Item nr.

Audium T8KL Hearing Aid

Early solid state hearing aid.

Data for Audium T8KL
ProductionGermany, 1958.
3x OC71.
CabinetSteel. Size 7x6x2cm. Weight 140 gr.
PowerBatt 1xAA.

The Design

With its volume of 84 cubic centimeters (ccm), this early transistorized hearing aid is just marginally smaller than its tubed predecessors, like the Phonak 330 of 114ccm. It is also a shirt pocket model, with one ear piece connected by a wire. The big gain for the user was, of course the much lower current and the use of relatively cheap AA cells. This transistorized unit takes just 2.3mA from one AA cell, while the tubed unit typically draws 20mA from the AA cell for the heaters, plus 1 to 2 mA from the plate battery.

Removal of the front panel reveals the back of the PCB, with the cores of three audio transformers and the large microphone. Opening the back reveals the three OC71 transistors; bottom left of the picture shows adjustments to align the machinery to the user's needs.
The OC71 was introduced in 1954, and succeeded by various transistors introduced in 1957 (OC75) and 1960 (AC125). Because hearing aids often use the modernest techniques, I assume this unit to be built between 1957 and 1959, but I couldn't find any direct sources about the building year.

Audium had an agency in Amsterdam (Singel 160). Mick sent me this schema of the very similar Audium 800, a model of year 1956/7.

Obtained8/2020 from Barneveld Market; sn=47864.
Condition8; neat and works.
Value (est.)6,8€.

This Object

There was a Philips battery inside, dateds as "Best before 4/2010", but which still worked fine. The hearing aid works. I think positions 1 and 2 of the switch use the microphone, and 3 is for Audio Induction Loops. With this loop, the unit makes it possible to "hear" magnetic fields, for example from a transformer, speaker, or electric clock.

The earpiece is a magnetic piece with DC resistance about 300 Ohms and 1k impedance. The plug has one of the pins slightly thinner, so it fits only in one position. It is not interchangeable with the plugs of the Phonak hearing aid or the Tecla pocket radio (both tubed).

Part of Gerard's Radio Corner.
Generated by SiteBuilder on 20/9/2023 by Gerard (g.tel@uu.nl)