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Last updated
25 Jan 1998

Amplifiers: 1960s

Sinclair founded his career on the production of cheap gadgets for hi-fi, electronics and radio enthusiasts. His company, Sinclair Radionics, soon became a highly successful outfit. It started off small - just Sinclair himself and a couple of others on the payroll - but as it expanded, it soon found itself moving to a series of new premises in and around the Cambridge area. The famous Sinclair logo was created in 1964; unfortunately, some of Sinclair's more notable flaws (such as exaggerated product claims and sometimes unreliable devices) were also apparent even at this early stage.

Micro-Amplifier (1962)

Sinclair's very first commercial product was the Micro-Amplifier, launched at the end of 1962. For a mere twenty-eight shillings and sixpence (roughly 1.42, but representing a lot more buying power in those far-off days), the hobbyist would receive a kit of parts which he (or, conceivably but improbably, she) could make up, ‘in under 2 hours using ordinary tools’ into ‘the smallest [amplifier] of its type in the world’. Pictured standing on a half-crown in the advert, the amplifier measured just by 3/8 by inches (about 19 by 9.5 by 12.7 mm) - an early example of Sinclair's long-running fetish for miniaturisation. Orders poured in for the amplifier to Histon Road, Cambridge, premises shared with Polyhedron Services and Cambridge Consultants Ltd (CCL), with whom Sinclair had much interaction in later years. CCL took on one Nigel Kember to pack the components and dispatch them to the eager public on Radionics’ behalf.

TR-5 and TR-750 (1964)

The TR-5 and TR-750 were cheap amplifiers designed and launched in 1964. Although suitable for a range of purposes, they were marketed as an add-on to the Micro-6 and Slimline radios. One Sinclair ad stated:


Because of the enormous interest in the Micro-6 we at Sinclair Radionics have been devoting much of our time to the development of accessories which add to its usefulness. The first of these, the TR750 Power Amplifier was introduced last month. This remarkably low-priced [39/6, ready-built and tested 45/-], high performance [frequency response within + 1dB from 30 to 20,000Hz] design has, of course, a great many applications but in conjunction with the Micro-6 or the Slimline it can form a really powerful car, home or portable radio.

X-10 (1964)

Sinclair's long-standing enthusiasm for technological-sounding names reveals itself in the X-10 (right), an audio amplifier designed by Gordon Edge which used a new technique called pulse width modulation. It aroused a lot of interest, as nothing of similar size, price or performance had been seen before. Unfortunately, Sinclair's claims were a little too extravagant and the amplifier ran into the first advertising standards problem of Sinclair’s career. A contemporary journalist recalls that Wireless World refused to take Sinclair’s subsequent advertising for the X-10 because of complaints over the performance claimed for the amp. Its stated output was 10 watts R.M.S., but in reality it was capable of only a quarter of that, and was temperamental to boot.

The debut of the X-10 was also fraught with problems. The original circuit design had been delivered to Sinclair Radionics accompanied by a working prototype; Jim Westwood took the prototype and engineered it into a marketable form which was sent to the firm in Hampshire which manufactured Sinclair's circuit boards. They promised that the first batch of boards would be delivered within a few days, and the first advertisement for the X-l0 was planned to coincide with promised deliveries.

The boards arrived exactly on time. They looked all right at first, and Westwood set about building the first real X-10 amplifier. But disaster struck - the board was a mirror image of what should have been and the whole batch was useless. By the time the correct boards had been made and an amplifier built, the X- 10 had been appearing in advertisements for some time, with many customers wondering when their amplifiers would arrive. This was the first but by no means the last time that technical problems were to cause delays to deliveries of a Sinclair product.

Z-120 (1965)

On the strength of the X-10 (which had been more of a PR than a commercial success), Sinclair Radionics won a contract to supply vibration test equipment to an aircraft equipment company; a high-powered amplifier which would drive a vibrator with power output up to 120 watts. Clive Sinclair was busy feeding ideas to Jim Westwood to try out. Richard Torrens was busy trying to solve some problem that had been encountered with the transistors which had been shipped to Hong Kong. Another engineer had to be found, since the Hong Kong problem needed someone's dedicated attention, and the deadline for the 120-watt amplifier contract was looming ever nearer.

