Integrated Amplifiers

While the anniversary celebrated by this 'Twenty' version of the AX-5 integrated has long since passed, comparisons with the original AX-5 [HFN Jun '14] reveal potentially far greater revisions to both the VGT (Variable Gain Transconductance) 'volume control' and 'Double Diamond' output stage than suggested by Ayre itself. Both these circuit innovations combine to create a very distinctive technical fingerprint for the AX-5 and its successor – noise, for example, is exceptionally low and the A-wtd S/N correspondingly wide at 101dB (re. 0dBW) while distortion is purely 2nd/3rd harmonic. Distortion is very consistent with frequency but not with output level or applied gain. For example, THD is as low as 0.009% at 1W/8ohm but increases to 0.04%/10W and 0.13%/100W (all from 20Hz-10kHz). The output impedance is also very flat at 0.150-0.155ohm (20Hz-20kHz), increasing slightly to 0.185ohm/100kHz, but the response [see Graph 2, below] gets flatter and more extended the lower the gain (lower VGT setting), from –2.0dB/100kHz (vol = 46) to –0.3dB/100kHz (vol = 32).

Power output is higher than achieved by the AX-5, and its load tolerance is significantly enhanced, despite the 125W/8ohm specification remaining unchanged. The AX-5 Twenty achieves 2x155W/8ohm and 2x250W/4ohm with a boost to 183W, 335W, 535W and 650W into 8, 4, 2 and 1ohm loads under dynamic conditions (the AX-5 managed 160W, 280W, 215W and 125W, respectively). This is clearly illustrated [Graph 1, below] as is the variation in distortion both with output level and decreasing load impedance. I would expect the sound of the AX-5 Twenty to vary slightly with speaker load and volume setting. PM