Sidebar 5: More Measurements
I used optical and coaxial S/PDIF data to test the DAC/streamer module's digital inputs, as well as network data played with Roon. The Roon app recognized the amplifier as "Onkyo A-50." While the optical and coaxial S/PDIF inputs—the latter is labeled "CD"—didn't lock to data with sample rates >96kHz, the A-50 accepted network data sampled at 176.4kHz and 192kHz. The digital inputs preserved absolute polarity at the speaker, Preamplifier, and headphone outputs.
With the volume control set to the maximum, the output level with a 1kHz tone at –30dBFS was 411mV from the Preamplifier output, 12.82V from the headphone output, and 11.42V into 8 ohms from the loudspeaker output. This is 9.2dB below the clipping voltage of 32.9V into that load; except where noted, I examined the digital inputs' behavior at the speaker outputs with Direct mode enabled and with the volume control set to –25dB in order to avoid overloading the A-50's output stages.
To confirm that the problem was the lack of a ground connection, I then repeated the jitter testing of the optical and network inputs with the coaxial cable disconnected but first with the AP's single-ended analog outputs connected to one of the A-50's line inputs, then with a separate wire connecting the analyzer ground to the grounding terminal on the amplifier's rear panel. The result with optical S/PDIF data was now the same as shown in fig.12 in both cases, though the supply-related sidebands were 5dB higher in level with network data.
Fig.12 Onkyo A-50, Direct mode, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit coaxial S/PDIF data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
I had problems when I tested the Onkyo A-50's rejection of data-related jitter. When the amplifier was fed 16-bit optical or network J-Test data, the resultant spectra were contaminated with high levels of supply-related sidebands around the central spike that represents the high-level tone at one-quarter the sample rate. Suspicious that the two-pin AC cable that was supplied with the Onkyo and which TF used for his auditioning doesn't have the usual ground connection, I therefore repeated the jitter test with coaxial S/PDIF data, which connects the analyzer ground to the amplifier with the signal cable's shield. The odd-order harmonics of the undithered low-frequency, LSB-level squarewave now all lay at the correct levels, shown by the green line in fig.12, and while there are still low-level supply-related sidebands at ±120Hz and ±240Hz present, these lay below –110dB (0.0003%).
Fig.13 Onkyo A-50, Direct mode, digital inputs, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.14 Onkyo A-50, Direct mode, digital inputs, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan) with data sampled at 44.1kHz (20dB/vertical div.).
I left the ground connection in place for the subsequent tests. The Onkyo's impulse response with PCM data sourced from Roon (fig.13) revealed that the review sample's reconstruction filter is a long linear-phase type, with equal amounts of ringing before and after the single sample at 0dBFS. The magenta and red traces in fig.14 show the ultrasonic rolloff of the Onkyo A-50's digital inputs with white noise data sampled at 44.1kHz. The traces reach full stop-band attenuation at 24kHz, above half the sample rate, which is indicated by the vertical green line. The aliased image at 25kHz of a 19.1kHz tone at 0dBFS (cyan, blue) is suppressed by almost 100dB and the harmonics associated with the 19.1kHz tone all lay at or below –70dB (0.03%).
Fig.15 Onkyo A-50, Direct mode, digital inputs, frequency response at –12dBFS 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.).
The digital frequency response with data sampled at 44.1kHz, 96kHz, and 192kHz (fig.14) was flat in the audioband with a sharp rolloff just below half of the two lower sample rates.
Fig.16 Onkyo A-50, Direct mode, digital inputs, spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS with: 16-bit data (left channel green, right gray), 24-bit data (left blue, right red) (20dB/vertical div.).
Fig.17 Onkyo A-50, Direct mode, digital inputs, waveform of undithered 16-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).
Fig.18 Onkyo A-50, Direct mode, digital inputs, waveform of undithered 24-bit, 1kHz sinewave at –90.31dBFS (left channel blue, right red).
An increase in bit depth from 16 to 24, with dithered data representing a 1kHz tone at –90dBFS and the volume control set to the maximum, dropped the Onkyo A-50's noisefloor by 12dB (fig.16), which implies a measured resolution of 18 bits. The presence of odd-order harmonics with 24-bit data (blue, red traces) suggests that the LSB is being truncated. With 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 clearly defined (fig.17). With undithered 24-bit data (fig.18), the Onkyo A-50 output a relatively clean sinewave.
Fig.19 Onkyo A-50, Direct mode, digital inputs, HF intermodulation spectrum (DC–30kHz), 19+20kHz, 24-bit data, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).
Intermodulation distortion with 24-bit data representing an equal mix of 19 and 20kHz tones, each at –6dBFS, was low in level, the difference product at 1kHz lying at –90dB (0.003%). Aliased images of the two tones were present, but their levels were close to –90dB.
The A-50's phono input can be set to moving coil (MC) or moving magnet (MM) with a rear-panel switch. I left the amplifier in Direct mode for the phono-input measurements and used the ground connection between the amplifier and the analyzer that I mentioned earlier. (This is recommended in the manual.) The MM input impedance was 46k ohms at 20Hz, 49k ohms at 1kHz, and 41k ohms at 20kHz. The MC input impedance was approximately 130 ohms, but I couldn't be specific, as the Audio Precision's voltage readings were disturbed by a high level of infrasonic noise. With the volume control set to the maximum, the gain in MM mode was 78.2dB at the loudspeaker output, 79dB at the headphone output, and 49.2dB from the Preamplifier output. The infrasonic noise made it impossible to estimate the gain in MC mode with the volume control set to the maximum. However, with the volume set to "50" with the app, which reduced the effect of the LF noise, the MC gain was approximately 20dB higher than in MM mode. Both modes preserved absolute polarity at all three output types.
Fig.20 Onkyo A-50, Direct mode, MM phono input, response with RIAA correction (left channel blue, right red) (1dB/vertical div.).
The Onkyo's RIAA equalization, measured in MM mode (fig.16), was superbly accurate, with very close channel matching. The A-50 phono stage's unweighted, wideband S/N ratio in MM mode, measured at the loudspeaker output with the input shorted to ground and the volume control set to the maximum, was a good 64.4dB in both channels ref. 1kHz at 5mV. Restricting the measurement bandwidth to 22Hz–22kHz didn't increase the ratio, while the A-weighted ratio was 70dB. I couldn't measure the S/N ratios in MC mode, as the level of the infrasonic noise was only a few dB below the MC reference level of 1kHz at 500µV. Spectral analysis of the phono stage's low-frequency noisefloor in MM mode revealed that power supply– related spuriae were very low in level, at >80dB ref. 1kHz at 5mV.
I measured the A-50 phono input's overload margins with the volume control set to "40" to make sure I was looking at actual input overload rather than output stage clipping. The margins in MM mode ref. 1kHz at 5mV were an excellent 24.6dB across the audioband. Harmonic and intermodulation distortion in MM mode were also very low. The difference product at 1kHz with an equal mix of 19kHz and 20kHz tones, with a peak level of 100mV at the volume control set to "50," lay below –100dB (0.001%).
The Onkyo A-50's measured performance with its analog line-level inputs was very good. The measured performance with digital inputs was respectable, but only when there was a ground connection between the amplifier and the audio data source, which will be the case with a coaxial S/PDIF connection. If an analog source is connected to the line-level inputs, which should make the necessary ground connection, the performance with TosLink and network data will be similar to that with coaxial data. The phono input in moving magnet mode did well on the test bench, but the moving coil mode was compromised by the presence of high-level infrasonic noise. This could be mitigated with the bass control, but that would also have a negative effect on the music being played. The optimal solution would be a true subsonic high-pass filter.—John Atkinson
