Pre/Power Amplifiers

If the inside shot looks more like what you’d expect under the bonnet of an AV receiver than a headphone amplifier, then a brief explanation is in order. There are really two fundamental parts to the P-750u – three if you count the substantial linear power supply with its individual points of regulation throughout the dual-mono amplifier layout. LECUA or ‘Luxman Electronically Controlled Ultimate Attenuator’ is the volume circuit that Luxman employs in various of its products. There are two of them here on top of the stack of three boards inside the P-750u. It’s a chip-based circuit that’s governed by the rotary control on the P-750u’s fascia and provides closely matched steps in gain to accommodate volume, the three-position ‘sensitivity’ control, and channel balance control. Under this lies Luxman’s ODNF or ‘Only Distortion Negative Feedback’ circuit, a somewhat axiomatic name for an amplifier comprising a three-stage input buffer/preamp that receives the error signal from a three-stage Darlington output amp. Much of the perceived complexity of the P-750u is due to the fact that there are four of these ODNF amplifiers inside – operating as balanced, floating pairs to drive the XLR outputs and in parallel to drive the single-ended 6.35mm headphone output. PM