Integrated Amplifiers

Patented in the 1960s by Prof. Richard Baker of MIT, the 'Gateable Bridge Network Having Power Gain' (or Diamond Circuit) describes two pairs of bipolar transistors with tied emitters and bases. Still used in unity gain op-amp ICs to boost current rather than voltage output, the technique remains rare in higher power amplifier circuits. While there are advantages in the reduction of common-mode noise and distortion, the bridged output is necessarily floating, so the AX-5 Twenty should not be connected to any speaker that has a ground (including electrostatics or many with active bass systems).

Its other 'feature', VGT or Variable Gain Transconductance, is a method of offering control over volume without simply attenuating the full gain of the preceding preamp section. Here a stepwise resistor ladder governs the gain of the power amp's FET-based input – innovative, but with variable gain comes variable compensation (feedback) and so distortion changes with volume setting even if the output power is unchanged. For example, for a fixed 10W power output, the lowest distortion (0.03%/1kHz) is achieved at the maximum volume position '46' (+31dB gain), increasing very slightly to 0.04% at '40' (+22dB gain) before advancing to 0.05% at '38' (+19dB gain), 0.14% at '34' (+13dB gain) and 0.26% at '32' (+10dB gain). This is clearly illustrated by the inset Graph where grey is 'Vol 46'; red, '40'; cyan, '38'; green, '34' and black is 'Vol 32'. PM