QUAD CKASSIC 2 Integrated Valve Stereo New Amplifier

The new integrated is, essentially, a pair of Quad II amplifiers (ca 1953) powered with silicon rather than glass rectification and bundled with a perfectionist-quality control unit and a solid-state phono section. According to de Paravicini, the output section runs in pure class-A and is modeled after Walker's Acoustical circuit, in which a portion of the output transformer's primary winding is coupled to the cathodes of the KT66 output pentode tubes (as opposed to their screen grids, as in Ultralinear output circuits). Those transformers, of course, are designed by de Paravicini, and made in England to his very exacting specifications. The amp's output section is configured as autobias, and a bit of global feedback is used—although a less-than-typical amount is required, given that the Acoustical circuit itself provides local feedback, with apparently little loss of gain and power.

The small-signal tubes in the Quad II Classic—four 12AX7 dual-triodes and two 6922 dual-triodes—function as the phase splitter/driver stage for the voltage-hungry output section; the Quad's line stage is passive. The phono section is described as "a simple and elegant circuit with five transistors per channel." It's powered by a 36V drop from the rail voltage, and uses a combination of active and passive filters to achieve RIAA equalization. Gain is 42dB in moving-magnet mode, and 62dB when switched to moving-coil mode.