Orchard Audio PecanPi+ Streamer Premium DAC Measurements

Link: reviewed by Matt Bonaccio on SoundStage! Hi-Fi on October 15, 2024

General Information

All measurements taken using an Audio Precision APx555 B Series analyzer.

The Orchard Audio PecanPi+ Streamer Premium was evaluated as a DAC and conditioned for 30 min at 0dBFS (volume set to 2Vrms out) into 200k ohms before any measurements were taken.

The Orchard Audio PecanPi+ Premium is marketed as a network streamer, but does offer one coaxial S/PDIF (RCA) digital input. There are two line-level outputs (balanced XLR and unbalanced RCA) and one headphone output (1/4″ TRS). There is a digital volume control for the headphone and line-level outputs via a potentiometer on the front panel. Comparisons were made between unbalanced and balanced line-level outputs and no appreciable differences were seen in terms of THD and noise, but 1kHz FFTs are provided for both balanced and unbalanced outputs.

The analyzer’s input bandwidth filter was set to 10Hz-22.4kHz for all measurements, except for frequency-response (DC to 1 MHz), FFT (10Hz to 90kHz), and THD vs frequency (10Hz to 90kHz) charts, the latter to capture the second and third harmonics of the 20kHz output signal.

The PecanPi+ digital volume control offers no visual on-screen feedback to the user in terms of level. Volume can be adjusted in 0.5dB steps. Channel-to-channel deviation was good, at around 0.013dB throughout the range.

Volume-control accuracy (measured at line-level outputs): left-right channel tracking

Primary measurements

Our primary measurements revealed the following using the coaxial input and the balanced line-level outputs (unless specified, assume a 1kHz sinewave at 0dBFS, 200k ohms loading, 10Hz to 22.4kHz bandwidth):

*due to very low noise of DUT, analyzer self-noise has been removed from measurement to more accurately report value

Our primary measurements revealed the following using the coaxial input and the headphone output (unless specified, assume a 1kHz sinewave at 0dBFS, 300 ohms loading, 10Hz to 22.4kHz bandwidth):

Frequency response vs. sample rate (16/44.1, 24/96, 24/192; Coherent Processing)

The plot above shows the PecanPi+’s frequency response as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital input signal from 5Hz to 22kHz, the purple/green traces are for a 24/96 dithered digital input signal from 5Hz to 48kHz, and finally orange/pink represents 24/192 from 5Hz to 96kHz. The behavior at low frequencies is the same for the digital input—perfectly flat down to 5Hz. The behavior at high frequencies for all three digital sample rates is soft filtering below 20, 30, and 50kHz (less than half the respective sample rate), The -3dB point for each sample rate is roughly 16, 35, and 70kHz, respectively. With 16/44.1 data, the -1dB point is at 13.1kHz. The PecanPi+ appears to utilize a reconstruction filter that prioritizes a clean impulse response (no pre-/post-ringing behaviour) with virtually no phase shift at the expense of more high-frequency attenuation in the frequency domain. Evidence for this can be seen in other graphs in this report. In the graph above and most of the graphs below, only a single trace may be visible. This is because the left channel (blue, purple or orange trace) is performing identically to the right channel (red, green or pink trace), and so they perfectly overlap, indicating that the two channels are ideally matched.

Phase response vs. sample rate (16/44.1, 24/96, 24/192)

Above are the phase response plots from 20Hz to 20kHz for a 0dBFS input signal as a function of sample rate. The blue/red traces are for a 16-bit/44.1kHz dithered digital, the purple/green traces are for a 24/96 dithered digital input signal, and finally orange/pink represents 24/192 from 5Hz to 96kHz.  There is essentially no phase shift in the audioband, even for the 16/44.1 data.

Digital linearity (16/44.1 and 24/96 data)

The graph above shows the results of a linearity test for the coaxial digital input for both 16/44.1 (blue/red) and 24/96 (purple/green) input data, measured at the balanced line-level output of the PecanPi+. The digital input was swept with a dithered 1kHz input signal from -120dBFS to 0dBFS, and the output was analyzed by the APx555. The ideal response is a straight flat line at 0dB (i.e., the amplitude of the digital input perfectly matches the amplitude of the measured analog output). The 24/96 input data is essentially perfect down to -120dBFS, while the 16/44.1 input data performed well, only over-responding by 3/2dB (left/right) at -120dBFS. The 24/96 data yielded such superb results that we extended the sweep down to . . .

. . . -140dBFS. Above we see that even at -140dBFS, the PecanPi+ is only over/undershooting by 1 dB between -140 and -130dBFS. This is an excellent linearity-test result.

