JVC A-X4 (Vintage)

摘要

While not a flagship amplifier, the A-X4 is still illustrative of late '70s/early '80s designs from Japan that featured variations on a sliding biason-switching output stage, rather than adopt an inefficient pure Class A topology. JVC's 'Super-A' regime has some of the properties of positive feedback and, indeed, the unusually near-zero negative output impedance of our sample reflected this. Nevertheless the A-X4's performance is certainly impacted by speaker load, so while its response actually 'brightens' slightly from -0.2dB/20kHz to -0.08dB/20kHz unloaded-to-8ohm before reaching +0.0dB/20kHz and +0.1dB/20kHz into 4 and 2ohm, its power output drops from 80W to 65W (8 to 4ohm) as does its dynamic output of 110W, 65W, 35W and 20W (8, 4, 2 and 1 ohm). So the A-X4 is not a speaker-wrestling powerhouse [see Graph 1, below]. Our sample was also a little noisy, and arguably noisier than it might have been in 1979, with a 76dB A-wtd S/N ratio (re. OdBW) and moderate 0.009-0.03% distortion at 10W/8ohm (re. 20Hz-20kHz). DC offset was very well controlled bearing in mind the near-DC response of the amplifier's output stage.