John
A breath of fresh Airist — five young guns, whose education background runs from Harvard to Columbia to MIT, are back from the quiet success of their Heron 5 headphone amplifier with a new product: the Massdrop x Airist Audio R-2R DAC. Almost everything we need to know is in the product name to make the Airist DAC doubly unusual: an old-school design selling through a new-school distribution channel.
‘R-2R’ refers to how the DAC handles D/A conversion: a series of 24 resistors (per channel) with each resistor outputting twice the voltage of the previous resistor. Each resistor is turned on or off by the ones and zeroes present in a chunk of PCM data arriving via the DAC’s USB, TOSLINK or coaxial input e.g. 1001 0101 0111 1011. These groups of 16 (or 24) bits are called words. Each word specifies a voltage – a plot point on a waveform. For CD-quality audio, words arrive at the DAC at 44,100 times per second to net 44,100 point plots per second. For 96kHz audio, they arrive at 96,000 times per second for 96,000 plots per second.
‘R-2R’ is how DACs were made before delta-sigma chips swept the majority aside with lower manufacturing costs and lower power requirements. That might be fine for the mass market but it proved not so good news for those who care about sound quality. Some audiophiles complain that delta-sigma’s data handling – where bit depth is traded in for a higher sample rate – often leads to a harsher, more metallic sound.
‘R-2R’ demands the resistors used be made with precision. And, predictably, precision manufacturing doesn’t come cheap — it’s why we often see resistor ladder DACs selling in the thousands (not hundreds). See: TotalDAC, MSB, Aqua, LampiztOr, Metrum and dCS. Airist seeks to buck that price trend with an R-2R DAC that will sell for US$350.
“The RDAC contains two PCB boards: the bottom board with power, USB, and different file-conversion technologies, and the top board with the resistor ladders, designed to have the shortest distance between parts for the cleanest results. The hardware includes a custom programmable logic device and augmented power regulation with voltage-stabilization circuits. There is a buffer/isolator stage to each bit on the R-2R ladders, while the output buffer stage boasts five op-amps per channel. Finally, the inputs, outputs, and USB/SPDIF conversion are all located on a separate PCB from the resistor ladders.”
The above info wasn’t lifted from Airist’s press release (there wasn’t one) but from the Massdrop page from which Airist will tap maximum audiophile FOMO during the next 23 days to sell the first production run of 960 units. With further echoes of Kickstarter and Indiegogo, those units won’t ship until December 2018.
Readers are (once again) reminded that a DAC’s sound quality falls to more than its decoding topology. The power supply, the input board’s data handling, its immunity to EMI/RFI and the analogue output stage all play a (greater?) part. And just like turntables and their phono stages, not all R-2R DACs sound “rich, warm and natural”.
Still, if you feel like dipping your toe in the water…
Further information: Massdrop x Airist Audio R-2R DAC
Specifications:
- Ladder width: 24 bits
- Decoding format: Sign-magnitude
- PCM formats: Up to 24 bit/384 kHz over USB or 24/192 over coaxial/optical
- DSD formats: DSD64/128 over USB and S/PDIF if native DSD stream, DSD64/128 over USB using DoP, DSD64 over S/PDIF for DoP
- DSD method: Converted to PCM
- Filters: Linear-phase FIR
- Galvanic isolation: Yes
- Clocks: Low phase-noise NDK
- Frequency response, 20 Hz–20 kHz: +/-0.1 dB
- THD+N at 1 kHz: 0.08%
- SNR: 118 dB
- Crosstalk at 1 kHz : -120 dB
- Maximum output: 2.5 VRMS
- USB controller: XMOS XU208
- Inputs: USB, coaxial, optical
- Output: RCA
- Dimensions, L x W x H: 8.1 x 8.1 x 1.2 in (207 x 206 x 30 mm)
- Length with buttons and RCA port: 8.7 in (220 mm)
- Height with feet: 1.5 in (39 mm)
- Weight: 48.2 oz (1,366 g)
