Outboard DACs

Following our Lab Report on the original Pro iDSD [HFN Sep '18], iFi Audio has moderated the spec. for its headphone amp. The rated output is now 1575mW/16ohm (and 5.9V/600ohm) which, along with a genuine uplift in performance in this Signature version, is inching closer to its 'real world' numbers. Tested at its maximum +18dB gain setting, at full volume and with a peak 0dBFs digital input, the maximum single-ended headphone output is 5.06V or 43mW/600ohm [blue trace, inset Graph].

Unlike many DAC/headphone amps, the Signature's output is not clipped at full 0dBFs input/max volume, settling at just 0.03%/1kHz THD into 32ohm. Distortion does increase with loading, however, and principally at low bass frequencies, from 0.008% into 600ohm, to 0.08%/32ohm and 0.35%/8ohm (all at 10mW/20Hz). The moderate 2.6-3.3ohm source impedance remains, resulting in a further ~0.7dB signal loss and power output held to 655mW/32ohm [black trace] and 950mW/8ohm [red trace] beyond which the Signature is protected...

This finite source impedance may also introduce changes in tonal balance with low/variable impedance headphones – otherwise the frequency response is determined by choice of digital filter. Importantly, residual noise remains very low in this model and the A-wtd S/N is 1dB wider than before at 109.5dB, so hiss and hum remains very low with sensitive headphones. PM