GB2166015A - Overload preventing circuit for headphones - Google Patents

Abstract

A circuit in or for use with headphones prevents overloading of the transducers in the earpieces of the headphones by monitoring at 21 the input signal and operating a cut-off switching device T1 when the input signal level exceeds a reference level. The circuit is made from passive components and utilises zener diodes Z1, Z2 to provide an output signal indicative of the input level exceeding the reference level. The signal output from the zener diodes is voltage multiplied, averaged, at 22, and used to gate an FET device T1 which is used to cut-off the input signal to the headphones. Alternatively an attenuation circuit which reduces the level of the input signal could be used. Mono and stereo circuits are disclosed. <IMAGE>

Description

SPECIFICATION Headphones The present invention relates to a circuit in or for use with headphones in order to prevent overloading of the transducers in the ear pieces of the headphones.

Circuits for prevention of overload are known in relation to loudspeakers but there are particular difficulties encountered with headphones in as much as the transducers in the earpieces tend to be of different efficiencies and the earpieces may be small which would make providing a built-in apparatus difficult.

From one aspect, the present invention provides an adapter for use with earphones, which adapter contains a circuit for modifying the signal to be applied to the earphones in order to prevent overloading of the transducers in the earpieces.

From another aspect, the present invention provides a circuit for use with or in earphones which will monitor the signal being applied to the earpieces and automatically reduce the input signal applied to the headphone to an appropriate level. The circuit may be built into the headphones or may be an adapter into which the headphones plug.

Preferably, the apparatus makes use of an electric circuit including a switching device and which utilises only passive components for generating a signal for controlling the switching device.

Further features and advantages of the present invention will become apparent from the following description of an embodiment thereof when taken in conjunction with the accompanying drawing, in which: Figure 1 shows one form of a general arrangement of a system including an adapter; Figure 2 shows a circuit diagram of the system shown in Fig. 1; and Figure 3 shows a block diagram of a further embodiment used for stereo headphones.

The embodiment of the invention shown in Fig. 1 incorporates an adapter 10 between a piece of electrical equipment 1, such as a cassette player, hi-fi video, or television, and headphones 13. Connection between the adapter 10 and the headphones 13 is by any suitable means such as a conventional jack plug 12 and socket 11. Likewise, the connection between the adapter 10 and the headphones 13 is also preferably by a jack plug and socket arrangement.

The adapter 10 contains circuitry for monitoring the sound level of the signal output from the equipment 1 and for modifying the input to the headphones transducer or transducers.

As shown in Fig. 2, the input audio signal (output from the equipment 1) is fed to an input modifying circuit 20 whose output is fed to the headphones transducer or transducers.

A detector circuit 21 also receives the input audio signal and generates a control signal in response to the level of the input audio signal for controlling the modifying circuit 20. A drive circuit 22 is interposed between the detector circuit 21 and the modifying circuit 20 and derives power from the input audio signal.

In this circuit, the drive circuit 22 is a voltage multiplier and is shown to be a voltage tripler.

The modifying circuit 20 may take any convenient form. For example, it could be a simple switch, such as the transistor T1, which disconnects the headphones 13 from the audio input in the presence of an input above a level determined by the detector circuit 21.

Alternatively, it could include an attenuation circuit which reduces the level of the input signal.

This is a passive circuit which, without in any way interfering with 'normal' moderate drive operation of a headphone, switches off the voltage drive to the headphones whenever the average sound level at the ear exceeds a predetermined level, e.g. about 93 dB (flat), so limiting the possibility of high sound levels inducing hearing loss or temporary threshold shift.

The circuit operates by using zener diodes Z1 and Z2 to detect the short term voltage drive level to the headphone during normal signal operation. When this level is reached, these diodes conduct and enable the voltage tripling circuit 22 (comprising C, to C3 and D and D3) to produce a negative voltage which, although it will vary to a certain extent during the moments which follow the circuit switching on, will remain at a sufficiently steady level of several volts, to bias 'off' the FET elements which constitutes transistor T,. This transistor has a very high drain/source resistance when the gate voltage is of the order of a few volts negative.

If the input signal now vanishes, or falls to a lower level, (say by being 'reset' by the user) then in about 1 second the negative voltage discharges and the 'zero' bias on the transistor T, gives the condition of the device being 'on', with a drain/source resistance of about 25 ohms. Hence the circuit produces only a small attenuation of the signal under 'normal' listening conditions, and no non-linearity or other kinds of distortion e.g. time or transient effects. Zener diodes Z3 and Z4 prevent very high short term signal peaks from causing significant bias changes on the FET gate, which would result in modulation effects and 'breakthrough' of signal when the device is operating to limit sound level excesses.

These peaks are not, however, clipped by the zener diodes, for their operating point is more than 20 dB above the 'normal' listening levels, and their loading effects are in any event small for low duty signals.

Fig. 3 shows a block diagram of a further embodiment of the present invention designed for use with stereo equipment. This embodiment uses two circuits, each as shown in Fig.

2; and which are connected as indicated in Fig. 3 with reference to connection points 1, 2 and 3 which are marked in Fig. 2. No further description of the stereo embodiment is thought necessary.

The diodes Z, and Z2 must be narrowly selected, for they determine the exact point at which the limiter operates, and would, if unbalanced as between channels, result in different switch off points. Note that this could otherwise occur on stereo programme, when there may sometimes be small signal differences which averaged over a second or so would produce higher dc voltage differences.

All other components are close tolerance (5%) types, with germanium diodes being used in the tripler to maximise the dc voltage derived from the signal.

While an adapter is disclosed, it is contemplated that the circuitry shown in Figs. 2 and 3 could be incorporated in the headphones which would render the adapter superfluous.

When an adapter is used, a visual indication may be given when the circuitry operates to modify the input signal. Also an on/off switch could be incorporated so that the modifying circuitry may be selected or not by the user.

Claims (7)

1. A circuit for monitoring an input signal to headphones comprising a monitoring circuit for detecting when the level of the input signal is greater than a reference level, a switching device for altering the input signal to the headphones, and a drive circuit responsive to the monitoring circuit for controlling operation of the switching device, the monitoring circuit and drive circuit being formed by passive components.

2. A circuit according to claim 1, wherein the drive circuit is a voltage multiplier.

3. A circuit according to claim 1 or 2, wherein the monitoring circuit comprises two zener diode connected in series but in polarity opposition.

4. A circuit according to claim 1, 2 or 3 and comprising means for preventing transient signals from operating the drive circuit.

5. A circuit substantially as hereinbefore described with reference to Fig. 2 or Fig. 3.

6. An adapter including a circuit according to any one of the preceding claims.

7. Headphones including a circuit according to any one of claims 1 to 5.