THRESHOLD SA1

Eleven years ago, Threshold Corporation entered the high-end audio market with the first amplifier ever to use sliding bias (footnote 1) in its output stages. Some 10 years later, Threshold spawned another innovation: their so-called Stasis circuitry, which yielded the S-series amplifiers. The SA-1 and its lower-powered sister SA-2 are the latest from Threshold, and are the first Threshold amps to abandon sliding bias for straight class-A operation. Both use the Stasis circuit. In most amplifiers, the voltage amplification stages are used to drive the output transistors, which in turn drive the loudspeaker. In a Stasis design, the voltage amplifiers connect directly to the loudspeakers. Obviously, this cannot work. If you recall Ohm’s law, or even if you never knew it, you may take my word that current equals voltage divided by resistance, and power equals current squared times resistance. That means that any resistance across the output of a voltage amplifier will require that amplifier to deliver power as well as voltage—something it is better off not being asked to do. The ideal load for a voltage amp is an open circuit—infinity ohms. The Stasis circuit allows the loudspeaker to behave like just such an open-circuit load.