Pre/Power Amplifiers

The Unico DM v2 is a triode/MOSFET hybrid amp, but it’s no ordinary tube/tranny mix. On the one hand, the 2xECC82 triode input stage is conventional enough, its relaxed local feedback setting the ‘tonal colour’ for the amplifier as a whole – the extended distortion harmonics, dominated by a strong 2nd, adding up to ~0.1% [re. 1kHz/10W; see Graph below]. The innovation, however, is left to the output’s three parallel pairs of MOSFET power devices, configured in complementary pairs but with a twist to the usual push-pull convention.

Unison’s ASHA (Analogue Synchronised Hybrid Amplification) refers to its ‘hybrid biasing’ of each ‘synchronised’ pair of N- and P-channel FETs. Here the N-channel FET carries the full waveform voltage – it never switches off and, as such, forms the Class A heart of Unison’s Class A-AB topology. This FET also carries the positive-going current. The partnering P-channel FET is synchronised to the leading N-channel device, so as the current falls in the latter it increases in the former during negative-going cycles of the musical waveform. This is the Class AB ‘half’ of the circuit. It’s the leading N-channel FET’s voltage waveform that sets the linearity of the output stage as a whole while both FETs deliver the current to support that voltage into ‘real’ loudspeaker loads. This ‘synchronising’ of current between the complementary FETs is achieved by what Unison describes as a ‘robust and fast control circuit’ that integrates the transistor drivers for the leading N-channel and following P-channel power FETs. PM