JPS5834077B2 - speaker protection circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a speaker protection circuit that prevents a speaker from being damaged due to exceeding the permissible input.

A speaker has a specified nominal impedance and permissible input.

Common nominal impedances are 4Ω and 8Ω.

The allowable input is generally about 30W to 100W.

On the other hand, there has recently been a trend toward higher output power for audio power amplifiers, and audio power amplifiers that output power greater than the permissible input of a speaker have appeared.

For this reason, there are many accidents where speakers are destroyed due to exceeding the permissible input of the speaker.

To prevent this, there is a method of equipping the audio power amplifier with a power limiter.

There are two methods for limiting power: a method for limiting voltage amplitude and a method for limiting current.

FIG. 1 is an example of a power limiter according to the prior art.

In the figure, an audio signal 1 is power amplified by an audio power amplifier 8 and applied to a speaker 12.

Here, the resistor 2, the Zener diodes 4, 5.6, and the switch 7 constitute a voltage amplitude limiting circuit.

Zener voltage Vz of Zener diodes 4, 5 and 6
By providing three types of , the voltage amplitude of the audio signal entering the audio power amplifier 8 is divided into three stages and the amplitude is limited depending on the position of the switch 7.

The speaker is protected by selecting the position of the switch 7 of the power limiter according to the permissible input of the speaker used here.

However, since the power limiter only limits the voltage amplitude of the audio signal entering the audio power amplifier 8, if an excessive audio signal is further applied to the power limiter, the input waveform of the audio output amplifier 8 will change. Although the amplitude is limited, the waveform is close to a rectangular wave, so the power applied to the speaker 12 is
The problem is that it increases.

Generally speaking, a speaker will not be damaged even if an input that exceeds the allowable input is momentarily applied to the speaker.

Human power that can be tolerated for only a short period of time is usually determined as the maximum allowable input for a speaker.

In some cases, the distortion rate of sound reproduced by a speaker is defined, and the input that achieves that distortion rate is the maximum permissible input for that speaker.

Therefore, since the above-mentioned power limiter always flips the peak value of the audio signal having a certain amplitude or more, the reproduced sound is distorted more than necessary, which causes discomfort to the listener.

Furthermore, if the voltage waveforms of a speaker with a nominal impedance of 4Ω and a speaker with a nominal impedance of 80Ω are the same, the power will be twice as different, which is inconvenient, and if the listener makes a mistake, there is a risk of damaging the speaker. .

The present invention eliminates the drawbacks of the prior art described above and provides a speaker protection circuit that prevents damage to the speaker by blocking the signal supplied to the speaker when a signal exceeding the permissible input of the speaker is supplied to the speaker. It is something.

The present invention detects the power applied to the speaker from the audio power amplifier, and limits the input signal to the audio power amplifier via a time constant circuit that is approximately equivalent to the time constant of the temperature rise of the speaker's voice coil. , which provides an accurate and reliable speaker protection circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENTS A speaker protection circuit according to the present invention will be explained below with reference to an embodiment shown in the drawings.

FIG. 2 shows an embodiment of the present invention.

The audio signal 1 is sent to the speaker 22 via the low frequency power amplifier 5.
is applied to

A voltage proportional to the power applied to the speaker 22 appears at the output terminal of the Hall element 13.

This voltage is amplified by an amplifier 15 and applied to a time constant circuit composed of a variable resistor 17 and a capacitor 18.

This time constant circuit constitutes a time constant circuit that is approximately equivalent to the time constant of the temperature rise of the voice coil of the speaker 22.

When the output of the low frequency power amplifier 5 increases and the voltage between the gate and cathode of the thyristor 21 reaches a certain value, the thyristor 21 operates, the coil current of the relay 19 flows, and the relay 19 is turned off. .

In this state, the power applied to the speaker 22 from the power amplifier 5 is cut off by the relay 1-9 to prevent damage to the speaker.

In the figure, 2.3.4 is the coupling resistor and capacitor, and 6
, 7.8 is a feedback resistor and capacitor, 9 is a compensation resistor,
20 is a protection diode, and 23 is a power supply.

Here, a power detection mechanism using a Hall element will be explained.

One of the input terminals of the Hall element 13 is connected to the output terminal of the power amplifier 5 via the variable resistor 12 and the resistor 11, and the other is grounded.

