Peachtree Audio decco65 D/A integrated amplifier Measurements
Sidebar 3: Measurements
I measured the Peachtree decco65 using Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system (see www.ap.com and the January 2008 "As We See It") for both analog and digital inputs. To test the decco65 as a digital processor, I took the measurements from its Preamplifier Output jacks.
Hooking up my MacBook Pro to the Peachtree's USB input, I used the Apple USB Prober utility to identify the decco65 as "XMOS USB Audio 2.0," XMOS being the supplier of the USB data-receiver chip. Operational mode was confirmed as "isochronous asynchronous," and the sample rates handled extended up to 192kHz with 24-bit word length. The coaxial S/PDIF inputs locked to data with sample rates up to 192kHzas did, rather surprisingly, the TosLink input. All the digital inputs preserved absolute polarity (ie, were non-inverting), and a 1kHz tone at 0dBFS resulted in a level of 4.41V at the preamplifier output with the volume control set to its maximum.
The digital inputs' frequency response with data having sample rates of 44.1, 96, and 192kHz is shown in fig.1. With each sample rate, a smooth rolloff above the audioband is interrupted by a sharp cut in output just below the Nyquist frequency (half the sample rate). Channel separation at low and middle frequencies for digital data was excellent, at 100dB in both directions, but this worsened at 20kHz to a still-adequate 72dB, due to the usual capacitive coupling between the channels, probably at the volume control.
Increasing the bit depth from 16 to 24 dropped the noise floor by around 12dB (fig.2), implying resolution of approximately 18 bits. The peak representing the 1kHz tone at 90dBFS just reached the correct level, suggesting minimal linearity error. With 24-bit data (blue and red traces), however, some low-level enharmonic spuriae are unmasked. Fig.2 was taken with TosLink S/PDIF data; repeating the test via the decco65's USB port gave an identical result (fig.3), confirming that the USB input does correctly pass 24-bit data. With undithered 16-bit data, the waveform of a 1kHz tone at exactly 90.31dBFS was correctly reproduced (fig.4), though with more noise than the best-measuring D/A processors. With undithered 24-bit data (fig.5), the result was a well-shaped, if noisy, sinewave.
I uncovered an anomaly in the D/A section's behavior: When fed a full-scale tone, the reconstructed analog signal began to clip, giving rise to a picket fence of distortion harmonics in its spectrum (fig.6) that was independent of the volume-control setting. Fortunately, with a tone at 1dBFS the spectrum was very much cleaner (fig.7), with the second harmonic at 90dB (0.003%) and the third at 96dB (0.0015%), the only harmonics of note. The same thing happened with the high-frequency intermodulation test: the full-scale mix of 19 and 20kHz tones gave rise to many distortion products (fig.8), but the same signal at 1dBFS produced a clean spectrum (fig.9). Given how rarely music has a sustained level greater than 1dBFS, this anomaly should have no subjective consequences.
The decco65 proved effective at rejecting word-clock jitter on both its S/PDIF and USB inputs. Fig.10 is the spectrum of the output at the preamplifier outputs while the Peachtree decoded a 24-bit version of the Miller-Dunn J-Test via the USB port. No significant sidebands can be seen.
Turning to the decco65's performance via its analog inputs, the maximum gain measured at the speaker outputs at 1kHz was 31.3dB, this dropping by 0.25dB when the tube was switched into circuit with the remote control. The maximum gain from the preamplifier output was 8.25dB. The output was non-inverting from both speaker and preamplifier outputs. The preamplifier output impedance was a low 50 ohms at high and middle frequencies, increasing to 1k ohm at 20Hz, presumably due to the presence of an undersized coupling capacitor. The analog input impedance was a moderately high 15.5k ohms at 20Hz and 1kHz, dropping slightly but inconsequentially to 13.5k ohms at 20kHz.
