JPS5910807Y2 - BTL circuit output switching circuit - Google Patents

Description

[Detailed description of the invention] This invention relates to an improvement of the output switching circuit of the BTL circuit, and its purpose is to prevent the terminal voltage of the BTL circuit from being applied to the jack fitting of the external output jack when not in use. It is an object of the present invention to provide an output switching circuit that prevents the occurrence of low-frequency noise and impairs the low-frequency characteristics of an external speaker.

The so-called BTL circuit can output approximately four times as much output as a normal output circuit with the same power supply voltage.
It became widely used in cassette tape recorders with radio receivers that required high output but wanted to keep the power supply voltage as low as possible.

FIG. 1 shows an output switching circuit of a conventional BTL circuit, in which the amplifier output is switched between a built-in speaker SP and an external output such as an external speaker using an external output jack J having a switching contact.

Although the jack fitting a of the external output jack J is an exposed metal part, it is connected to one output terminal of the BTL circuit, and a DC voltage of 9 of the power supply voltage Vcc and a signal output voltage of h are applied. .

By the way, jack fittings (not shown), such as microphone jacks or external input jacks, which are installed in close proximity to the above jack fitting a, are electrically grounded, so if there is a short circuit between the jack fittings for some reason. In some cases, an accident may occur in which the power supply voltage is shorted.

In addition to the above case, such a short-circuit condition is more likely to occur if a grounded metal panel or the like is provided near the external output jack J.

FIG. 2 shows an output switching circuit for a BTL circuit that has been improved to address the above problems, in which a capacitor C is connected between the jack fitting a of the external output jack J and the output terminal of the BTL circuit.

Therefore, even if the jack fitting a is short-circuited to the ground, the power supply voltage will not be short-circuited because the direct current is cut off.

However, in order to produce sufficient low-frequency output from an external speaker, the capacitance of capacitor C must be 470 to 1000.
A μF capacitor is required, and due to the nature of the BTL circuit, a non-polar capacitor must be used.

Such non-polar large capacity capacitors are about twice as large as polar ones, and are also more expensive, making it difficult to achieve both low-frequency characteristics and cost.
Furthermore, although the capacitor C can prevent the power supply voltage from being short-circuited, the signal output h will be short-circuited, so it is possible that the external output will be drastically reduced. Regarding the speaker, there are drawbacks such as the fact that it cannot be called an OCL output circuit.

This invention is intended to improve the drawbacks of the conventional example as described above, and FIG. 3 shows a circuit of this embodiment.

In this circuit, a switch S is added which is closed when the plug of an external speaker is inserted into the external output jack J, and is opened when the plug is not inserted into the external output jack J.

Therefore, when the speaker plug is not inserted into the external output jack J, the jack fitting a is cut off from the BTL circuit, and when the speaker plug is inserted next, the switch S is closed. Since the output of the BTL circuit is applied to the external speaker, there is no need to use the capacitor C as in the conventional example.

In addition, in the circuit of this embodiment, when the speaker plug is inserted, the power supply voltage is applied to the jack fitting a, but the jack fitting a is larger than this and is a plug made of resin or the like. covered by the base of
There is no risk of this shorting out to the grounded metal part.

As described above, in the output switching circuit of the BTL circuit of this invention, when the external output jack is not in use, neither DC voltage nor signal voltage is applied to the jack metal fitting, which is an exposed metal part, so if it is connected to other grounded metal parts, Even in the event of a short circuit, it has the advantage of not causing a short circuit in the power supply voltage or a drop in signal output, and since it does not use a DC cut capacitor, the low frequency characteristics of the external speaker will not be impaired. The advantage of the OCL amplifier is that it can fully utilize the advantages of the OCL amplifier even when used with external speakers.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of the output circuit of a conventional BTL circuit, Figure 2 is a circuit diagram of a conventional BTL output switching circuit with measures against power supply short circuits, and Figure 3 is the output of a BTL circuit showing an embodiment of this invention. FIG. 3 is a circuit diagram of a switching circuit. J... Jack for external output, a... Jack fitting for external output jack, S... Switch, SP... Built-in speaker.

Claims (1)

[Scope of utility model registration request] External output jack with switching contacts allows BTL
In a circuit that switches the output of the circuit between a speaker and an external output, a switch that closes when an output plug is inserted into the external output jack is provided in addition to the switching contact above, and the exposed metal part of the external output jack is The jack fitting forming the above is connected to one output terminal of the BTL circuit through the switch, and when the output plug is not inserted into the external output jack, the jack fitting is cut off from the BTL circuit by the switch. An output switching circuit for a BTL circuit characterized by the following.