Power amplifiers mono — Sugden Audio MASTERCLASS MPA-4

The Masterclass MPA-4 is a balanced Class A mono power amplifier from Sugden Audio, the British company based in Heckmondwike, West Yorkshire, whose founder James Sugden designed the A21 Class A integrated amplifier in 1967. The Masterclass series has been in continuous production for more than seventeen years, and the MPA-4 is its top power amplifier, sold as a pair.

Key specs that matter

Each MPA-4 delivers 165W into 8 ohms. Input sensitivity is 1V for full output through a mono balanced XLR input. Frequency response is flat (+/-0dB) from 15Hz to 30kHz, and the -3dB bandwidth extends from 2Hz to 200kHz. Signal-to-noise ratio exceeds 85dB. Each amplifier measures 250 x 430 x 360mm and weighs 25kg, and a single pair of multi-way locking binding posts is provided.

Engineering and use context

Each MPA-4 contains the two output stages of Sugden's stereo power amplifier connected in balanced (bridged) mode, which is why a single mono unit delivers more than twice the power per channel of the stereo SPA-4 built on the same Class A circuit. Running both halves in balanced form also lowers the noise floor and cancels even-order distortion products common to both halves. The gain stages are symmetrical, current-feedback and DC-coupled, with servo-controlled DC offset so that no coupling capacitors sit in the signal path, and the output stage uses multi-emitter transistors. The balanced input uses a discrete virtual transformer circuit, giving the common-mode noise rejection of an input transformer without its magnetic core. Because the circuit runs in Class A, the output devices conduct continuously whether music is playing or not, so the amplifiers draw substantial mains power and generate significant heat. They need ventilated placement rather than an enclosed cabinet.

Pairing

The MPA-4 is designed as the engine room of a fully balanced system, taking a balanced preamplifier signal through its XLR input, and Sugden's own Masterclass preamplifiers are its natural partners. With its high Class A output and wide bandwidth, it drives most passive loudspeakers in medium to large rooms, and its input sensitivity is easy to meet with almost any preamplifier. Two heavy chassis need rack or floor space with good airflow.

Alternatives

Sugden Audio's own Masterclass SPA-4 uses the same Class A circuit in one stereo chassis, with 75W per channel into 4 ohms and both balanced and single-ended inputs.