JPH02177705A - Acoustic amplifier - Google Patents

Abstract

PURPOSE:To prevent thermal destruction of a power transistor(TR) by connecting a speaker terminal to a reference resistor at application of power, selecting a level of a power voltage generated from a power supply circuit in response to a pure resistance component of a speaker connecting to the speaker terminal and connecting the speaker terminal to an output terminal of the power amplifier after a prescribed time elapses from the application of power. CONSTITUTION:A 1st relay contact 12 is thrown to the position of a contact (a) just after application of power. Even when a noise signal is outputted from a power amplifier 7 due to the transient state, the noise signal is not fed to a speaker 10. The speaker 10 connecting to a speaker terminal 9 is connected to a reference resistor 11 when the 1st relay contact 12 is thrown to the position of the contact (a). When a prescribed time elapses after application of power, the 1st relay contact 12 is thrown to the position of a contact (b). Since a voltage over the Zener voltage is applied to a Zener diode 13 without any change only with the speaker 10 disconnected from the reference resistor 11, the ON state of a 1st TR 15 is continued.

Description

DETAILED DESCRIPTION OF THE INVENTION (A) Field of Industrial Application The present invention relates to an acoustic amplifier designed to provide desired rated outputs to speakers having different nominal impedance values.

(b) Conventional technology In the past, speakers with a nominal impedance of 8Ω were the mainstream for speakers that could be connected to an audio amplifier, but recently,
The nominal impedance is replacing those with 6Ω and 4Ω. Therefore, as the impedance of speakers becomes lower, acoustic amplifiers can be set to have a rated output for low impedance loads, such as 4Ω, in addition to the rated output for an 8Ω load, and the power supply circuit is designed to satisfy the rated output. Designed.

By the way, if the power supply voltage applied to the power amplifier of the acoustic amplifier is constant, the lower the impedance of the speaker that can be connected to the power amplifier, the more current will flow.
With the same power supply circuit, the rated output can be higher at low impedance.

However, if the impedance of the speaker connected to the power amplifier is low, a large current will flow through the power transistor constituting the output stage of the power amplifier, causing a problem of heat generation in the power transistor. Therefore, conventionally,
For example, as shown in Japanese Unexamined Patent Publication No. 61-251214, a power supply that generates a high power supply voltage and a power supply that generates a low power supply voltage are provided, and are used according to the nominal impedance value of the speaker connected to the power amplifier. Power supply circuits that switch power sources are known. That is, when a speaker with a large nominal impedance value is connected, the power supply circuit is switched so that a high power supply voltage is applied to the power amplifier, whereas when a speaker with a small nominal impedance value is connected, the power supply circuit is switched so that a high power supply voltage is applied to the power amplifier. It is switched so that a low power supply voltage is applied to the Therefore, if the above power supply circuit is used, when the nominal impedance of the speaker connected to the power amplifier is large, the output of the power amplifier will be as large as possible within the capacity of the above power supply circuit, and on the other hand, when the nominal impedance of the speaker is small. At this time, heat generation in the power transistors constituting the output stage of the power amplifier was suppressed, and thermal destruction of the power transistors was prevented.

(c) Problems to be Solved by the Invention However, in the power supply circuit of the above-mentioned publication, by comparing the voltage drop across the emitter resistance of the power transistor and the voltage generated according to the current flowing through the load (speaker), The power supply voltage applied to the power amplifier is switched, and the voltages that can be compared both vary depending on the output current that drives the speaker, making the design difficult and the operation unstable. Moreover, even when using an acoustic amplifier to drive a speaker, the circuit for comparison described above is connected to the speaker, and that circuit becomes a load on the power amplifier, which has a negative effect on speaker drive. It had the disadvantage of causing

(d) Means for Solving the Problems The present invention is an acoustic amplifier that has been made in view of the above points, and has a reference resistor that can be connected in series with a speaker connected to a speaker terminal and to which an operating voltage is applied. , a voltage divider circuit that divides the operating voltage by the reference resistor and the speaker, a transistor that is controlled according to a voltage generated at a voltage dividing point of the voltage divider circuit, and a transistor that is controlled depending on whether the transistor is turned on or off. It consists of a power supply circuit that generates power supply voltages of different levels, and a switch that selectively connects the speaker terminal to the output terminal of the power amplifier and the reference resistor, and when the power is turned on, the speaker terminal is connected to the reference resistor. By connecting to the speaker terminal, the level of the power supply voltage generated from the power supply circuit is switched according to the pure resistance component of the speaker connected to the speaker terminal, and the speaker terminal is connected to the power amplifier after a predetermined period of time has elapsed since the power was turned on. The output terminal of the

(Function) The present invention divides the operating voltage between the reference resistor and the speaker using the voltage divider circuit within a predetermined time after the power is turned on, and adjusts the power supply voltage generated from the power supply circuit according to the divided voltage value. By switching the level, the level of the power supply voltage applied to the power amplifier is switched according to the nominal impedance of the speaker connected to the power amplifier, and when the speaker is driven, the power supply voltage level from the above-mentioned power supply circuit is switched from the speaker. The circuit for controlling this is separated.