So desperate was Sinclair that he nearly sent Torrens to Hong Kong . . . until he found Martin Wilcox, who joined the company as chief engineer in February 1965. His first job was to sort out the 120-watt amplifier contract. After Sinclair had left for Hong Kong, Wilcox decided that the prototype amplifier couldn't possibly be ready in time for the client company's engineer who was visiting in a few days. He therefore set to and designed, built and tested a conventional 20-watt amplifier to demonstrate instead. At least there was something to show; he carried on and four months later the Z-120, as the larger amplifier was called, had been developed and tested. Built to a military specification to be acceptable to the aircraft industry, these eventually sold for 75 each.

X-20 (1965)

The X-20 was much better than its precessor: it was putatively rated at 20 watts and it would indeed deliver 20 watts, but only with its dying breath, for the output transistors that arrived for manufacture were somehow never as good as the prototypes supplied for development. In any case, Sinclair already had a 10W amplifier: the new one obviously had to be better because it was more expensive! Apart from this, the X-20 was a good amplifier. There was none of the unpredictable performance of the X-l0, no two of which ever behaved in quite the same way. Later- just about the time at which the X-20 was discontinued - larger output transistors became readily available. Some of these were tried in the X-20 and it gave 20 watts without the slightest troubles or sign of distress.

The X-20 continued to be the lead item in June and July advertisements, when a double-page spread trumpeted 'in step with the SPACE AGE!' in rather wobbly lettering backed by a drawing of the Post Office tower. Only a year old and 619 feet high, the tower symbolised London's step into the space age; not only did it handle satellite communications (indirectly); it had a revolving restaurant. The adverts declared that the X-20


The Sinclair X20 enables you to enjoy for the first time ever, the advantages of using a high power, high fidelity audio amplifier truly in step with today's space age electronics . . . and it is easier to build and install than any amplifier you have ever owned. Best of all it costs far less.

Z-12 (1966)

The Z-12 amplifier replaced the X-10 - by now something of an embarrassment - with the number being rather more meaningful this time, and was supplied ready built. This achieved ‘laboratory standards of performance’, whatever those may be, but advertising aside it is remembered as quite a good amplifier. 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, and filling the houses of worship in a secular fashion; unkind commentators muttered phrases such as 'cheap labour'.

The PZ-3 and Stereo 25, introduced later the same year, were, respectively, a power supply unit and control unit providing volume balance and tone controls for the Z-12.

IC-10 (1968)

The IC-10 was an audio integrated-circuit power amplifier, created under a marketing agreement with Plessey, who were to produce the devices for Radionics. The first batch was scheduled for delivery on 1 June 1968 and the advertising for the IC-10 duly appeared as a double-page spread in all the usual magazines. Deliveries of packaging material and instruction leaflets began on time, and all was set for a June launch.

June arrived - but no ICs: there had been production problems at Plessey. No - they didn't know how long these would take to sort out. No, Sinclair couldn't have even a small dclivery immediately - not for an unspecified time.

Since it takes about three months to change advertising, Sinclair decided not to modify the plans, for surely the devices would be available by the time the adverts changed? Eventually, it became apparent that the delay was going to be prolonged, but orders - and cash - were flowing in from customers. The delay was lengthy and all the customers' payments would certainly have to be refunded. Luckily, Chris Curry - later of Acorn fame - was working on the design of a minute FM radio which contained a small device - the IC-4 - which was wheeled out as a stop-gap alternative to the IC-10. Each customer who had ordered an IC-10 received an IC-4 as a free gift with an apology for the delay and a request to be patient, since the IC-10 was well worth waiting for. This was not strictly true, as Alfred Marks remembers:

It was Clive who first made an integrated circuit available to the public - the IC-10. Then an integrated circuit was a miracle, but the IC-10 wouldn’t peak at 3 watts really, and flopped. It was probable, although only rumour, that the IC-10 was a product which Plessey were not too happy to have in their stable, anyway.

(Information from The Sinclair Story, by Rodney Dale (1985), and Sinclair and the Sunrise Technology, Ian Adamson & Richard Kennedy (1986); photos from The Sinclair Story.)