Sugden's first commercial amplifier was the A21 in the mid-1960s. It is generally recognised as the first commercial, solid state Pure Class A amplifier to be available for audio enthusists. It was a very special amplifier with a single-ended Class A output configuration and current feedback. This design concept did not appear generally in commercial audio amplifiers until over a decade later. Capable of recreating a musical performance well in advance of its peers, it made the name of Sugden synonymous with musical performance. The true potential of the output configuration was realised in the 1980s when it was coupled to modern components, reduced internal filtering and multi emitter solid state output devices. The result was an amplifier of extended and more controlled bass and treble but still retaining the wonderfully detailed mid range of the original design. The amplifier became the A21a, a reference amplifier on many continents. Further improvements have achieved significant enhancement of performance and the amplifier has been rebranded as the Sugden A21a Series 2.