The Z12 amplifier replaced the X-10 - which was by now something of an embarrassment for Sinclair - with a pre-built device. Its manufacture was subcontracted to one Harvey Hall who was running a factory in a Nonconformist chapel in Thetford, half way between Cambridge and Norwich. The factory had been started by Cathodeon in the 1950s, as part of the policy of the Pye Group of bringing employment to a wider area.

Although it was claimed to achieve "laboratory standards of performance", whatever those may be, its performance and reliability were questionable. It reportedly only achieved a maximum output of three watts, despite being advertised as being capable of 12 (hence the name). Its lifetime was also problematic - if run continuously at full output, as little as 12 hours (presumably not alluded to by the name). The problem probably lay in the fact that, like many Sinclair products of the time, it was built with cheap transistors that had been discarded by other manufacturers as rejects. Despite these difficulties, the Z12 was a reasonably successful product and remained a part of the Sinclair line until manufacturing difficulties ended production in 1968.

The Z12 eventually became part of a wider range of hi-fi products comprising the PZ3 and PZ5 power supply units plus the Stereo 25, a control unit providing volume balance and tone controls for the Z12.