The Stereo 25 pre-amp control system was Sinclair's first significant step into the hi-fi market. Available only as a pre-assembled unit, it marked the beginning of the company's diversification away from the electronics hobbyists who had until then been the backbone of Sinclair's customer base. The Stereo 25 was designed to be used with the PZ3 power supply unit and Z12 amplifier to provide the core of a home hi-fi system.

The device was supplied as a complete chassis and front panel, which needed to be mounted in a case. It was designed by Clive Sinclair's brother Iain. The device was reasonably useful, but Sinclair's first adverts were a bit over the top when describing its virtues:

A COMPLETE HIGH-FIDELITY STEREO ASSEMBLY FOR £22.18.0

All you need is one Stereo 25 preamp Control Unit (£19.9.6), two Z12s (£8/19/-) and one PZ3 Mains Power Supply Unit (£3/19/6) to possess the finest hi-fi stereo installation. As a very desirable optional extra you could include the Micro FM (£5/19/6). The overall saving in cash will be staggering and you will have an installation second to none irrespective of price.

What the advert failed to mention was that this was actually far from being a "complete" hi-fi system; it would also need at least a pair of speakers and a record turntable or tape deck, none of which were (at that time) in the Sinclair range.

A more fundamental problem was that Sinclair ran out of parts to built the Stereo 25. The transistors in the device came from a consignment of about 100,000 transistors bought cheaply in 1964 as rejects from other manufacturers. These ran out in the spring of 1968 without any alternative suppliers having been found. Production of the Stereo 25 ground to a halt in the spring of 1968, leaving the company in an awkward position while it tried to find new suppliers. In the event, the Stereo 25 was superseded the following year by the more advanced Stereo Sixty.