Outboard DACs

Two DIP switches on the base of the preamp confer a range of four alternate gain settings: 0dB, +10dB, +18dB and +20dB that amount to –0.25dB, +9.7dB, +17.7dB and +19.8dB, respectively. There is no difference in the amplifier's maximum power output or source impedance – a flat 6ohm from 20Hz-20kHz – with the various gain settings although, for a given output voltage, distortion falls with increasing gain reaching a minimum of 0.0003% through bass and midrange at 1V/600ohm, increasing to 0.0023%/20kHz. Distortion does increase as the amp drives lower impedances, however, up to 0.02% (20Hz-20kHz) at 30mW/32ohm.

Maximum output is 260mW/32ohm (Lehmannaudio specifies 400mW/60ohm) with distortion climbing from 0.002%/1mW to 0.006%/10mW, 0.1%/100mW and 0.5%/200mW/32ohm [black trace, Graph 1]. There's sufficient current available to support 35mW into a very low impedance 8ohm insert earphone, for example [red trace, Graph 1]. The response is flat to within ±1dB from 3Hz-43kHz, the A-wtd S/N is sufficiently wide at 93dB and residual noise usefully low at –92dBV (25µV/+20dB gain setting).

The USB/preamp was tested at the +10dB gain setting, yielding a maximum 3.265V output (re. 1kHz/0dBFs) from the rear RCAs. Note that the output is phase inverting and that digital files up to 96kHz are accepted with a default Windows USB driver. The chosen slow roll-off linear phase digital filter delivers a –9.1dB/20kHz and –13.8dB/45kHz response (more audible with CD/48kHz files than 96kHz) while the A-wtd S/N is a generous 108dB and jitter suppressed to a spectacularly low <10psec. Distortion falls as low as 0.0003% over the top 20dB of its dynamic range [black trace, Graph 2] but increases at HF. PM