Pre/Power Amplifiers

Though Engström makes no Class A claims, its ARNE draws 350W from the wall whether at idle or when delivering its rated 2x30W power output. So it's inefficient and very hot, the very costly cast glass cover registering 70oC on its top surface after a couple of hours of use... With some tube amplifiers the relationship between distortion and power output is a progressive one with worthwhile gains in level achievable at 1%, 2%, 3%... THD. Less so here as the ARNE meets its rated 30W/8ohm at 1% THD but with only a marginal increase to 32W/2% THD and 36W/3% THD. Output is higher into 4ohm at 2x39W, increasing to 2x50W at 3% THD, but there's no further headroom under dynamic conditions where it delivers 30W, 40W, 25W and 15W into 8, 4, 2 and 1ohm loads at 1% THD, increasing to 36W, 50W, 55W and 53W into the same 8, 4, 2 and 1ohm loads at 3% THD [Graph 1].

While these figures suggest the ARNE is best partnered with high (90dB+) sensitivity speakers, they are still generous when paralleled SE 300Bs are typically rated at ~15W and push-pull 300Bs at ~20W or, perhaps 30W with a very high plate voltage and rather higher distortion than achieved by Engström. Of course, THD necessarily increases at low frequencies [see Graph 2, below] through transformer core saturation (4.7%/20Hz/10W) and at HF with falling feedback (2.2%/20kHz/10W), but these are not unusual figures. Engström continues to push at the technical boundaries by achieving very low noise and a wide 90.7dB A-wtd S/N ratio. The fairly steady ~1.5ohm output impedance is low by tube/transformer standards too, so the ARNE's response – stretching from 10Hz-52kHz ±1dB – is also less influenced by swings in speaker impedance than the 'average' tube amp. PM