Impulse response

The graph above shows the impulse responses for a looped 24/44.1 test file that moves from digital silence to full 0dBFS (all “1”s) for one sample period then back to digital silence, fed to the coaxial digital input, measured at the balanced outputs, into a 200k ohm-load for the left channel only. We can see that PecanPi+ yields an impulse response with essentially no pre- or post-ringing behaviour, or one that emulates a non-oversampling DAC.

J-Test (coaxial input)

The plot above shows the results of the “J-test” test for the coaxial digital input measured at the balanced line level output of the PecanPi+. The “J-test” was developed by Julian Dunn the 1990s. It is a test signal, specifically a -3dBFS undithered 12kHz squarewave sampled (in this case) at 48kHz (24bit). Since even the first odd harmonic (i.e. 36kHz) of the 12kHz squarewave is removed by the bandwidth limitation of the sampling rate, we are left with a 12kHz sinewave (the main peak). In addition, an undithered 250Hz squarewave at -144dBFS is mixed with the signal. This test file causes the 22 least significant bits to constantly toggle which produces strong jitter spectral components at the 250Hz rate and its odd harmonics. The test file shows how susceptible the DAC and delivery interface are to jitter, which would manifest as peaks above the noise floor at 500Hz intervals (e.g. 250Hz, 750Hz, 1250Hz, etc). Note that the alternating peaks are in the test file itself, but at levels of -144dBrA (fundamental at 250Hz) down to -170dBrA for the odd harmonics. The test file can also be used in conjunction with artificially injected sinewave jitter by the Audio Precision, to also show how well the DAC rejects jitter.

The coaxial input shows a strong J-Test result, with peaks visible, but only below the -140dBrA level.

J-Test (coaxial input, 2kHz sinewave jitter at 100ns)

The plot above shows the results of the J-Test test for the coaxial digital input measured at the balanced line-level output, with an additional 100ns of 2kHz sinewave jitter injected by the APx555, which would manifest as sideband peaks at 10kHz and 14kHz without perfect jitter immunity. The results show no visible sidebands. This is further evidence of the PecanPi+s strong jitter immunity.

Wideband FFT spectrum of white noise and 19.1kHz sinewave tone

The plot above shows a fast Fourier transform (FFT) of the PecanPi+’s balanced-line level output with white noise at -4 dBFS (blue/red) and a 19.1 kHz sinewave at 0dBFS fed to the coaxial digital input, sampled at 16/44.1 (purple/green). There is a soft roll-off above 20kHz in the white-noise spectrum. There are absolutely no imaged aliasing artifacts in the audioband above the -135dBrA noise floor. The primary aliasing signal at 25kHz is barely suppressed at -20dBrA.

THD ratio (unweighted) vs. frequency vs. load (24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms (blue/red) and 600 ohms (purple/green) as a function of frequency for a 24/96 dithered 1kHz signal at the coaxial input. The 200k and 600 ohms data are identical throughout the audioband, which is an indication that the PecanPi+’s outputs are robust and can handle loads below 1k ohms with no difficulty. There was an evident difference in THD ratios between the left and right channels, with the right channel outperforming the left by about 10-15dB from 20Hz to 1kHz. THD ratios (right channel) ranged from 0.00003-0.00005% from 20Hz to 2kHz, then up to 0.0005% at 20kHz. Despite the left/right THD ratio discrepancy, these values are extremely low and nearing the limits of what the APx555 can measure.

THD ratio (unweighted) vs. frequency vs. sample rate (16/44.1, 24/96)

The chart above shows THD ratios at the balanced line-level output into 200k ohms as a function of frequency for a 16/44.1 (blue/red) and a 24/96 (purple/green) dithered 1kHz signal at the coaxial input. The 24/96 data (right channel) consistently outperformed the 16/44.1 data by 10-15dB from 20Hz to about 1kHz due to the inherently higher noise floor at 16 bits (i.e., the analyzer cannot assign a THD value below the noise floor). THD ratios with 16/44.1 data ranged from 0.0001 to 0.0005% throughput the audioband. 24/96 THD ratios (left channel) were essentially the same as the 16/44.1 data.

THD ratio (unweighted) vs. output (16/44.1, 24/96)

The chart above shows THD ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green), with the volume set to maximum. Once again, the 24/96 outperformed the 16/44.1 data, with a THD range from 0.2% at 300uVrms to 0.00005% at 2Vrms, while the 16/44.1 ranged from 3% down to 0.0005% at the maximum output voltage of 5.19Vrms.

