Integrated Amplifiers

Tested in its 'High Power' setting (default for KT120 and KT150 tubes) and with 'Ecomode' off, the V70 met its 2x40W/8ohm and 2x50W/4ohm specification at 2x43W and 2x55W, respectively, once the THD limit was relaxed from 1% to 2%. The 'hard clip' point for the V70 is closer to 3% as illustrated by the dynamic output profiles [Graph 1, below] that show 50W, 80W and 88W into 8, 4 and 2ohm, with a limit of 25W into 1ohm. I discuss Octave's 'Class A' circuit innovations in my boxout but linked to this is its use of a single secondary winding, rated across 4-16ohm, in its 'wideband' silicon-steel core output transformers – hence the single set of 4mm terminals. In practice it looks as if the windings, and limited feedback, are optimised for 4ohm loads – as evidenced by the reduction in distortion above 10W output from 1.15% to 0.25% (re. 30W) from 8ohm to 4ohm loads [black vs. red traces, Graph 1].

As for 'wideband', the V70 certainly offers a very flat and extended response – free of the oft-seen ultrasonic transformer resonance – that reaches from –0.25dB/20Hz to +0.1dB/20kHz into 8 and 4ohm loads before peaking slightly at +0.4dB/50kHz and rolling away to –6.2dB/100kHz/8ohm (–5.8dB/100kHz/4ohm). Of course, the moderate 1.75-2.0ohm output impedance (20Hz-20kHz) will invoke far larger variations in the system response according to the variable impedance trend of the attached loudspeaker. Noise, too, is lower than 'average', delivering an 86dB A-wtd S/N ratio (re. 0dBW), while distortion into all loads shows the expected increase versus level and low bass frequencies at 0.45%/20Hz/1W to 0.96%/20Hz/10W [see Graph 2, below]. PM