Усилители мощности стерео — Sugden Audio MASTERCLASS SPA-4

The Masterclass SPA-4 is a stereo power amplifier from Sugden Audio, hand-built at the company's works in Heckmondwike, Yorkshire, where Sugden has made amplifiers since 1967. It shares its Class A circuitry with the MPA-4 mono block of the same Masterclass series, and Sugden has kept this series in uninterrupted manufacture for over seventeen years. The UK price is GBP 6,900, and the casework is offered in Titan, Graph and Stealth finishes.

Key specs that matter

The SPA-4 is rated with both channels loaded, and it delivers 50 W per channel into 8 ohms and 75 W into 4 ohms. Sugden specifies a flat response of plus or minus 0 dB between 15 Hz and 30 kHz, with the minus 3 dB points placed at 2 Hz and 200 kHz. An input level of 1 V drives it to full output, with noise more than 85 dB down. The enclosure stands 250 mm high and 430 mm wide, with a depth of 350 mm. Packed weight is 25 kg.

Engineering and use context

Inputs come in two forms, RCA for unbalanced sources and XLR for balanced ones, and the balanced path uses a discrete virtual transformer input circuit in place of a physical input transformer. Each gain stage uses a symmetrical dc-coupled topology with current feedback, which removes series capacitors from the audio chain, and a servo circuit keeps the dc offset at the speaker terminals under control. Multi emitter transistors form the output devices. Because the design is biased in Class A, the output stage draws full current at idle and dissipates much of it as heat, which is why a chassis of this rating ends up at this height and weight. Sugden states that the circuit is fully balanced and that the SPA-4 is intended to cope with small loudspeakers as well as large ones. Loudspeaker leads attach to a single stereo set of locking multi-way terminals, and a mains cable is packed with the unit.

Pairing

Preamps fitted with differential outputs can connect over longer runs via XLR, and the amplifier is a natural partner for the LA-4 preamplifier from the Sugden catalogue. Owners who later want more power can move to a pair of MPA-4 mono blocks built on the same circuit. The extra current available into low impedances makes lower impedance floorstanders a workable load, provided the cabinet has space around it for ventilation.

Alternatives

The Sugden Sapphire FBA-800 offers 40 W of Class A output in a two-channel chassis, built from four 20 W amplifiers in a floating bridge arrangement.