THD+N ratio (unweighted) vs. output (16/44.1, 24/96)

The chart above shows THD+N ratios measured at the balanced output as a function of output voltage for the coaxial input into 200k ohms from -90dBFS to 0dBFS at 16/44.1 (blue/red) and 24/96 (purple/green), with the volume set to maximum. The 24/96 outperformed the 16/44.1 data, with a THD+N range from 2% down to 0.0002% at 3-5Vrms, while the 16/44.1 ranged from 30% down to 0.002% at the maximum output voltage of 5.19Vrms.

Intermodulation distortion vs. generator level (SMPTE, 60Hz:4kHz, 4:1; 16/44.1, 24/96)

The chart above shows intermodulation distortion (IMD) ratios measured at balanced output for 16/44.1 (blue/red) input data and 24/96 input data (purple/green), from -60dBFS to 0dBFS. Here, the SMPTE IMD method was used, where the primary frequency (F1 = 60Hz) and the secondary frequency (F2 = 7kHz) are mixed at a ratio of 4:1. The SMPTE IMD analysis results consider the second (F2 ± F1) through the fifth (F2 ± 4xF1) modulation products. The 16/44.1 data yields IMD ratios from 2% down to 0.002% at 0dBFS. The 24/96 data yields IMD ratios from 0.1% down to 0.0005% from -10 to 0dBFS.

FFT spectrum – 1kHz (digital input, 16/44.1 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1. We see signal harmonics at 2 and 3kHz. The second (2kHz) harmonic for the left channel (right channel cannot be seen above the noise floor) is at -120dBrA, or 0.0001%, and the third harmonic (3kHz) is at -130dBRa, or 0.00003%. There are also no power-supply noise peaks to speak of to the left of the main signal peak.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/96. Due to the increased bit-depth, the noise floor is much lower compared to the 16/44.1 FFT at a very low -160dBrA. We see signal harmonics ranging from -120dBrA to -150dBrA, or 0.0001% to 0.000003%, all the way to 20kHz (and beyond). The second (2kHz) signal harmonic shows the significant discrepancy between the left (-120dBrA) and right (-150dBrA) channels. Here also, there are no powersupply noise peaks to speak of to the left of the main signal peak.

FFT spectrum – 1kHz (digital input, 24/96 data at 0dBFS, unbalanced output)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the unbalanced output into 100k ohms for the coaxial digital input, sampled at 24/96. The main difference with the balanced output is at the second (2kHz) signal harmonic for the right channel, where here we find a level of -125dBrA, or 0.00006%, instead of the -150dBrA for the balanced outputs. The third (3kHz) harmonic is also higher here, at -120dBrA, or 0.0001%, instead of -130dBrA for the balanced outputs.

FFT spectrum – 1kHz (digital input, 16/44.1 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 16/44.1 at -90dBFS. We see the signal peak at the correct amplitude, and essentially no signal harmonics or noise peaks.

FFT spectrum – 1kHz (digital input, 24/96 data at -90dBFS)

Shown above is the fast Fourier transform (FFT) for a 1kHz input sinewave stimulus, measured at the balanced output into 200k ohm for the coaxial digital input, sampled at 24/961 at -90dBFS. We see the signal peak at the correct amplitude, and essentially no signal harmonics or noise peaks.

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 16/44.1)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 16/44.1. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is perhaps barely visible above the noise floor at -130dBrA, or 0.00003%, and the third-order modulation products, at 17kHz and 20kHz, are at -135dBrA, or 0.00002% (left channel).

Intermodulation distortion FFT (18kHz + 19kHz summed stimulus, 24/96)

Shown above is an FFT of the intermodulation distortion (IMD) products for an 18kHz + 19kHz summed sinewave stimulus tone measured at the balanced output into 200k ohms for the coaxial input at 24/96. The input dBFS values are set at -6.02dBFS so that, if summed for a mean frequency of 18.5kHz, would yield 2Vrms (0dBrA) at the output. We find that the second-order modulation product (i.e., the difference signal of 1kHz) is at -130dBrA, or 0.00003%, and the third-order modulation products, at 17kHz and 20kHz, are at the same level.

Intermodulation distortion FFT (coaxial input, APx 32 tone, 24/192)

Shown above is the FFT of the balanced line-level output of the PecanPi+ with the APx 32-tone signal applied to the input. The combined amplitude of the 32 tones is the 0dBFS reference, and it corresponds to 2Vrms into 200k ohms. The intermodulation products—i.e., the “grass” between the test tones—are distortion products from the amplifier and below the -140dBrA, or 0.00001%, level. This is a very clean IMD result.

Diego Estan
Electronics Measurement Specialist