Outboard DACs

Although the Pro iDSD packs a capable headphone amp, the rated 1500mW/64ohm (and 4000mW/16ohm) cannot quite be achieved in practice because of its limited voltage output. While the balanced line outputs achieve 8.45V, the maximum single-ended headphone output is 4.2V (the Pro iDSD was tested at its maximum +18dB gain setting, at full volume and with a maximum 0dBFs digital input). Unlike most DAC/headphone amps, the output is not clipped at this full 0dBFs input/maximum analogue output (just 0.14% THD, in fact), but with the moderate ~3ohm source impedance resulting in a further 0.8dB signal loss, the power output is 578mW/25ohm (or 900mW/16ohm).

This finite source impedance will also emphasise any swings in headphone response with low impedance models – otherwise the frequency response is determined by choice of digital filter [see Filter Flavours sidebar]. Importantly, residual noise is very low and the A-wtd S/N extremely wide at 108dB, so hiss, hum and other noise will still be low with sensitive headphones.

Distortion depends on digital level, loading and choice of solid-state or tube output [Graph 2]. Via the line outputs, THD falls to as low as 0.0003% at –30dBFs (20Hz-20kHz) but is closer to 0.11-0.18% at 0dBFs, merely doubling in ‘Tube’ and ‘Tube+’ modes (these are almost indistinguishable). Via the headphone out, THD increases from 0.009% to 0.09% at low bass frequencies when loaded (10mW/25ohm) and from 0.006% to 0.025% at 1kHz [black vs. red traces, Graph 1]. Ifi Audio’s digital engineering has improved significantly of late [xDSD, HFN Jul ’18] – and this is true here with jitter extremely well controlled down to <25psec with 48kHz-192kHz/24-bit data. PM

Max output/Imp. (<1% THD, DAC): 8.45Vrms / 70ohm
Maximum output (headphone): 4.2V/600ohm / 578mW/25ohm
Headphone Output Imp. (20Hz-20kHz): 2.5-3.4ohm
A-wtd S/N ratio (DAC / headphone): 114.0dB / 107.8dB
Distortion (20Hz-20kHz, DAC/headph.): 0.0086-0.028% / 0.019-0.086%
Freq. resp. (20kHz/45kHz/90kHz, DAC): +0.0dB to –0.0dB/–0.2dB/–3.4dB
Digital jitter (48kHz / 96kHz, DAC): 20psec / 25psec
Power consumption: 11W
Dimensions (WHD) / Weight: 220x63x213mm / 2kg
Price: £2500