Merason DAC1 USB DAC Lab Report

Lab Report

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


Distortion vs. 48kHz/24-bit digital signal level over a 120dB dynamic range (1kHz, black; 20kHz, blue)


High resolution jitter spectrum via USB (black, 48kHz/24-bit, with markers; red, 96kHz/24-bit)

Maximum output level / Impedance 2.955Vrms / 145-335ohm
A-wtd S/N ratio (S/PDIF / USB) 111.4dB / 111.4dB
Distortion (1kHz, 0dBFs/–30dBFs) 0.018% / 0.0003%
Distortion & Noise (20kHz, 0dBFs/–30dBFs) 0.028% / 0.0012%
Freq. resp. (20Hz-20kHz/45kHz/90kHz) +0.0 to –0.7dB/–2.8dB/–6.6dB
Digital jitter (48kHz / 96kHz) 111psec / 85psec
Resolution (re. –100dBFs / –110dBFs) ±0.2dB / ±0.5dB
Power consumption 17W
Dimensions (WHD) / Weight 450x100x290mm / 8kg
Price £4195

Niedal Audio Lab
Supplied by: Whole Note Distribution, Angus, Scotland
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