Pre/Power Amplifiers

This is an uncommonly ‘well sorted’ triode tube amp, operable in either push-pull or (parallel) single-ended modes. User-defined negative feedback influences all parameters [see boxout] so the max ‘10’ setting was used for all the measurements here. Manley rates the Neo-Classic 300B at 25W (PP) and 12W (SE) at 3% and 1.5% THD, respectively, which, in practice, is met at 33W (10W SE) and 40W (16W SE) into 8 and 4ohm loads, via the 4-8ohm tap, at <1% THD. Relaxing the THD limit to 3% does not deliver any significant increase in power – 37W and 41W – while the limited headroom is realised in dynamic outputs of 39W, 46W, 46W and 37W into 8, 4, 2 and 1ohm loads, respectively [see Graph 1, below]. In SE mode the dynamic output reaches 14W, 18W, 11W and 5.5W, respectively, but note that distortion is lower into 4ohm (0.45%/10W) than 8ohm (1%/10W) in SE mode [dashed traces, Graph 1].

By way of trend, distortion rises gently with power output, from 0.025%/1W to 0.10%/10W and 0.18%/20W (PP mode) and from 0.2-1.0% (1-10W, SE mode) – all re. 1kHz/8ohm. Versus frequency there’s an inevitable and sharper increase in distortion through the high treble – 5.3% and 6.3% (PP and SE; re. 20kHz/10W) – and in the low bass at 0.19% and 15% (PP and SE; re. 20Hz/10W) [see Graph 2, below]. Bass performance, and overall system response, is necessarily modified by the Neo-Classic 300B’s ~0.65ohm output impedance [see boxout] but, into a non-reactive load, retains –0.1dB/+0.4dB limits between 20Hz-20kHz/8ohm. Noise is also well managed and the 82dB A-wtd S/N ratio (re. 0dBW) remains respectable for this class of tube amplifier. PM