S-30 Mk. 3.1

About the S-30 Mk. 3.1

The Atma-Sphere S-30 Mk. 3.1 is an earlier version of Atma-Sphere’s compact stereo OTL (output-transformerless) vacuum-tube power amplifier. It uses the company’s fully balanced Differential and Circlotronic direct-coupled OTL circuit, with a Class-A output stage and no output transformer. The design is effectively dual-mono in one chassis, is hand wired with star grounding, uses automatic biasing, and provides RCA single-ended plus true-balanced XLR inputs. It is best partnered with loudspeakers whose impedance is benign and remains relatively high; its output power increases into a 16-ohm load rather than following the usual solid-state power-doubling behavior. The Mk. 3.1 designation is an older revision; some published performance data applies broadly to Mk. 3 and later S-30 versions, so revision-specific differences may exist.