Early Personal Audio: The Regency TR-1 Transistor Radio

Early Personal Audio: The Regency TR-1 Transistor Radio

The first portable transistor radio became available in time for Christmas in 1954 and was the result of a conscious effort by a young Texas Instruments to create a mass market for transistors. Up to this time, the only commercial use for transistors had been in hearing aids. As the father of the project, Patrick Haggerty, later noted, the thinking was that “...a dramatic accomplishment by [us would] awaken potential users to the fact that...we were ready, willing, and able to supply [transistors]”. TI arranged a deal with a small company called IDEA (Industrial Development Engineering Associates), whose Dick Koch modified TI’s first-pass circuit (principally designed by Paul D. Davis and Roger Webster) to reduce cost and improve manufacturability. The task was challenging as no one had much expertise with transistors yet. To make a tough job even more difficult, the germanium transistors then available were quite poor by today’s standards (fT’s of only a few MHz at best, and β’s of 10-20), while their cost was high. Compounding those difficulties was the lack of off-the-shelf miniature components to complement the small transistors. It was quite a struggle to cram all of the circuitry into a case small enough to fit in a shirt pocket (indeed, early advertisements used a custom-made shirt with oversized pockets). In a first for consumer electronics, printed-circuit technology was chosen for the TR1 in order to facilitate interconnecting such densely packed circuit elements. The newness of the technology presented many daunting manufacturing challenges.

Calculations showed early on that no more than four transistors could be used or IDEA and its Regency division would not be able to make a profit at the targeted sale price of $49.95. The four transistors accounted for about half of the cost of the materials. At a time when an All-American Five could be purchased for about $15, it was difficult to imagine that there would be a significant market for such an expensive device. As it happened, demand outstripped production capacity for quite some time.

As seen in Fig, four transistors were enough. In this circuit, the first transistor, Q1, functions as an oscillator-mixer, just as the first tube does in an All-American Five. Transformer coupling between collector and emitter circuits provides the positive feedback necessary for oscillation.

Fig: Schematic of the Regency TR1

The incoming RF signal is tuned using a mechanism called “absorption,” developed by the German company Telefunken around World War I. In this technique, an LC tank coupled to the input circuit shorts out (absorbs) signals at all frequencies other than the resonant frequency of the tank. The RF signal can pass to the base of Q1 only when this shorting disappears, at the absorbing tank’s resonant frequency (determined by C2). The inherent nonlinearity of the base-emitter diode provides the mixing action. Hence, in addition to the local oscillator signal, the collector current also has a component at the sum and difference heterodyne terms. The difference signal is then fed to the first IF amplifier, Q2, through an LC band­pass filter tuned to the IF of 262kHz. The unusually low IF allows the low-fT transistors to provide useful amounts of gain, but exacerbates an already bad image rejection problem. The variable capacitor in the absorptive LC front-end tank is ganged with the LO variable capacitor. The degree of image rejection achieved here is best described as adequate.

The second IF amplifier, Q3, is connected in a manner essentially identical to Q2. The large Cμ values (probably about 30-50 pF) are partially cancelled by positive feedback through C10 and C14 (a technique introduced in the 1920’s as the Neutrodyne circuit).

A standard envelope detector performs demodulation, and then feeds a single stage of audio amplification. Transformers couple signals into the detector and out of the audio amplifier.

AGC action is provided in a familiar manner: the demodulated audio is further RC filtered (here by R11 and C9), and the resulting negative-polarity feedback signal controls the gain of the first IF stage by varying its bias.

The success of the TR-1 had important consequences beyond establishing TI as a leader in the semiconductor business. Of particular significance is that IBM quickly abandoned development of new vacuum tube computers, with Thomas Watson, Jr. reason­ing that if transistors were mature enough for high-volume consumer gear, it was time to consider them for computers. As he later told the story, every time one of his subordinates expressed doubt about transistors, he’d give him a TR1, and that usually settled the argu­ment.

A young company called Sony introduced their own transistor radio, the TR55, soon after Regency’s TR1 debuted. The company would soon dominate the consumer market for portable electronics.


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