Sidebar 2: Measurements
The D 3020 was voted Stereophile's "Budget Component of 2014." Getting the review sample onto my test bench had been on my to-do list for way too long, but I was reminded of the unfinished task when I picked up the PS Audio Sprout for measurement from Herb Reichert, whose review of the latter appears elsewhere in this issue. I first checked the performance of the D 3020's digital inputs, looking at the signal from the headphone output jack with my Audio Precision SYS2722 system (see www.ap.com, and the January 2008 "As We See It").
The headphone output preserved absolute polarity for both analog and digital input signals, and the output impedance was on the high side, at 200 ohms at all audio frequencies. For analog input signals, the D 3020's headphone output offers a maximum gain of 28.6dB, but a digital signal with a level of –12dBFS clipped the headphone output with the volume control set to its maximum. At –14dBFS, the unclipped output level was 6.81V; so, except as noted, for the digital-input measurements I reduced the volume-control setting, to avoid clipping. The D 3020 locked to an S/PDIF datastream with sample rates up to 192kHz, even via TosLink, though the USB input was restricted to 96kHz and below. Apple's USB Prober utility identified the D 3020 as "NAD USB Audio" with the serial-number string "(C) 2011 Wavelength Audio, ltd." This implies that the D 3020 uses Gordon Rankin's "Streamlength" asynchronous USB protocol and USB Prober did indeed confirm that the USB input operated in the optimal isochronous asynchronous mode.
The reconstruction filter's impulse response (fig.1) was a conventional FIR type, with time-symmetrical ringing, while wideband spectral analysis of the analog output as the NAD decoded 44.1Hz data representing white noise at –4dBS (fig.2, magenta and red traces) indicated that the response rolled off quickly above half the sample rate (vertical green line). The sampling image at 25kHz of a full-scale tone at 19.1kHz (cyan, blue) was suppressed by more than 90dB. Note the rise in the ultrasonic noise floor in fig.2, suggesting that the D 3020's DAC stage uses some kind of sigma-delta upsampling. Fig.3 is a more conventional response measurement, taken with 44.1, 96, and 192kHz data. The overall shape of the response at ultrasonic frequencies is the same—a small, 0.4dB rise above the audioband—broken by a sharp rolloff just below 22 and 47kHz with the two lower rates.
Only when it came to jitter did I encounter some anomalous behavior via the D 3020's digital inputs. Fig.7 shows the spectrum of the headphone output with 24-bit J-Test data fed to the D 3020's USB input. No sidebands are visible around the 11.025kHz tone, and the noise floor is superbly clean. By contrast, fig.8 shows the spectrum of the analog output with the same data fed to the optical S/PDIF input. Some sidebands of unknown origin are visible, and the noise floor now has a peculiar sculpted appearance. This modulation of the noise floor was not apparent with the 19.1kHz tone in fig.2, which was taken with USB data. Repeating that test with S/PDIF data, I got a similar noise-floor modulation to that seen in fig.8.
Whereas the original 3020 used a conventional output stage, based on a complementary pair of 2N2055/MJ2955 bipolar transistors, the D 3020, as its name implies, is a class-D design, and so produces ultrasonic noise that would overload the Audio Precision's input circuitry. I therefore performed most of the tests of the D 3020 as a power amplifier using, ahead of the analyzer, an Audio Precision AUX-0025 passive low-pass filter, which eliminates noise above 200kHz. (Without the filter and with no signal, there was 140mV of ultrasonic noise with a center frequency around 485kHz present at the NAD's speaker terminals.)
Assessed at the speaker outputs, the D 3020 offered a modest maximum gain for line-level sources of 33.8dNB and was non-inverting. The input impedance ranged from 14.3k ohms at 20Hz to 11k ohms at 20kHz; the output impedance was around 0.15 ohm at all audio frequencies, resulting in a modulation of the amplifier's frequency response of less than ±0.15dB with our standard simulated loudspeaker (fig.9, gray trace). The response was –0.5dB at 20kHz, above which it rolled off sharply. (The headphone output was flat to 200kHz.) Commendably, the response didn't change at different volume-control settings, and switching the Bass equalization into circuit gave a boost of just over 6dB, centered between 70 and 80Hz, with a sharp rolloff below that region (fig.10). The D 3020's reproduction of a 10kHz squarewave with the Audio Precision low-pass filter in circuit was well defined, with just the slightest hint of overshoot on the leading edges (fig.11).
