Pre/Power Amplifiers

Luxman's diminutive SQ-N150 tube amplifier has a power output to match: just 2x12W at 1% THD into 8-4ohm loads. Even under dynamic conditions, its maximum output of 14W, 15.5W, 10W and 5.8W into 8, 4, 2 and 1ohm loads, at up to 3% THD, suggests it is most 'useable' with 95dB+ sensitivity/6ohm+ loudspeakers [see Graph 1, below]. I discuss the response, and sweetened treble, in my boxout but distortion is well managed, increasing gently with power through the midrange from 0.06%/ 1W to 0.15%/5W and 0.4%/10W. The output transformers are not massive, so distortion increases quickly at low bass frequencies from 0.5%/1W to 1%/5W at 20Hz. Gain is sensibly moderate at just +33dB but, most impressive of all, the SQ-N150 is very quiet for a tube design. Residual hum and noise is just –76dBV while the A-wtd S/N ratio is a very wide 95.3dB (re. 0dBW) – 10dB wider than that achieved by many solid-state designs.

The partnering D-N150 player/DAC is based around TI's 'Directpath' PCM5102A chip, its charge-pump (capacitorless) output servicing a generous 108dB A-wtd S/N from a maximum 2.1V output. The 225ohm source impedance is not too high and while distortion is at its highest over the top 10dB of its dynamic range via both CD and 24-bit LPCM inputs (the latter 0.004-0.03%, 20Hz-20kHz), it relaxes between –10dBFs to –30dBFs where THD is just 0.0004-0.009% [see Graph 2, below]. Luxman has chosen the 'normal latency' version of TI's linear phase digital filter – stopband rejection is just 64dB but the response extends to –0.2dB/20kHz, –1.1dB/45kHz and –3.0dB/90kHz (CD/48kHz, 96kHz and 192kHz media). Jitter is acceptably 'average' at 340psec, 330psec and 290psec (CD, 48kHz and 96kHz). PM