Vintage

That Aiwa describes the amplifier section of its AX-7400 receiver as an OTL (output transformerless) complementary SEPP (single-ended push-pull) design reflects what a novelty the everyday amplifier circuit of today was back in 1978. The specification is conservative too, Aiwa's modest 25W/8ohm and 30W/4ohm rating beaten on test to the tune of 2x30W/8ohm and 2x40W/

4ohm with sufficient in reserve to support peaks of 38W, 61W, 74W and 59W into 8, 4, 2 and 1ohm loads respectively [see Graph 1, below]. As befits an amplifier of this pre-CD vintage, when line-level sources meant 500mV rather than 2V, the overall gain is a very high +46.7dB and the A-wtd S/N ratio slightly below average at 80dB (re. 0dBW). With just 65mV required to raise the rated 25W/8ohm output, an in-line attenuator would get better use out of Aiwa's volume control if the AX-7400 were pressed into a modern system.

Distortion increases with level at and above 10W, although principally at bass and midrange rather than high frequencies, for example from 0.022%/1W to 0.06%/10W at 20Hz [see Graph 2, below]. With all tone controls defaulted to '0' the response shows a bass roll-off of –1.5dB/20Hz and a more gentle treble roll-off of –0.5dB/20kHz to –3.9dB/100kHz but these features are exaggerated into lower impedances. Into 2ohm the treble falls away to –1dB/8kHz and –3dB/20kHz while distortion, at 1kHz/10W, increases from 0.016% (8ohm) to 0.07%. Into easier 8ohm loads the AX-7400's loudness contour has a significant impact – raising bass to +9dB/50Hz and treble to +8dB/20kHz while the 'high filter' has a –6dB point at 8kHz and a second-order roll-off thereafter. PM