 Wattson Madison LE, 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.1 shows the Madison's impulse response with data sampled at 44.1kHz. This is typical of a very short linear-phase reconstruction filter, with one cycle of ringing before and after the single sample at 0dBFS. The magenta and red traces in fig.2 show the Wattson's wideband spectrum with 44.1kHz white noise data at –4dBFS. As expected from the impulse response, a gentle rolloff starts at the top of the audioband with full stopband attenuation not reached until 41kHz. With this slow rolloff, the image at 25kHz of a 19.1kHz tone at 0dBFS (cyan, blue traces) is suppressed by just 10dB.
Fig.3 Wattson Madison LE, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), and 192kHz (left blue, right red) (1dB/vertical div.).
Fig.4 Wattson Madison LE, frequency response at –12dBFS into 100k ohms with data sampled at 192kHz with equalization set to "free field" (left channel blue, right red), "near a wall" (left cyan, right magenta), and to "in a corner" (left green, right gray), (1dB/vertical div.).
The Madison's frequency response with 44.1kHz data (fig.3, green and gray traces) is flat up to 10kHz but is then down by 3dB at 20kHz. With data sampled at 96kHz (cyan, magenta) and 192kHz (blue, red), the response is –3dB at 38kHz and 70kHz, respectively. The frequency response was identical from all three output types and was not affected by the volume control setting. Fig.4 shows the effect of the equalization set to "free field" (left blue, right red), "near a wall" (left cyan, right magenta), and "in a corner" (left green, right gray). There is no low-frequency rolloff in the "free field" setting; "near a wall" rolls off the midrange and bass by up to 3dB; "in a corner" rolls these regions off by up to 6dB.
Fig.5 Wattson Madison LE, spectrum of 24-bit 1kHz tone at 0dBFS, DC–1kHz, with volume control set to the maximum (left channel blue, right red) and to –12dB (left green, right gray) (20dB/vertical div.).
Channel separation was superb, at >120dB in both directions below 3kHz and still 111dB at the top of the audioband. The low-frequency noisefloor was free from power supply–related spuriae (fig.5). The blue and red traces in this graph were taken with the volume control set to the maximum; lowering the volume by 12dB (green, gray traces) reduced the level of the noisefloor by close to the same 12dB, which implies that the LEEDH volume control topology preserves resolution at its lower settings.
Fig.6 Wattson Madison LE, left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 20dB/vertical div.); linearity error (red, 1dB/small vertical div.).
Fig.7 Wattson Madison LE, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with 16-bit data (left channel green, right gray) and 24-bit data (left blue, right red) (20dB/vertical div.).
The red trace in fig.6 plots the error in the analog output level as a 24-bit, 1kHz digital tone stepped down from 0dBFS to –140dBFS. The amplitude error is 0.8dB down to –135dBFS, which implies superbly high resolution. When I examined the spectra with 16- and 24-bit dithered data representing a 1kHz tone at –90dBFS, the increase in bit depth lowered the noisefloor by 30dB (fig.7), which suggests a measured resolution of 21 bits.
Fig.8 Wattson Madison LE, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.9 Wattson Madison LE, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red)
When I played undithered data representing a tone at exactly –90.31dBFS, which consists of data at –1LSB, digital zero, and +1LSB, the waveform was symmetrical, and the three DC voltage levels described by the data were clearly defined (fig.8). With undithered 24-bit data, the Madison output a superbly clean sinewave (fig.9).
Fig.10 Wattson Madison LE, spectrum of 24-bit 1kHz sinewave, DC–1kHz, at 0dBFS into 200k ohms, volume control set to –12dB (left channel blue, right red, linear frequency scale).
Fig.11 Wattson Madison LE, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms, 24-bit, 44.1kHz data (left channel blue, right red; linear frequency scale).
The Wattson Madison LE produced primarily third-harmonic distortion, this lying at –104dB (0.0006%) with a 1kHz signal at 0dBFS. With the volume control set to the maximum, the second and higher-order harmonics were present at lower levels; repeating the spectral analysis with the control set to –12dB lowered the level of the third harmonic by 3dB and the other harmonics were now negligible (fig.10). Intermodulation distortion with 24-bit data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was low in level, even into 600 ohms (fig.11). Though a large number of aliasing products were present, other than images of the signal tones at 24.1kHz and 25.1kHz, these all lay below –120dB (0.0001%).
Fig.12 Wattson Madison LE, 16-bit network data, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.12 Wattson Madison LE, 24-bit TosLink data, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
The Madison was immune to jitter with all its inputs. Fig.12 shows the spectrum of its output when it was fed 16-bit J-Test data via my network. The odd-order harmonics of the undithered low-frequency, LSB-level squarewave lie at the correct levels; the spectrum was similarly excellent with the S/PDIF inputs and with 24-bit data (fig.13).
