Integrated Amplifiers

Few brands take advantage of the high 2V+ output of modern line-level sources – most still design amps with an unnecessarily high 35-45dB overall gain – so it is refreshing to see MF offering 'Normal' and 'Direct' modes of operation, the latter bypassing the A1's preamp stage to offer a reduced +24.1dB gain ('Normal' is +35.7dB). 'Direct' requires a perfectly achievable 885mV input to raise its rated 2x25W/8ohm output while also optimising the full arc of the A1's volume control. Tested in 'Direct' mode (there's no appreciable difference in noise, distortion or response in 'Normal' mode) the A1 delivers 2x34W/8ohm but drops to 2x18W/4ohm at 1% THD (increasing to 30W only at 3% THD). Under dynamic conditions there's some Class AB headroom that takes it to 36W, 36W, 21W and 11W into 8, 4, 2 and 1ohm loads (2 and 1ohm are at 2% THD), so while the 'Planar Audio transistors' may have a potential '25A max output', in practice only 3.5A is realised at <1% THD [see Graph 1, below].

As might be anticipated, distortion is lowest at low levels (0.026% re. 1kHz/1W/8ohm) and then increases linearly with power output to 0.14%/10W, 0.23%/20W and 0.27% at the rated 25W. While this is 'high' for a typical transistor amp, the harmonics are simple second-order and – more important still – are very uniform with frequency [see Graph 2, below]. The 85.5dB S/N ratio (re. 0dBW) also shows a hint of 'warming' hum punching through the A-weighting curve. The response shows a subsonic roll-off amounting to –0.4dB/10Hz and –6dB/2Hz while the HF reaches out to –0.3dB/20kHz and –6.0dB/100kHz, all from a moderate, but uniform, 0.33ohm source impedance. PM