Therefore, the power amplifier 5 is connected to the current terminal of the Hall element 13.
A current proportional to the output voltage flows.

Further, since the Hall element 13 is placed near the parasitic vibration prevention coil 10 connected to the output terminal of the power amplifier 5, it is affected by a magnetic field proportional to the output current of the power amplifier 5.

Therefore, a voltage proportional to the output voltage and output current of the power amplifier 5 is generated at the voltage terminal of the Hall element 13, and this voltage becomes a voltage proportional to the output power of the power amplifier 5.

That is, the speaker 22 is connected to the voltage terminal of the Hall element 13.
A voltage proportional to the applied power is generated.

This voltage is amplified by an amplifier 15 and guided to a time constant circuit composed of a variable resistor 17 and a capacitor 18 , and the output of the time constant circuit is supplied to the gate of the thyristor 21 .

Here, the resistor 14 and the resistor 16 are resistors that determine the amplification degree of the amplifier 15.

Further, the variable resistor 12 is a resistor for adjusting the sensitivity of the Hall element.

This variable resistor 12 is used to adjust the output voltage of the Hall element in accordance with the permissible input of the speaker, and is actually designed to be compatible with speakers having different permissible inputs.

On the other hand, as a result of examining the failure mode of the speaker, the following points were found.

When power is applied to the speaker, the temperature of the voice coil increases, the coating on the coil ruptures, the coil shorts out, and the impedance of the speaker decreases.

As a result, the current flowing through the voice coil further increases,
As the temperature of the voice coil further increases, the adhesive that bonds the voice coil and bobbin eventually deteriorates, and eventually the voice coil separates from the bobbin, resulting in damage to the speaker.

Therefore, in the present invention, a time constant circuit having a time constant equal to the time constant of the temperature rise of the voice coil is provided, and the protective operation is performed in accordance with the temperature rise of the speaker.

In the embodiment shown in FIG.
is made variable so that the time constant can be determined depending on each speaker.

The coil 10 to which the output signal of the power amplifier 5 is supplied is actually attached to the printed wiring board, but it is preferably attached to the printed wiring board as shown in FIG. 3 in order to provide effective magnetic flux to the Hall element 13. .

In FIG. 3, the coil 10 is mounted perpendicularly to the printed wiring board 24 so that its axial direction is perpendicular to the printed wiring board 24.

Therefore, by attaching the Hall element 13 between the coil 10 and the printed circuit board 24, the Hall element 13 can be placed in a place where the magnetic flux density is high.

As described above, according to the present invention, through the time constant circuit equal to the time constant of the temperature rise of the voice coil of the speaker,
Since the input signal of the audio power amplifier is controlled, a speaker protection circuit that does not provide over-protection or under-protection of the speaker can be realized.

Further, since the protection operation is performed by detecting the power applied to the speaker by the Hall element, even if speakers with different nominal impedances are connected, there will be no inconvenience that the listener will operate the protection circuit incorrectly.

In the above description, the time constant associated with the temperature rise of the voice coil was used as the time characteristic leading to the destruction of the speaker, but it goes without saying that a time constant associated with mechanical destruction or the like may also be used.

[Brief explanation of drawings]

FIG. 1 is a circuit diagram of a conventional power limiter, FIG. 2 is a circuit diagram showing an embodiment of a speaker protection circuit according to the present invention, and FIG.
The figure is a perspective view showing how the Hall element and the coil are attached. 5: Low frequency power amplifier, 10: Coil, 13: Hall element, 19: Relay, 21: Cyrisk.

Claims (1)

[Claims] 1. A coil that is supplied with an output signal of a low frequency power amplifier and generates a magnetic flux according to the output signal current, and a coil that is arranged within the magnetic field of the coil and is supplied with the output signal, and that generates a voltage of the output signal and the magnetic field. a Hall element that generates a voltage according to the strength of the signal, a time constant circuit that is supplied with the output voltage of the Hall element and has a time constant that is approximately equal to a time constant of a temperature rise of the voice coil of the speaker; A speaker protection circuit comprising: a switch that is controlled by an output voltage and cuts off an output signal of the low frequency power amplifier supplied to the speaker when a steady output voltage of the time constant circuit exceeds a predetermined value.