GB2157917A

GB2157917A - Active tone controls for hearing aids

Info

Publication number: GB2157917A
Application number: GB08509750A
Authority: GB
Inventors: Fritz Hueber
Original assignee: Viennatone GmbH
Current assignee: Viennatone GmbH
Priority date: 1984-04-16
Filing date: 1985-04-16
Publication date: 1985-10-30
Also published as: DK164345B; CH668153A5; ATA127084A; DK171785A; GB8509750D0; DK171785D0; DE3513197C2; AT379724B; JPS60233917A; DK164345C; DE3513197A1; GB2157917B

Abstract

An active tone control circuit for a hearing aid having at least two amplifier stages, (V1), (V2), is provided in the signal path between the microphone (14) and the loud speaker (13) of the hearing aid. To achieve high and low frequency control by only one common adjustment, the tone control comprises a bridge circuit (B) having a resistor (1,2) in each of two opposite branches and a series connection consisting of a resistor (3,4) and a capacitor (5,6) in each of the other two opposite branches. The diagonal connection of the bridge circuit is formed by the resistance path (7) of a variable resistor (P1), which is formed as a common adjustment for low and high frequencies and whose tapping (8) is connected to the input of transistor (T) whose main current section is connected to the output of the bridge circuit (B). The tone control circuit may alternatively be located before amplifier stage (V<1>) or after amplifier stage (V2) (see figures 3 and 4). <IMAGE>

Description

SPECIFICATION Active tone controls for hearing aids The invention relates to an active tone control circuit for hearing aids for interposition in a transistor amplifier having at least two stages.

Hearing aidsforthe deaf or persons hard of hearing are mostly equipped with tone controls, i.e., adjusting devices (trimmers or potentiometers) with the aid of which the electric frequency response of the amplifier can be controlled. In many cases, in orderto achieve as good an adaptation as possible of the hearing aid to the individual hearing loss curve ofthe wearer, two separate adjustments are provided for controlling the low and high frequencies.

In the case particularly of small hearing aids to be worn on the head, for example, behind-the-ear appliances or hearing-aid glasses, the fitting of two separate adjustments causes difficulties for reasons of space. Attempts have therefore been madetofind circuitsfor a tone balance, i.e., with one adjustment which, starting from a linear frequency response in the centre position, gives a boost to the low frequencies or, on the other hand, a boost to the high frequencies on rotation to right or left.

Such circuits operateforthe most part as active tone controls, i.e., with the inclusion of a transistor amplifier stage in which the desired effect is achieved by utilisation offrequency-dependent positive and negative feedbacks. An example of such an active tone control is disclosed, for instance, in EU-A 54608.

Because itis importantthatthe partsof a hearing aid should be of low cost it is, however, common to the majority of known circuits that they have a relatively high degreeof distortion in the positivefeeezack range, the effectiveness as a whole is not very great, or the centre position of a linear adjustment does not coincide with the linear positon ofthe frequency response, for which reasons adjustments with special curves (eg. logarithmic) are then needed.

The object of the invention is to provide a tone control circuitwhich avoids the mentioned disadvantages ofthe known circuits.

According to the invention, this is achieved by providing a tone control circuit in the signal path between the microphone and the loud speaker comprising a bridge circuit having a resistor in each oftwo opposite branches and a series connection consisting of a resistor and a capacitor in each of the othertwo opposite branches, the diagonal connection of the bridge circuit being formed by the resistance path of a variable resistor, which series as a control element for low and high frequencies and the tapping point of said variable resistor being connected to the input of a transistorthe main current section of which is connected to the output ofthe bridge circuit.

Preferably the output stage of the transristor ampli fier ofthe hearing aid is formed by the transistor of the tone control circuit.

The active tone control circuit according to the invention avoids distortion by constantly operating in the negative feedback range and gives good effective ness of any setting oradjustmentfora justifiable cost of parts, the centre position of the linear adjusting device also corresponding to the linear position ofthe frequency response.

The invention will now be described in greater detail by way of example, with reference to the drawing. In the drawing: Fig. 1 shows a hearing aid circuit with an embodiment of an active tone control according to the invention; Fig. 2 is a graph showing the frequency response of the tone control circuit according to Fig. 1% and Fig. 3 and 4 show two more embodiments of a circuit according to the invention.

