Pre/Power Amplifiers

This fabulous example of 'audio architecture' is ordered with either 8ohm or 4ohm output transformer secondaries – our sample, judging by the 72W/4ohm power delivery, was configured with the latter. Into 8ohm, the ERIC Encore delivered 56W with 56W, 72W, 78W and 83W achieved under dynamic conditions into 8, 4, 2 and 1ohm [see Graph 1, below]. This equates to a maximum current of 9.1A which is not unreasonable for a pair of 845 tubes operating with a 1000V plate voltage. These tubes are running as close to Class A as practical – the power consumption at idle and full 70W output unchanged at 430W while the 185oC triode temperature cools under load. Anyway, all my power figures were obtained with a slightly relaxed 2% THD limit although, as Graph 1 also illustrates, the amp is clearly optimised for 8-4ohm loads with distortion increasing markedly into 2 and 1ohm. At 1W/8ohm distortion is as low as 0.04% through the midband, increasing to 0.09% at 10W/8ohm, but jumps up at HF to reach 1.2%/10kHz and 9.1%/20kHz/10W [see Graph 2, below].

The 1.45-1.26ohm (20Hz-20kHz) source impedance of the '4ohm tap' will have some impact on damping and system freq. response, depending on choice of speaker, but the bandwidth of the amplifier – flat to within ±0.5dB from 48Hz-45kHz and –10.5dB/60kHz – is otherwise set by one of the interstage coupling transformers. Overall gain is a sensibly modest +26.3dB (balanced XLR input), requiring an 810mV input to raise the rated 70W/4ohm output. However, a moderate (rectifier) noise of –51dBV pulls the A-wtd S/N down to 76.2dB (re. 0dBW) – comparable with a vinyl rather than 'digital' system. PM