The output impedance was moderately high over most of the audioband, at 0.18 ohm, rising to >1 ohm at 20kHz, due to the presence of a low-pass filter to reduce the level of ultrasonic noise. As a result of the interaction between this high source impedance and the impedance of our standard simulated loudspeaker, there was a variation in response of ±0.4dB (fig.11, gray trace), and an increasing rolloff at the top of the audioband as the load impedance decreased. The output at 20kHz is 1dB into 8 ohms (blue and red traces) but 3dB into 4 ohms (cyan and magenta traces). This rolloff slows the rise of a 10kHz waveform (fig.12). Like all subsequent graphs other than fig.13, fig.12 was taken with an Audio Precision AUX-0025 passive low-pass filter between the dummy load and the analyzer's input, to prevent ultrasonic switching noise generated by the decco65's output stage from corrupting the measurements.
Fig.11 was taken with the volume control set to its maximum; the matching between channels can be seen to be excellent. However, at lower control settings, an imbalance of up to 1dB appeared. Fig.13, for example, was taken with the volume control set to 12 o'clockthe right channel is now 0.75dB lower than the left. The tube was switched into circuit for this graph; as well as reducing the level of both channels by 0.25dB, it rolls off the top octave by 3dB at 20kHz. Channel separation measured at the speaker outputs was modest, at 70dB in both directions in the midrange, decreasing to 42dB at 20kHz (not shown).
The decco65 had much less ultrasonic noise in its output than other class-D amplifiers I have measured recently: just 23.6mV with a center frequency of 401kHz with the inputs shorted and the volume control at its maximum compared with 790.5mV for the Bel Canto CR7 reviewed elsewhere in this issue and 851.5mV for the Anthem M1 that was reviewed in the December 2012 issue. The decco65's noise level is equivalent to an unweighted, wideband signal/noise ratio of 41.6dB ref. 2.83V into 8 ohms, which increased to 70.4dB when the Audio Precision passive low-pass filter was switched into circuit. Further restricting the measurement bandwidth to the audioband increased the S/N ratio to 74.7dB, while an A-weighting filter gave 77.5dB. These are fairly modest results, mainly due to the presence of some low-level, power-supplyrelated spuriae in the decco65's output (fig.14).
Figs. 15 and 16 show how the THD+noise percentage in the Peachtree's output varied with power into 8 and 4 ohms, respectively. Specified as giving a maximum power of 65Wpc into 8 ohms (18.1dBW) or 95Wpc into 4 ohms (16.8dBW), the decco65 is shown by these graphs as clipping at 90Wpc into 8 ohms (19.5dBW) and at 120Wpc into 4 ohms (17.8dBW). (Clipping is defined to be when THD+N reaches 1%.)
Fig.17 shows how the THD+N varied with frequency into 8 ohms (blue and red traces) and 4 ohms (cyan and magenta). The decco65 turned itself off with sustained drive into 2 ohms, which is why I haven't plotted its behavior into this low impedance. (Turning the amp off and on again restored operation.) As is typical of class-D amplifiers, the THD rose at higher frequencies, especially into 4 ohms. This graph was plotted without the tube in circuit; switching it in made no difference.
At a moderate output level, 30W, into 4 ohms (fig.18) and 8 ohms (fig.19), the distortion was predominantly fifth harmonic in nature, though other harmonics are also present. At lower powers, however, the third harmonic dominated (figs.20 & 21). Finally, despite the decco65's decreasing linearity at high frequencies, it didn't perform as poorly as I was expecting on the high-frequency intermodulation test. With an equal mix of 19 and 20kHz tones a few dB below visible waveform clipping on the oscilloscope (fig.22), the difference component at 1kHz lay below 80dB (0.01%), with the higher-order components at 18 and 21kHz at 70dB (0.03%). Again, switching the tube into circuit produced no appreciable difference in these results.
Considered in the light of its $999 price, the decco65's measured performance is better than you might expect. Its digital section offers relatively high resolution, and while the predominance of the fifth harmonic at high powers is not something I like to see, its amplifier offers more subjectively benign distortion products at lower powers. The only thing that bugged me was the poor volume-control tracking, which could be considered inevitable given the designer's cost constraints.John Atkinson