Sidebar 3: Measurements
I measured Peachtree Audio's nova300 with my Audio Precision SYS2722 system (see the January 2008 "As We See It"). Before performing any tests, I ran it at 80Wpc into 8 ohms for an hour. At the end of that time, the vents on its top panel were warmer than I'd anticipated for an amplifier with a class-D output, at 92.3°F (33.5°C), though this is cooler than would be expected for an amplifier with a conventional output stage. Because of it uses ICEpower class-D modules, all measurements of the nova300 were taken using Audio Precision's auxiliary AUX-0025 passive low-pass filter, which eliminates noise above 200kHz that might otherwise overload the SYS2722's input circuitry. Without the filter, there was almost 1V of ultrasonic noise present at the Peachtree's output, with a center frequency of 472kHz.
Looking first at the nova300 via its line-level (Aux 1) input, the maximum gain was a moderate 33.3dB at the speaker terminals, 14.9dB at the preamplifier output, and 5.9dB at the headphone output. All three outputs preserved absolute polarity (ie, were non-inverting). The input impedance was 10k ohms at all audio frequencies. The output impedance from the headphone jack was very low, at 2.1 ohms, which will be appropriate for low-impedance cans. The impedance from the preamplifier output was a low 100 ohms, again constant across the audioband. The speaker output impedance was very low, at 20Hz and 1kHz at 0.086 ohm—but, very unusually, became negative at 20kHz; that is, a drop in load impedance resulted in a rise in output. I assume that this was due to the action of the necessary low-pass filter that removes most of the class-D stage's switching noise.
Fig.1 shows that while the response into 8 ohms (blue and red traces) is down by 0.5dB at 20kHz, into 2 ohms (green) the output is flat to 30kHz. And because of the low output impedance at low and middle frequencies, the modulation of the amplifier's response by the impedance of our standard simulated loudspeaker is negligible (gray). However, because of the low-pass filter before the speaker terminals, all the responses roll off sharply above 50kHz, which slows the rise of the waveform's leading edges with a 10kHz squarewave (fig.2). Two cycles of well-damped ringing can also be seen in this graph. The response from the headphone jack was flat to well above 100kHz.
The Peachtree comfortably exceeded its specified maximum output powers of 300W into 8 ohms (24.8dBW) and 450W into 4 ohms (23.5dBW), clipping (defined as 1% THD+noise), with both channels driven, at 350Wpc into 8 ohms (25.4dBW, fig.4) and 500Wpc into 4 ohms (24dBW, fig.5). At very high powers, however, the nova300 loudly buzzed and hummed. The THD+N was very low at low and middle frequencies into both 8 and 4 ohms (fig.6, blue, red, cyan, and magenta traces), but rose significantly in the top octave and into 2 ohms (gray). Fortunately, the distortion signature was heavily second-harmonic in nature (fig.7) and low in level, even at high powers (fig.8). High-frequency intermodulation was also very low (fig.9).
The impulse response with 44.1kHz-sampled data (fig.13) revealed the nova300's digital reconstruction filter to be a conventional FIR type, with symmetrical ringing either side of the single sample at 0dBFS. With 44.1kHz-sampled white noise (fig.14, red and magenta traces), this filter rolled off fast enough to almost eliminate the aliased product at 25kHz of a full-scale tone at 19.1kHz (blue, cyan). With spot frequencies sampled at 44.1, 96, and 192kHz (fig.15), the nova300's response followed the same basic shape, but with sharp rolloffs just below half of each sample frequency. With USB data sampled at 384kHz, the response was very similar to that at 192kHz, but with a slightly higher output above 50kHz, reaching –16dB at 80kHz. Channel separation via the digital inputs was good, at >90dB below 1kHz, but decreased to 70dB at the top of the audioband.
Following our positive review in 2010 of Peachtree's class-AB iDecco, I bought the review sample. It has given sterling service ever since, driving a 35-year-old pair of Celestion SL6 speakers in our 2.0-channel bedroom home-theater system. In most ways, the measured performance of Peachtree's nova300 is better than that of the iDecco, and it has a respectable MM phono stage. However, the high level of switching noise on its output and the less-good behavior of its S/PDIF inputs compared with its USB input raise my eyebrows a little.—John Atkinson
Fig.1 Peachtree nova300, 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.2 Peachtree nova300, small-signal, 10kHz squarewave into 8 ohms.
The Peachtree's channel separation via its Aux 1 input was very good, at >90dB in both directions below 1kHz, though it worsened slightly at 20kHz, to 70dB L–R and 78dB R–L. With the auxiliary Audio Precision filter in circuit, the unweighted, wideband signal/noise ratio was okay, at 66.5dB. This improved to 84dB when the measurement was restricted to the audioband, and to 87.5dB when A-weighted. Though there are some supply-related spuriae present in the nova300's output, these are very low in level (fig.3).
Fig.3 Peachtree nova300, spectrum of 1kHz sinewave, DC–1kHz, at 1W into 8 ohms (linear frequency scale).
