Outboard DACs

The PCM1794 and later 'A' version has been used in many successful, fine-sounding players and DACs – it supports LPCM and I2S, but not DSD, inputs and sample rates up to 200kHz (in practice this means 192kHz). In a further twist, the DAC1 does not support 176.4kHz over S/PDIF but it does via USB. The PCM1794A is known for its excellent S/N, low-level linearity and potential for very low distortion. In this implementation it achieves a wide 111.4dB A-wtd S/N ratio via a custom, balanced analogue output stage that max's out at 2.95V from a 145ohm source impedance (increasing below 100Hz to 335ohm/20Hz). Low-level resolution is good to ±0.5dB over a full 110dB dynamic range while distortion falls to a low 0.0003-0.0012% at –20dBFs across 20Hz-20kHz [see Graph 1, below]. Distortion increases from –20dBFs to 0.018-0.028% at peak output (0dBFs) but this is a function of the I-to-V and output buffer. While TI recommends using a generic NE5534 op-amp for this purpose, Merason's discrete, Class A circuit permits far finer 'sound tuning'.

The PCM1794A DAC chip comes with integral 'sharp' and 'slow' roll-off 8x oversampling digital filters, and will also accommodate external upsampling solutions, but Merason has chosen a standard, sharp roll-off linear-phase type offering an 84dB stopband rejection with 48kHz inputs. The frequency responses are gently and sensibly tailored, reaching out to –0.7dB/20kHz, –2.8dB/45kHz and –6.6dB/90kHz with 48kHz, 96kHz and 192kHz inputs, respectively. The dual-mono layout also maintains a superb >120dB channel separation (20Hz-20kHz). Back in the digital domain jitter, too, is more than adequately suppressed down to 111psec with 48kHz/24-bit data and 85psec with 96kHz/24-bit data [see Graph 2, below]. PM