25-03-2022: Electronics 14¶

Hearing loop system¶

Hearing loop systems are used for broadcasting audio information to individuals with T-Coil equipped hearing aids. They are used in public areas such as theatres, churches, meeting and conference rooms, etc.

The architecture of a hearing loop system is shown in the Fig 1.

Fig. 10 Architecture of a hearing loop system: A loop antenna converts the electrical audio signal into magnetic flux. A receive coil in the hearing aid converts this magnetic flux into a voltage. An amplifier in the hearing aid amplifies this voltage to drive the loudspeaker in the hearing aid.¶

The send and receive antennas will be provided. Behavioral models need to be based on measurement data, acquired during the demonstrations. Test and measurement equipment for characterization of the antennas is available in the Tellegen hal.

Target specification¶

1. Maximum audio input level: 1V peak

2. Maximum audio output level with the receive coil placed in the center of the transmit coil: 0.25V peak

3. -3dB bandwidth: 60Hz … 15 kHz

4. Full-power bandwidth: 5kHz

5. Input impedance transmitter: > 10kOhm

6. Load impedance receiver: > 2kOhm

7. Receiver output noise (no input signal, frequency range 60Hz … 15 kHz) < 100uV RMS

8. Power supply voltage +/- 15V

Tasks¶

Groups of four to six participants will design, build and test a hearing loop system. Two or three participants design and build the transmit coil driver, while the other two or three design and build the receive coil amplifier. All group members participate in the selection of the system architecture (amplifier types) and in the the definition and the execution of the test plan.

The design comprises the following steps:

1. Design a lumped element network model of the transmit coil

2. Design a lumped element network model of the receive coil

3. Set-up selection criteria for the electrical quantity driving the transmit coil and for the electrical quantity to represent the information from the receive coil

4. Design the system architecture, model the transfer, and derive specifications for the amplifiers:

   • type and gain

   • noise requirements

   • signal levels and drive capability requirements

   • frequency response

5. Design a test plan

6. Design the transmit coil driver amplifier

7. Design the receive coil amplifier

8. Build the system

9. Execute the test plan

10. Discuss the results with each other and with the instructors

Data sheets of available components¶

• Data sheet 100mH receive coil: RL622-104K-RC

• Data sheet proposed operational amplifier TL081C

• Data sheet high current driver BUF634