Outboard DACs

Announced by dCS back in 1991, its Ring DAC converter is now a highly evolved and practical technology, combining the pure monotonic conversion of a genuine 'single-bit' DAC with the operation of a PWM bitstream-style converter. The current Ring DAC hosts all its custom DSP, including the six digital filters that are unchanged here, on Xilinx Spartan 6 processors. The proprietary code truncates incoming LPCM data (and converts DSD) to an average wordlength of 4.6 bits and these 24 (or 24.6) possible values are then mapped across 48 'identical' current sources that comprise the Ring DAC. This matrix of current source/resistor elements is clearly visible in our pictures, as are the Xilinx processors and fully balanced analogue output stages.

In practice the Ring DAC approach differs from traditional PWM/bitstream/hybrid DACs that would use these bits to control the length of time a single current source is held open or closed. However, for the Ring DAC to be truly monotonic – like a pure 1-bit DSD DAC – its 48 current sources would need to be absolutely identical in size. Even with laser-trimmed chip resistors, this is not practical so dCS randomises the mapping of bits to its current sources. Clever thinking – so fixed errors, that would otherwise appear as harmonic distortions, are thus traded for an inaudible increase in noise.

Although all the APEX upgrades are 'analogue' some still impact on the DAC's digital performance. This doesn't include the thermally matched compound transistor pairs now used in the output – these influence analogue noise and distortion only. However, dCS has focused particular attention on the latches that address the matrix, tightening up the clocking and reducing the source impedance of the reference PSU, resulting in a real, and measurable, reduction in low-level phase noise. The uncorrelated noise 'skirt' seen with the previous Vivaldi [red trace, inset Graph] is reduced in the APEX variant [black trace] and although the difference is at a very low level – this is a very high-resolution/zoom jitter spectrum – it is nonetheless proof of generational gain. PM