With its independently configurable outputs, this A‑D converter is a little different from most of its competitors...
The pro‑audio world is awash with A‑D converters across a wide range of prices and with an equally wide range of features. However, the British manufacturer Drawmer's A2D2 stands out from the crowd because of its unusually versatile feature set and design topology.
This two‑channel, high‑quality A‑D converter is housed in a 1U rackmount case and accepts stereo line inputs via electronically balanced XLRs. It provides two independent sets of outputs, each with AES3, S/PDIF coaxial and Toslink optical ports, all being active all of the time. There is also a quartet of BNC sockets, providing one word clock input and three buffered outputs. A fused IEC mains inlet feeds a linear power supply, which is factory‑set for the local supply voltage (but is switchable internally), and completes the rear-panel facilities.
The front panel follows Drawmer's house style for its DMS products — a brushed aluminium facia, with a large blue illuminated logo at the centre. The right‑hand side is dominated by a huge stereo bar-graph meter, which spans a 50dB range, with 1dB increments down to ‑10dBFS, 2dB steps onwards to ‑30dBFS and 5dB intervals for the rest. As standard, the meter shows green up to ‑7dBFS, with yellow to ‑1dBFS and red for the zero and overloading lights.
The left‑hand side of the central logo carries the configuration controls, which start with a pair of rotary input‑level controls scaled to accept peak levels from +28 down to ‑2dBu. An illuminated button switches between these controls and a fixed calibrated input level, set via a pair of front‑panel, 24‑turn level trimmers, with adjacent indicators identifying the current mode. The converter is aligned at the factory such that the calibrated mode conforms to the EBU specification, providing ‑18dBFS for a 0dBu input.
The sample rate of each of the two sets of outputs can be configured independently via two pairs of up‑down buttons, each cycling through six standard sample rates from 44.1 to 192kHz, plus the external clock input — and the current mode is shown on two rows of LEDs. A further pair of red LEDs illuminates when the system has locked to an external clock. The final two buttons configure the two output sets to provide 24 bits of properly re‑dithered, 16‑bit output formats, the latter suiting legacy-format recorders.
Internally, the analogue front end is constructed from very high-quality Burr Brown OP275 op‑amps. These feed a Cirrus CS5381 A‑D converter running from its own crystal clock, operating at a fixed sample‑rate of 210.9kHz. The digital outputs are delivered via Burr Brown SRC4192 sample-rate converters (which also provide the word-length reduction function) and Cirrus CS8406 transmitters. The output sample rate clocks for these two SRCs are derived from two separate, temperature‑compensated AES Grade 1 crystal clocks generating the six standard rates.
This unusual arrangement (which is similar to that employed by Benchmark's ADC1) ensures that the A‑D conversion is always running optimally and extremely stably, regardless of the clock source or rate, with the sample-rate converters handling the number crunching for the required outputs and isolating any external clock jitter neatly in the process. As a result, the A2D2 can be used to provide a high-quality 24-bit/96kHz output to feed a high resolution recorder, while simultaneously also feeding a 16/48 signal to a video system, or 16/44.1 format to a real‑time CD‑R recorder.
Hooking the A2D2 into my system was very straightforward, the calibrated alignment matching my own house reference levels perfectly. I have always found the quality of Drawmer's clock circuitry to be excellent, and the A2D2 maintains that standard with ease. The provision of three buffered Word Clock outputs also enables the converter to serve as the clock master for a modest digital system, if required.
The AES17 dynamic range test returned a figure of 100dB, which is below the state of the art, but still credit-worthy, and the THD+N noise figure was better than 110dB for normal signal levels. Although there are quieter converters on the market, in practice the A2D2 is more than quiet enough for real-world applications, and its versatility and configurability make up for the very minor limitations to dynamic range performance in many applications.
Careful listening tests revealed the A2D2 to be a fundamentally neutral, clean converter, with a strong bottom end and an open and airy top end, while the mid-range came across accurately and with masses of detail. Stereo imaging was precise, stable and spacious, with excellent width and depth. The excellent metering allowed levels to be monitored and controlled precisely, and the clocking flexibility accommodated every situation I could contrive with ease.
One final point worthy of mention: a persistent issue I have with digital meters is the way most fail to indicate the normal operating range and safety headroom margin in an operationally useful way. The standard A2D2 does better than most in this respect, with the top six meter segments glowing yellow to warn of incursions into the headroom zone. However, with such a large and detailed scale, I felt there was a better option... and after discussing this with Drawmer I'm delighted to say that the company were very willing to supply a unit to my precise specifications. As a result, I bought a customised unit, which is now installed in my test reference system. The 'Robjohns Meter,' as I believe it is known in Wakefield, uses green LEDs from ‑50 up to ‑20dBFS, yellow from ‑18 to ‑9dBFS, and red all the way up to zero. This arrangement conforms with the broadcasting level standards (alignment level of ‑18dBFS and maximum permitted level of ‑9dBFS) and makes it very easy to see at a glance when signals are sitting in the 'normal' yellow region, and when they have crept into the (red) headroom margin.
The A2D2 is a clever, versatile, and competitively priced A-D converter — and I'm impressed!