Outboard DACs

Key to the Phonitor's analogue performance are the proprietary 'VOLTAiR' discrete op-amps that feature in all its crossfeed/angle/laterality and main headphone preamp circuits. Running off high ±60V rails, the VOLTAiR-based headphone amp offers a 44.5V single-ended output, sufficient to develop an unprecedented 2.95W into 600ohm [black trace, Graph 1]! This voltage falls across lower impedances where maximum output is 865mW/32ohm (or 847mW/32ohm in the Low gain setting) with THD climbing from 0.015%/1mW to 0.018%/10mW, 0.043%/100mW, 0.4%/500mW and 0.7%/800mW/32ohm [orange trace, Graph 1]. There's sufficient current available to support 93mW into a very low impedance 8ohm insert earphone, for example [red trace, Graph 1] but note how distortion continues to increase with reducing headphone load. The effect is particularly marked at high frequencies as THD increases from just 0.0005%/20kHz unloaded (the VOLTAiR amps offer a vanishingly low 0.00005% THD at 1V/1kHz, unloaded) to 0.25%/20kHz into 32ohm.

The response extends from 1Hz-90kHz (–1dB) and gain is +0.5dB (Low) to +12.5dB (High) with ±3.4dB laterality (balance) either side. Residual noise is extremely low at 10µV (perfect for very sensitive 'phones) and the A-wtd S/N usefully wide at 99dB (re. 0dBV). Tested in the high gain setting, the USB DAC/headphone pathway yields a maximum 18.7V output (re. 1kHz/0dBFs) while the choice of fast roll-off/minimum phase filter in the AKM4490 DAC defines the digital/analogue response of –0.4dB/20kHz, –2.7dB/45kHz and –9.2dB/90kHz with 48/96/192kHz files. THD reaches a minimum of 0.0002-0.002% over the top 20dB of its dynamic range [see Graph 2], the A-wtd S/N is a wide 111dB and jitter moderate at 500-560psec (48kHz-192kHz/24-bit). PM