Sinclair followed up the highly successful Project 60 modular hi-fi system with the Project 605. As the name suggests, it was essentially an incremental development of its predecessor rather than anything radically new. It was a single pack containing a PZ5 power supply unit, two Z30 amplifiers, one Stereo Sixty pre-amp and one Masterlink.

The latter was a new module - apparently unique to the Project 605 - containing all of the input sockets and components, with all of the leads cut to the correct lengths and fitted with sockets. The idea was that the complicated and messy business of soldering would thus be eliminated, leaving the customer with nothing more arduous than clipping together the various components. As this suggests, it was another step in Sinclair's journey towards the mass market and away from the less user-friendly world of the electronics hobbyists.