Integrated Amplifiers

Designer Nelson Pass, known affectionately as 'Papa' within the online/DIY audio community [see www.passdiy.com], has been innovating for over 45 years. 'I see myself as primarily a circuit topologist', he says. 'I like very simple topologies, so the simpler you can make an amplifier, the more likely there is to be good correspondence between the sonic performance and what you measure on a bench.' Over the years, Nelson's designs have typically utilised FET power devices and featured the minimum number (but no fewer) components, implemented with the absolute minimum corrective feedback.

However, before Nelson had developed his 'Super-Symmetry' circuit, used in some high-power Pass Labs amps [HFN Apr '10] and, by way of contrast, his very low-power single-ended Class A 'First Watt' [HFN May '12], he had already caught the audiophile's eye, and ears, with 'Stasis' – the core technology of Threshold Electronics, founded in 1974. The S500 and monoblock S1000 Stasis power amps were the flagships of the range – imported into the UK by Absolute Sounds in the early '80s before Krell's Class A behemoths distracted our attention. Replacing Nelson's earlier 'Dynamic Bias' designs, the Stasis amps employed a limited-gain stage operating with as near constant-voltage/constant-current conditions as possible. This defined the linearity – the low distortion – of the amplifier while a current-mirror bootstrap did the 'work' driving the loudspeaker load. They sounded... fabulous! PM