Sidebar 3: Measurements
I measured the Weiss Helios with my Audio Precision SYS2722, repeating some of the tests with the higher-resolution APx555. All the measurements were performed with the Helios's DSP bypassed. Apple's USB Prober utility identified the Helios as "Helios" from "Weiss_Engineering_Ltd." with the serial number string "0.0.1." The USB port operated in the optimal isochronous asynchronous mode, and Apple's AudioMIDI utility revealed that the Helios accepted 32-bit integer data sampled at all rates from 32kHz to 384kHz. The AES3 and coaxial S/PDIF inputs accepted data sampled at rates up to 192kHz; the TosLink input was restricted to sample rates of 96kHz and lower.
With the balanced output mode set to Loudspeaker, the Helios's maximum output level at 1kHz feeding 100k ohms was 16.4V with the level set to "0dB"; 10.3V set to "–4dB"; 6.5V set to "–8dB"; 4.1V set to "–12dB"; 2.6V set to "–16dB"; 1.63V set to "–20dB"; 1.03V set to "–24dB"; and 648mV set to "–28dB." Each nominal reduction of 4dB in the output level reduced the actual level by 4dB. As expected, the maximum levels from the unbalanced outputs were half those from the balanced outputs. Switching the output to Headphone reduced all the maximum levels by 12dB. With its polarity button set to Normal, the Helios preserved absolute polarity (ie, was noninverting) from all of its outputs. The balanced output impedance was 93.5 ohms at all audio frequencies in both Loudspeaker and Headphone modes; the unbalanced output impedance was 47 ohms in both modes.
Weiss's DAC502 performed supremely well on the test bench, but its measured performance was exceeded by that of the Weiss Helios!—John Atkinson
Fig.1 Weiss Helios, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.2 Weiss Helios, 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 Helios's impulse response with 44.1kHz data. It is typical of a conventional linear-phase filter with a symmetrical ringing before and after the single full-scale sample. This filter's ultrasonic rolloff (fig.2, magenta and red traces) reaches full stop-band attenuation at 24kHz with complete suppression of the aliased image at 25kHz of a full-scale tone at 19.1kHz (cyan, blue). The harmonics associated with the 19.1kHz tone all lie below –107dB.
Fig.3 Weiss Helios, 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.3 shows the Helios's frequency response with data sampled at 44.1, 96, and 192kHz. The response with all three sample rates is down by just 0.1dB at the top of the audioband with then a sharp rolloff just below half of the two lower sample rates. The response with 192kHz data continues the relatively gentle ultrasonic rolloff, reaching –3dB at 71kHz.
Fig.4 Weiss Helios, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS with volume control set to the maximum (left channel blue, right red; linear frequency scale).
Channel separation was superb, at >122dB in both directions below 3kHz, decreasing to a still-superb 113dB at 20kHz. Fig.4 shows the spectrum of the Helios's low-frequency noisefloor as it drove a full-scale 1kHz tone with the volume control set to its maximum and the level set to "0dB." The level of the random noise is extremely low, and there are no AC supply–related spuriae present. Reducing the maximum level didn't increase the level of the noisefloor.
Fig.5 Weiss Helios, left channel, 1kHz output level vs 24-bit data level in dBFS (blue, 20dB/vertical div.); linearity error (red, 2dB/small vertical div.).
Fig.6 Weiss Helios, 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.).
The red trace in fig.5 plots the error in the analog output level as a 24-bit, 1kHz digital tone stepped down from 0dBFS to –140dBFS. Even at the lowest level, the amplitude error is 1.1dB, which implies very high resolution. An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS, dropped the Helios's noisefloor by 33dB (fig.6). This implies a resolution between 21 and 22 bits, which is the highest I have encountered, greater even than that of the Weiss DAC502 and of the Merging Hapi MKII that KR reviewed in January 2024.
Fig.7 Weiss Helios, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.8 Weiss Helios, 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, the waveform was symmetrical, with negligible DC offset, and the three DC voltage levels described by the data were free from noise (fig.7). With undithered 24-bit data (fig.8), the Helios's very low analog noisefloor means it can output a clean sinewave, even at this very low signal level.
Fig.9 Weiss Helios, 24-bit data, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 600 ohms (left channel blue, right red; linear frequency scale).
Fig.10 Weiss Helios, 24-bit data, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 600 ohms, 44.1kHz data (left channel blue, right red; linear frequency scale).
Even set to its highest output level, the Helios produced very low levels of harmonic distortion with full-scale data even into the punishing 600 ohm load (fig.9). The subjectively benign second and third harmonics were the highest in level, but each lay close to a negligible –120dB (0.0001%)! Intermodulation distortion with an equal mix of 19 and 20kHz tones at –6dBFS was similarly extremely low (fig.10), with the difference tone at 1kHz into 600 ohms lying at –130dB (0.00003%)!
Fig.11 Weiss Helios, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit AES3 data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.12 Weiss Helios, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit network data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
The Helios offered excellent rejection of word-clock jitter. Fig.11 shows the spectrum of the Helios's output when it was fed high-level 16-bit J-Test data via AES3. All the odd-order harmonics of the undithered low-frequency, LSB-level squarewave lie at the correct levels, and no other sideband pairs are visible. The central spike that represents the high-level tone at one-quarter the sample rate (Fs/4) is broadened at its base, which was not the case with USB and network data (fig.12).
