Transistor hearing aids

The machine a person used to wear

For most of the first half of the century, a hearing aid was furniture you carried. The first wearable vacuum tube hearing aid went on sale in England in 1936, and a year later in the United States, but tubes were the whole problem. They were hot, fragile and greedy with power. A serious aid needed a heavy battery strapped to the chest, and the amplifier that ran on it was just as bulky. Hearing was restored, but not with any elegance, and not for long stretches between charges.

The transistor arrived from Bell Laboratories in 1948, the work of John Bardeen, Walter Brattain and William Shockley. It did everything a vacuum tube did, but it was tiny, cool, cheap to keep running and far less fragile. Its problem was that nobody had a use for it at a price consumers would pay. A transistor cost far more than the tube it replaced, so the obvious products stayed away. The one place all that did not matter was a device small enough and low-powered enough that the trade was worth making.

A transistor in the pocket

That device was the hearing aid. In 1952 Sonotone, an American hearing aid maker, put the first transistorized consumer product in the United States on sale at $229.50. The Sonotone 1010 was a hybrid, using a transistor in its output stage supported by vacuum tubes to stretch battery life, and the transistor came from Germanium Products Corporation.

The arithmetic made sense. Tubes drained a battery in hours; a transistor, used to replace the most power-hungry stage, gave an aid hours more listening time between charges and let the whole unit shrink toward something that clipped on. The same shrinking opened the door to miniature microphones, small enough to be mounted on an eyeglass frame. For the first time, amplification was possible in a package that did not announce itself as a machine strapped to the body.

Raytheon and the first production transistor

The company that pushed the component into volume was Raytheon, under its vice-president Norman Krim. Raytheon was among the first to mass-produce the transistor in America from 1951, and it made the CK718, the first transistor to be built in serious quantity. The company supplied three of these to the Maico Company for an all-transistor hearing aid introduced in 1953, and around the same time the first fully transistorized models, the Microtone Transimatic and the Maico Transist-ear, reached wearers and dropped the vacuum tubes entirely.

The hearing aid was not a sideline for the young industry. It was the market that funded the component. Norman Krim's aggressive pursuit of the hearing aid business is credited with making Raytheon the largest transistor manufacturer in the world between 1952 and 1955, at a time when almost nobody else had a commercial use for the device at all. Where radios and computers still balked at the price, hearing aid wearers paid a premium for portability and low power, and that stream of money kept transistor production alive while prices fell toward everything else.

Growing pains: dampness and body heat

The transition was fast, and for a reason. The rush to transistors was so quick that the new aids were not properly tested before release. Early germanium transistors proved sensitive to moisture: an aid would work for a few weeks and then die, and manufacturers had to learn to coat the transistor to protect it from dampness. Zenith was the first to notice that body heat was doing the damage, not the weather, a finding that would shape how the devices were sealed and worn.

Those early troubles are why the industry moved as carefully as it did at first, and they explain why Raytheon briefly went back to selling its old vacuum tube aids alongside the transistor ones while the reliability questions were settled. It was a hiccup, not a reversal. Once the dampness problem was solved, there was no returning to tubes.

A market that transformed a component

The payoff was fast. By 1954, according to the Computer History Museum, 97 percent of all hearing aids used only transistors, and the machines that had once been chest-mounted boxes had become small enough to wear. The tube hearing aid, barely two decades old, was gone.

The hearing aid's real legacy was that it proved the transistor as a consumer part. The pocket radio that followed, the Regency TR-1 in October 1954, rode on the same economics the hearing aid had established, and the hearing aid business gave the young industry the volume and the revenue it needed to bring component prices down. Within a generation the integrated circuit, introduced at Texas Instruments in 1958, would push the transistor hearing aid itself into history. But the wearable hearing aid is the device where consumer electronics first met the transistor, and the timeline building block that made the rest of the portable age possible.