Peachtree DAC•iT D/A converter Measurements

Sidebar 3: Measurements

I used Stereophile's loan sample of the top-of-the-line Audio Precision SYS2722 system to measure the Peachtree DAC•iT (see www.ap.com and the January 2008 "As We See It"); for some tests, I also used my vintage Audio Precision System One Dual Domain.

The Peachtree uses the popular Cirrus Logic 8416 S/PDIF receiver chip. The coaxial S/PDIF input successfully locked to datastreams with sample rates ranging from 44.1 to 192kHz. The TosLink input would not lock to datastreams with sample rates greater than 96kHz; this is normal behavior, however, as the TosLink specification doesn't include sample rates above 96kHz. The USB input correctly handled data with sample rates of 32, 44.1, 48, and 96kHz, but not 88.2kHz; the DAC•iT's Tenor TE7022L USB receiver chip is unable to operate at that sample rate, or at any rate above 96kHz. The Mac's USB Prober utility identified the DAC•iT as being a "Peachtree USB DAC" from "Peachtree audio," and revealed that the DAC•iT could operate as a two-channel processor with 16- or 24-bit capability in the normal "isochronous adaptive" mode. I found one anomaly: Not every music player program set the bit depth to 24. I was using Bias Peak Pro to play my test-signal WAV files; when I played 24-bit files, I got obvious truncation at the 16th bit, so I checked AudioMidi Set-Up: the DAC•iT had defaulted to 16 bits. I believe that Amarra, which Jon Iverson used for the review, correctly handles this, but the Peachtree's bit rate can always be manually set to 24.

The Peachtree's maximum output level at 1kHz was 2.04V, sourced from a usefully low impedance of 250 ohms across the audioband, and it preserved absolute polarity (ie, was non-inverting). Fig.1 shows the DAC•iT's frequency responses with data sampled at 44.1, 96, and 192kHz; each response follows the same curve, but with a sharp rolloff just below half the same rate. Channel separation (not shown) was asymmetrical: the R–L crosstalk was less than –105dB below 5kHz, but the L–R leakage was –95dB at all frequencies below 10kHz. Channel separation at 20kHz was >90dB in both directions at 20kHz, which is still excellent.

Fig.1 Peachtree DAC•iT, 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) (0.25dB/vertical div.).

For reasons of consistency with the reviews I've published since 1989, my first test of a D/A processor's dynamic range is to sweep a 1/3-octave bandpass filter from 20kHz to 20Hz while the processor decodes a dithered 1kHz tone at –90dBFS. The results of this test are shown in fig.2. With 16-bit data (top pair of traces), all that can be seen above 1kHz is the spectrum of the dither noise used to encode the signal; below that frequency the DAC•iT's noise floor starts to intrude, with a very slight bump at the AC line frequency of 60Hz. With 24-bit data (middle pair of traces), the noise floor drops by about 15dB in the treble, implying resolution of between 18 and 19 bits, though the noise floor looks rather granular. At low frequencies, however, the 24-bit data's resolution is obscured by the Peachtree's self noise. A similar picture of the DAC•iT's resolution can be seen in fig.3, which repeats the spectral analysis with a more insightful FFT technique, though some curious low-level spuriae are apparent with the 24-bit version of the signal at frequencies unrelated to the signal frequency.

Fig.2 Peachtree DAC•iT, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –90dBFS, with: 16-bit data (top), 24-bit data (middle), dithered 1kHz tone at –120dBFS with 24-bit data (bottom) (right channel dashed).

Fig.3 Peachtree DAC•iT, FFT-derived 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).

This anomalous behavior can also be seen in the bottom pair of traces in fig.2, which show the spectrum of the DAC•iT's output while it reproduced a dithered 24-bit tone at –120dBFS. While there is a small peak at 1kHz, the traces are disturbed by higher-level peaks at 800Hz and 1.7kHz—the ESS 9023 D/A chip used in the Peachtree appears to be producing "idle tones" with these data. I looked at this behavior in more detail: As you can see in fig.4, a dithered 24-bit tone at –110dBFS was correctly decoded, with no idle tones apparent in this 1/3-octave–smoothed spectrum. It was only when the signal dropped below this level that the D/A chip misbehaved. As real music never has information at this level without higher-level information also being present, this misbehavior will probably have no audible consequences.

Fig.4 Peachtree DAC•iT, 1/3-octave spectrum with noise and spuriae of dithered 1kHz tone at –110dBFS with 24-bit data (right channel dashed).

Though its noise floor is higher than usual at low frequencies, the Peachtree still managed to correctly reproduce the waveform of an undithered tone at exactly –90.31dBFS with both 16-bit data (fig.5) and 24-bit data (fig.6), and with no significant linearity error until below –110dBFS (not shown), which is good performance. A small degree of noise modulation was apparent, the noise floor rising as the level of a 1kHz tone increased from –60dBFS and –40dBFS (fig.7, green and gray, cyan and magenta traces, respectively) to 0dBFS (blue and red). Spectral peaks can also be seen in this graph at 60Hz and its odd harmonics, though these are all much too low in level to be audible.

