Unfortunately, the power requirements of these aids made it necessary to use two relatively large batteries—an "A" battery to power the filaments of the vacuum tubes and a "B" battery to supply the high-voltage for the plate current.
The batteries were worn external to the hearing aid itself and the name "Two-piece" hearing aid was used to describe them.
Until about 1945 "Two-piece" hearing aids were the norm. However, improved vacuum tube design reduced current requirements. As a result, battery manufacturers were able to reduce the size of the batteries used in hearing aids.
At the same time, electronic components were also being miniaturized. This allowed the hearing aid manufacturers to design hearing aids with internal batteries. After 1945, these "One-piece" hearing aids quickly replaced the more cumbersome "Two-piece" hearing aids. The progression from large to small over the years. From left to right:
- Large vacuum tube from early table model hearing aid (4½" high)
- Peanut Tube from Vactuphone hearing aid
- Miniature radio tube from 1940s
- Two sub-miniature Raytheon hearing aid vacuum tubes
- Integrated circuit (⅜")
With continuing improvements in both batteries and components, hearing aids continued to reduce in size until 1953 when the vacuum tube was suddenly supplanted by the transistor.