The hearing aid circuit according to Fig. 1 has a two-stage amplifierwith a preamplifier stage VI and a final amplifier stage V2. The two amplifier stages V1, V2 are connected to the operating voltage B+ via the lead 12. The useful signals picked up by the microphone 14 are amplified by the preamplifier VI and fed via a direct-current isolating capacitor 9to a tone control circuit. The tone control circuit consists of a bridge circuit B in which a resistor 1,2 is arranged in each oftwo opposite branches of the bridge. Aseries connection consisting of a resistor 3 or4 and a capacitor 5 or 6 is provided in each of the othertwo branches of the bridge.The diagonal of the bridge circuit B is formed by the resistance path 7 of a potentiometer P1,whose slider is connected to the input (base) of an n-p-n transistorT. The output (collector) ofthe transistorTis connected to the output of the bridge circuit B and via the working resistor 11 to the battery voltage B+. The output of the bridge circuit B is furthermore connected via a direct-current isolating capacitor 10 and a volume potentiometer P2 to the input of the final amplifier V2. The useful signal is reproduced attheoutputofthefinal amplifierV2 by a speaker (earphone) 13.

A negative feedback branch forms between the output (collector) and the input (base) of the transistor Via the branch 3,5 and part of the potentiometer resistance, the extent of the negative feedback being variable according to the position of the slider 7 of the potentiometer.

In Fig. 2, the frequency response of the tone control circuit according to Fig. 1 in the two extreme positions of the potentiometer P1 is represented with the aid of a graph, the frequency in Hz being plotted on the abscissa and the amplification in db being plotted on the ordinate. In the graph,the curve 15 corresponds to the upper position oftheslider8 and the curve 16 corresponds to the lower position of the slider 8 of the potentiometer P1.

The following components were employed forthe tone control circuit in a hearing aid circuit carried into effect: Resistorl 33kin Resistor2 33kO Resistor3 10 k n Resistor4 10k0 CapacitorS 4.7 F Capacitor6 4.7 nF TransistorT BC 146 Potentiometer P1 100 kn Capacitor9 1,uF CapacitorlO 1pF Resistor 11 3.3 kn With the above-indicated dimensioning, the cros soverfrequencyof about 1200 Hzwhich can beseen in Fig. 2 is obtained.

Fig. 3 shows the arrangement of the active tone control circuit between the microphone 14 and the first amplifier stage VI. The direct-current isolation between the microphone and the tone control circuit is again effected by a capacitor 9. The output of the tone control circuit is connected via another isolating capacitor 10 to the inputofthefirstamplifier stage VI.

The arrangement ofthetransistorTwiththe working resistor 11 and the parts of the bridge circuit is identical to the circuit according to Fig 1.

Fig. 4 shows the inclusion of the active tone control circuit in thefinal stage of an amplifier. From a preamplifier (not shown),the signal reaches another amplifierstage V2 via an isolating capacitor 10 and the volume control P2. Via another isolating capacitor 9, the signal finally reaches the bridge circuit ofthetone control, whosefunction is again identical to that of the circuit according to Fig. 1. Only the working resistor 11 ofthetransistorTis now replaced by the speaker (earphone) 13.

Claims

1. Atone control circuitforhearing aidsfor interposition in a transistor amplifier of the hearing aid, said amplifier having at least two Stc ;es, said tone control circuit comprising a bridge circuit having in each of two opposite arms a resistor and in each of the othertwo opposite arms a series connection consist ing of a resistor and a capacitor, the diagonal connection of the bridge circuit being formed bythe resistance path of a variable resistorwhich serves as a control elementforlowand high frequencies and the tapping point of said variable resistor being coupled to the input of a transistor whose main current path is connected to the output of the bridge circuit.

2. Atone control circuit according to Claim 1, wherein the outputstage ofthetransistoramplifier is formed by the transistor of the tone control.

3. A hearing aid having a tone control circuit according to Claim 1 or Claim 2.

4. Atonecontrol circuitfora hearingaid substan- tially as described herein with reference to the drawings.

5. A hearing aid having circuitry substantially as described herein with reference to any one of Figs 1,2, or3 ofthe drawings.