Fig.5 Peachtree DAC•iT, waveform of undithered 1kHz sinewave at –90.31dBFS, 16-bit data (left channel blue, right red).

Fig.6 Peachtree DAC•iT, waveform of undithered 1kHz sinewave at –90.31dBFS, 24-bit data (left channel blue, right red).

Fig.7 Peachtree DAC•iT, spectrum of 1kHz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red) –40dBFS (left cyan, right magenta), and –60dBFS (left green, right gray); (linear frequency scale).

When I fed the DAC•iT data representing a full-scale 50Hz tone. I was surprised to see that the output was actually just starting to clip, with a picket fence of low-level harmonics apparent in the spectrum, and these higher in the right channel (fig.8, red trace) than in the left (blue). This graph was taken into the very kind 100k ohm load; reducing the load to 600 ohms resulted in the Peachtree's output stage being fully clipped (not shown). Reducing the signal level to –10dBFS decreased the distortion harmonics to a vanishingly low level, with the second harmonic dropping from –73dB left and –70dB right to, respectively, –94 and –96dB (fig.9). A similar picture emerged with the high-frequency intermodulation test: dropping the signal level below 0dBFS dramatically reduced the amount and level of intermodulation products (figs. 10 and 11). Though there appears to be a DC–DC converter on the Peachtree's circuit board, presumably to increase the incoming 9V from the wall-wart supply, there appears to be insufficient power-supply headroom to cope with full-scale signals without adding some distortion. (Though it is fair to note that this distortion looks worse than it will sound on the spectral plot, due to the fact that other than the third, all the higher harmonics lie at or below –80dB.) This may well not matter with real music, or at least with classical and jazz, in which peaks only occasionally hit 0dBFS. However, it might be a problem with modern pop recordings, which bang their heads against the 0dBFS limit more or less continuously.

Fig.8 Peachtree DAC•iT, spectrum of 50Hz sinewave, DC–1kHz, at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.9 Peachtree DAC•iT, spectrum of 50Hz sinewave, DC–1kHz, at –10dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.10 Peachtree DAC•iT, HF intermodulation spectrum, DC–30kHz, 19+20kHz at 0dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Fig.11 Peachtree DAC•iT, HF intermodulation spectrum, DC–30kHz, 19+20kHz at –10dBFS into 100k ohms (left channel blue, right red; linear frequency scale).

Though the DAC•iT offers good resolution, this is offset to some extent by its jitter rejection. Although it doesn't suffer from data-related jitter, thus not amplifying the level of the high-order harmonics of the 16-bit J-Test signal's low-frequency LSB-level squarewave (fig.12, cyan and magenta traces), some spectral spreading is evident due to the presence of random low-frequency jitter, which also affects the DAC•iT's reproduction of the 24-bit J-Test signal (fig.12, blue and red). This graph was taken with TosLink data; USB data gave the same result.

Fig.12 Peachtree DAC•iT, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 16-bit data via TosLink from AP SYS2722 (left channel cyan, right magenta), 24-bit data (left blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

Considering its affordable price, the compromises in the Peachtree DAC•iT's measured behavior seem to be well arranged in that, other than that lack of high-level headroom, they will have no audible consequences. And perhaps that lack of headroom would disappear with an upgraded external power supply?—John Atkinson

COMPANY INFO
Peachtree Audio
Signal Path International
2045 120th Ave NE
Bellevue, WA 98005
(704) 391-9337
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COMMENTS
aopu.mohsin's picture

Thanks Jon for the review. Sounds like a great product and comes recommended. However, I was wondering if you already have a list or can provide a "Highly-recommended" or "Top-10" list of budget DACs for budget-minded listeners (within different price range: $300, 500, $800 and $1000 something like it)?

Thanks in advance.

Jon Iverson's picture

Good question - I've not really developed one and am pondering how such a list could be created rationally. With the dozens of well-designed and reasonably priced DACs out there, this might be a big project. Would sure make an interesting shoot-out though. hmmm.

Stephen Scharf's picture

Knowing Peachtree's products, I'm sure it's a fine-sounding DAC, but I think the new Schiitt Bifrost is a better DAC. It's also $50 less expensive, can use a real power cord rather than a wall wart (try it with a Shunyata Venom3 for superb results), is upgradeable for both USB and DAC boards, and supports asynch USB up to 24/192. Having owned one for three months, I personally think the Bifrost is better than anything else in the $500 price class and runs VERY close in performance to my $1300 Wadia 121. 

A review of the Bifrost should be on Stereophile's short list.

Jon Iverson's picture

Hi Stephen - yes that's a good suggestion. I'm hoping to hear the BiFrost soon. Have you heard the DacIT yet? I wasn't expecting much at the price point but was taken aback with how much it sounded like a high-end DAC. They did a great job voicing it.

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