Figs. 13 and 14 show how the percentage of THD+noise in the NAD's output changed with output power into 8 and 4 ohms, respectively. The minimum distortion level was low, and the D 3020 clipped (defined as when the THD+N reaches 1%) at 58Wpc into 8 ohms (17.6dBW) and 68Wpc into 4 ohms (15.3dBW). These two graphs reveal that the actual distortion is buried beneath the noise floor at levels below a few watts. I therefore examined how the THD+N percentage changed with frequency at 9V, equivalent to 10Wpc into 8 ohms or 20Wpc into 4 ohms. The result (fig.15) revealed no change in the distortion at different frequencies, a commendable result.
I was impressed by the NAD D 3020's technical performance. It packs a usefully powerful amplifier into a tiny package, and offers digital performance that is close to the state of the art, though its USB input is to be preferred. In fact, the only problems I had with this little gem was that the touch switch on its top panel, for bringing the amplifier out of and into standby, didn't always respond. And the first time I tried disabling the Bass boost with the tiny rear-panel pushbutton, the D 3020 locked up and wouldn't respond to any commands, necessitating a hard reboot.
Stephen Mejias concluded his original review of the D 3020 by asking, "Will [the D 3020] be the component that introduces a new generation of music lovers to true high-fidelity sound?" I'd like to think that the answer to that question is "Yes!"—John Atkinson
Fig.1 NAD D 3020, headphone output, impulse response at 44.1kHz (4ms time window).
Fig.2 NAD D 3020, headphone output, wideband spectrum of white noise at –4dBFS (left channel red, right magenta) and 19.1kHz tone at 0dBFS (left blue, right cyan), with USB data sampled at 44.1kHz (20dB/vertical div.).
Fig.3 NAD D 3020, headphone output, frequency response at –12dBFS into 100k ohms with data sampled at: 44.1kHz (left channel green, right gray), 96kHz (left cyan, right magenta), 192kHz (left blue, right red) (1dB/vertical div.).
To assess the D 3020's digital resolution, I set the volume control to its maximum, an unrealistic situation that did allow me to examine by how many dB the noise floor dropped when I changed from 16-bit data representing a dithered 1kHz tone at –90dBFS to 24-bit data. Fig.4 shows that the noise floor dropped by almost 20dB, indicating ultimate resolution of better than 19 bits—superb performance, even without taking into account the D 3020's very affordable price. The NAD's reproduction of an undithered 16-bit, 1kHz sinewave at exactly –90.31dBFS (fig.5) was essentially perfect, with the three DC voltage levels described by the data easily visible. With undithered 24-bit data, the result was a superbly well-defined sinewave (fig.6).
Fig.4 NAD D 3020, headphone output, 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.5 NAD D 3020, headphone output, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.6 NAD D 3020, headphone output, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).
Fig.7 NAD D 3020, headphone output, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data from MacBook Pro via USB (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.8 NAD D 3020, headphone output, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit data from MacBook Pro via TosLink (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.9 NAD D 3020, volume control set to maximum, frequency response at 2.83V into: simulated loudspeaker load (gray), 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (green) (0.5dB/vertical div.).
Fig.10 NAD D 3020, volume control set to –20dB, frequency response at 2.83V into 8 ohms with Bass EQ activated (left channel blue, right red) and inactive (left green, right gray) (2dB/vertical div.).
Fig.11 NAD D 3020, small-signal, 1kHz squarewave into 8 ohms.
Channel separation was >80dB in both directions below 1kHz, while the unweighted, wideband signal/noise ratio, ref. 1W into 8 ohms and taken with the volume control set to its maximum, but the input shorted and with the AP filter in-circuit, was an excellent 80.2dB; this improved to 91.3dB when A-weighted. Fig.12 shows a spectral analysis of the low-frequency noisefloor.
Fig.12 NAD D 3020, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale, 0dB ref. 1W).
Fig.13 NAD D 3020, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.14 NAD D 3020, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.15 NAD D 3020, THD+N (%) vs frequency at 9V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta).
The distortion at lower powers was primarily the third harmonic (fig.16), with the second harmonic becoming predominant at higher powers (fig.17), though lower-level, higher-order harmonics are also present. Intermodulation distortion was also low, even at a level a few dB below visible waveform clipping (fig.18).
Fig.16 NAD D 3020, 1kHz waveform at 10W into 4 ohms, 0.014% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.17 NAD D 3020, spectrum of 50Hz sinewave, DC–1kHz, at 35W into 4 ohms (linear frequency scale).
Fig.18 NAD D 3020, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 23W peak into 8 ohms (linear frequency scale).
