Patent GB-1452564-A (GB)
Title: Hearing aid control circuit
IPC: H04R25/00
Filed: 1974-11-15
Abstract: 1452564 Hearing aid circuits; muting VIENNATONE HORGERATE PRODUKTIONS GmbH 15 Nov 1974 [21 Nov 1973] 49578/74 Headings H4J and H3T A hearing aid circuit has, between the microphone 1 (Fig. 2) and the sound emitter 4, e.g. an earphone, a muting switch arranged to block transmission of the output signal from the microphone below a threshold level. Undesired background noises are suppressed until the sound signal has reached a threshold level, e.g. provided by speech or music. The microphone 1 is coupled to the base of an output stage amplifying transistor T 3 , a capacitor C 2 , potentiometer 3 and amplifier 2 being interposed. The collector of transistor T 3  is connected to a battery positive terminal through the sound emitter 4. An amplifying transistor T 1  and a switching transistor T 2  are also provided. In operation, an audio signal received by microphone 1 is amplified by amplifier 2 and the magnitidue of the signal is adjusted by a potentiometer 3 and applied to the base electrode of transistor T 3 . When the input to the microphone 1 is negligible, e.g. ambient noise, the voltage applied to switching transistor T 2  via a a voltage divider R 3 , R 4  and a resistor R 5  is such that transistor T 2  is blocked. No appreciable voltage is produced at the base of transistor T 3  and this is blocked. Thus the sound emitter 4 produces no output signal. Amplifying transistor T 1  and amplifier 2 are in operation independantly of the sound level. When the input signal reaches a threshold level the output voltage of transistor T 1  is sufficient to make transistor T 2  partially conductive. Current then flows from transistor T 2  through resistor R 6  to the base of output transistor T 3  which makes this transistor partially conductive, too. The resultant voltage drop at sound emitter 4 makes the voltage at point 9 more negative, thus causing transistor T 2  to be rendered more fully conductive and placing transistor T 3  into full operation. Resistor R 4  provides regenerative feedback, hastening the transition from blocked condition of transistors T 2 , T 3  which operate as a bi-stable flip-flop circuit. When the signal voltage falls below the threshold level the rectified signal voltage at the base of transistor T 2  is reduced again, direct current through resistor R 5  cannot keep the transistor T 2  conductive and the circuit flips back into blocked condition. Capacitor C 4  presents audio frequency signals passing via transistors T 1 , T 2  to transistor T 3  and delays the flip-flop action so that very short error signals will not switch on the circuit. A switch 6 enables adjustment of the threshold. In the position shown for switch 6 the potentiometer 5 makes only a small portion of the amplified signal voltage effective at switching transistor T 2  so that the input signal must have a high level to open transistor T 2 . In the centre position for switch 6 the entire signal will be picked off so that transistor T 2  becomes open for a low level input signal. With switch 6 in its lowest position the threshold switch is inoperative and the circuit operates acts as a conventional hearing aid.