Pre/Power Amplifiers

With neither of the DAC or phono stage modules on board, this test focuses on the performance of the LS-2 as a fully analogue, balanced line preamplifier. The logic- (relay-) governed R-2R volume control ‘number’ does not reflect absolute gain but the full ‘01’ to ‘99’ step sequence offers a very useable 71dB range with its channel balance holding true to within 0.04dB at every step. In practice these steps are roughly ±0.5dB from ‘99’ down to ‘89’ where the total change is 4.75dB. The next 40 steps from ‘89’ down to ‘49’ are in precise ±0.5dB increments, resulting in a 20.0dB overall change in level. The increments increase to ±1.0dB at volume number ‘46’ down to ‘09’, a level change of 37dB, becoming only slightly less accurate below ‘09’ with a total change in level rounding out to 7.9dB down to ‘01’.

The LS-2’s maximum balanced output is a substantial 16.8V from a flat and cable-agnostic 5ohm source (20Hz-100kHz). This low source impedance comes courtesy of the LS-2’s solid-state XLR buffer, but increases to 330-406ohm via the tube-driven RCA outs (distortion, response, noise, etc, are otherwise unchanged). The R-2R volume control is not so strictly buffered, hence the very slight difference in ultrasonic response [red trace, Graph 1] at the –6dB (‘87’) position. Otherwise the response has an infra-bass roll-off of –3dB/2Hz while remaining flat to –0.3dB/100kHz. Noise is low, and the 91dB A-wtd S/N ratio especially impressive, but the LS-2 does exhibit evidence of microphony so should ideally be sited away from sources of vibration. Distortion [Graph 2] is principally 2nd harmonic in nature and a moderate, and impressively ‘flat’, 0.013-0.019% (re. 0dBV, 20Hz-20kHz). This increases uniformly with level to 0.026-0.042% at 6dBV/2V [pink trace] and 0.052-0.085% at 12dBV/4V output [red trace]. PM