The amplifiers and filters in the HC-100 were designed using commonly available IC amplifiers from an OEM manufacturer in Canada (LTI now known as Gennum), see Figure 12. We had some severe problems in our application using their IC´s with “Motor boating” and high start up current especially at lower battery voltages that took some time to solve.

Figure 12: Amplifiers used in the HC-100.

Minor individual adjustments in each of the patients devices were needed to avoid feedback problems. Because of the problems it was quite clear that a new transducer element had to be developed for the next generation of the BAHA that was more adapted to this new mechanical load situation at the bone anchored implant. However, it was not obvious what would be the optimum transducer principle to be used: electromagnetic (moving coil or variable reluctance), piezoelectric or magnetostrictive to mention those principles that were more carefully tested. Various designs of piezoelectric transducers that were tested are shown in Figure 13.
 

Figure 13: Different piezoelectric transducers tested for a BAHA.

It was found that piezoelectric transducer were superior at high frequencies but had too low output capability at low frequencies in patients with a predominant conduction hearing loss. A moving coil type was tested but could not drive the relative high impedance of the bone anchored implant efficiently. We found that best combination of low and high frequency response was offered by a damped electromagnetic transducer of the variable reluctance type.

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