Advertisement
Fig.4 Peachtree nova300, distortion (%) vs 1kHz continuous output power into 8 ohms.
Fig.5 Peachtree nova300, distortion (%) vs 1kHz continuous output power into 4 ohms.
Fig.6 Peachtree nova300, THD+N (%) vs frequency at 20V into: 8 ohms (left channel blue, right red), 4 ohms (left cyan, right magenta), 2 ohms (left gray).
Fig.7 Peachtree nova300, 1kHz waveform at 100W into 8 ohms, 0.0033% THD+N (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.8 Peachtree nova300, spectrum of 50Hz sinewave, DC–1kHz, at 100W into 8 ohms (linear frequency scale).
Fig.9 Peachtree nova300, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 100W peak into 8 ohms (linear frequency scale).
I had thought about measuring the nova300's phono input (Aux 2) at the preamplifier outputs, but as the speaker outputs are still active, that's where I examined its behavior, making sure the volume control was set to a level where the amplifier was not being overloaded. With the volume control set to its maximum, the overall gain was 70.8dB, which is appropriate for a moving-magnet cartridge. The input impedance was an MM-compatible 49k ohms at 20Hz and 1kHz, dropping slightly to 42k ohms at 20kHz. The RIAA-equalized response (fig.10) showed some mild boost in the midrange, and the output rolled off above the audioband due to the low-pass filter mentioned above.
Advertisement
Fig.10 Peachtree nova300, phono input response with RIAA correction (left channel blue, right red) (1dB/vertical div.).
Channel separation was good, at 60–75dB across the audioband, and the MM input was quiet: the unweighted wideband S/N ratio, ref. 1kHz at 5mV and measured with the inputs shorted but the volume control wide open, was 76dB (average of both channels), this improving to 84.4dB when A-weighted. Distortion was also low via the phono input, the third harmonic being the highest in level with a 1kHz tone at 5mV (fig.11), as was intermodulation distortion (fig.12). Overload margins were good, at 18dB across the audioband.
Fig.11 Peachtree nova300, phono input, spectrum of 1kHz sinewave, DC–1kHz, at 10mV input (linear frequency scale).
Fig.12 Peachtree nova300, phono input, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 10mV peak input (linear frequency scale).
Turning to the nova300's digital inputs and the Audio Precision's digital outputs, I used WAV and AIFF test-tone files sourced via USB from my MacBook Pro running on battery power with Pure Music 3.0. Apple's USB Prober utility identified the amplifier as "Peachtree nova300" from "Peachtree Audio" with the serial number "161020042." The USB port operated in the optimal isochronous asynchronous mode. Apple's AudioMIDI utility revealed that, via USB, the nova300 set to USB 2.0 accepted 16- and 24-bit integer data sampled at all rates from 44.1 to 384kHz. The S/PDIF inputs, including TosLink, accepted data sampled up to 192kHz.
With a 1kHz tone at –20dBFS and the volume control set to its maximum, the output level measured at the speaker terminals was 23.1V into 8 ohms, equivalent to 66.7W, which suggests that the digital inputs are more sensitive than they need to be. As I had for the phono input, other than for the low-level signal tests I set the volume control to avoid overloading the output stage for the digital tests.
Advertisement
Fig.13 Peachtree nova300, USB data, impulse response (one sample at 0dBFS, 44.1kHz sampling, 4ms time window).
Fig.14 Peachtree nova300, USB data, 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.).
Fig.15 Peachtree nova300, S/PDIF data, 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.).
Increasing the bit depth from 16 to 24 with a dithered 1kHz tone at –90dBS dropped the noise floor by almost 12dB, which implies resolution of 18 bits (fig.16), and the Peachtree readily resolved the three DC voltages with an undithered tone at exactly –90.31dBFS (fig.17), though with more HF noise than the norm. With undithered 24-bit data, the results was a rather noisy sinewave (fig.18). Harmonic and intermodulation distortion were both low in level for digital data.
Fig.16 Peachtree nova300, 44.1kHz S/PDIF data, 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.).
Advertisement
Fig.17 Peachtree nova300, S/PDIF data, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).
Fig.18 Peachtree nova300, S/PDIF data, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).
When I tested the S/PDIF inputs for their rejection of word-clock jitter with 16-bit J-Test data, I got somewhat anomalous results (fig.19). Though there is a strong pair of sidebands spaced 229Hz to either side of the spectral spike that represents a high-level tone at one-quarter the sample rate, the usual odd-order harmonics of the LSB-level tone seem arbitrarily boosted or attenuated. (The correct levels are shown by the sloping green line.) Things looked better with 16-bit USB data (fig.20), though the sidebands at ±229Hz are still higher in level than they should be. They were still present with 24-bit J-Test data with both USB and S/PDIF data (fig.21).
Fig.19 Peachtree nova300, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.20 Peachtree nova300, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Fig.21 Peachtree nova300, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.
Advertisement