(v) Embodiment The figure is a circuit diagram showing an embodiment of the present invention, in which (1) is a commercial power supply, and (2) is a negative winding connected to the commercial power supply (1) via a power switch (3). a power transformer (4) connected to the secondary winding and having a center tap and a grounded center tap for rectifying the alternating current voltage generated from the secondary winding of the power transformer (2); bridge type rectifier circuit,
(5) and (6) are smoothing capacitors that smooth the DC voltage rectified by the rectifier circuit (4), and (7) is the terminal voltage of the smoothing capacitor 5) and (6) that is applied as the power supply voltage. (9) is a power amplifier that amplifies the power of the acoustic signal input to the input terminal <8>, and (9) is a speaker (10).
) is connected to the speaker terminal, (11) is the reference resistor to which the operating voltage (+ V cc+ ) is applied to one end, and (12
) is a first relay contact for selectively connecting the speaker terminal (9) to the output end of the power amplifier (7) and the other end of the reference resistor (11); (13) is a resistor whose cathode is a resistor; (14) is a Zener diode connected to the other end of the reference resistor (11); (15) is a common-emitter first transistor whose base is connected to the anode of the Zener diode (13); ) is a pull-down resistor for generating a base bias of the first transistor (15), and (17) and (18) are the power transformer (2).
) a second relay contact for selectively connecting the terminal end and intermediate tap of the secondary winding of ) to the rectifier circuit (4), (19)
is a relay coil that is connected to the collector of the first transistor (15) and excites the second relay contacts (17) and (18); (20) is composed of a resistor (21) and a capacitor (22); and the diode (
A time constant circuit (24) is connected to the collector of the first transistor (15) via the time constant circuit (23), and the terminal voltage of the capacitor (22) of the time constant circuit (20) is applied to the base and the collector is The Zener diode (13
) is a common emitter type second transistor connected to the cathode of the transistor.

By the way, the first relay contact (12) is connected to the power amplifier (7) due to the transient state of the acoustic amplifier during the period until it reaches a steady state when the power is turned on and until the operation completely stops when the power is turned off. ) is provided for memuting to prevent the noise signal output from the speaker (10) from being supplied to the speaker (10). 10)
plays a role in protecting. Therefore, the first relay contact (12) is configured to be connected to the a contact until a predetermined period of time (for example, 2 seconds) has elapsed after the power is turned on, and then to the b contact. .

Further, in a state where the first relay contact (12) is connected to the a contact, the reference resistor (11) is connected to the speaker (10
), and the reference resistor (11) and the speaker (10) constitute a voltage divider circuit that divides the operating voltage (+Vcct).

On the other hand, the Zener diode (13) is connected to the reference resistor (11
> and the speaker (10) constitute a voltage divider circuit, the reference resistor 1) and the speaker (10)
A Zener voltage determined according to the voltage generated at the voltage dividing point A between the two is used. In other words, if we set the rated output for an 8Ω and 4Ω load as an acoustic amplifier, the Zener diode (13) will be It is configured to be in an off state and to be in an on state when a speaker with a nominal impedance of 4Ω and a pure resistance component of approximately 4Ω is connected to the reference resistor (11).
As the reference resistor (11), the resistance value can be set according to the nominal impedance of speakers currently on the market, and for example, a resistor of 8 Ω is used. In that case, the speaker terminal (
If you ignore the elements (resistance (14), Zener diode (13), and pull-down resistor (16)) connected in parallel to the speaker (10) connected to the reference resistance (
When a speaker with a pure resistance of 8 Ω is connected to 11), a voltage of 1/2 VCel is generated at the voltage dividing point A, and on the other hand,
When a speaker with a pure resistance of 4 Ω is connected to the reference resistor (11), a voltage of 1/3·vcc will be generated at the voltage dividing point A. Therefore, as a Zener diode (13), the Zener voltage is smaller than 172·VCCI,
and larger than 1/3・VCCI, for example, 1/2・V
, c, and 5712 at a voltage intermediate between 173 V CCI.
・Use V CCI.

Therefore, the reference resistor (11) has a nominal impedance of 8
When a speaker of Ω is connected, a Zener diode (1
3) is turned on, and when a speaker with a nominal impedance of 4Ω is connected to the reference resistor (11), the Zener diode (13) is turned off.

Now, when the power switch (3) is closed and the power is turned on, the secondary winding of the power transformer (2) is connected to the commercial power supply (1
) is transformed by the power transformer (2) to generate an AC voltage. Here, the second relay contact (1
7) and (18) are each connected to a b contact in a state where they are not excited by the relay coil (19). Therefore, immediately after the power is turned on, the AC voltage generated from the middle tap of the secondary winding of the power transformer (2) is rectified by the rectifier circuit (4), and the smoothing capacitor (5) and The DC voltage obtained by smoothing according to (6) is applied as the power supply voltage. That is, immediately after the power is turned on, the power supply voltage of the lower level is applied to the power amplifier (7).

On the other hand, immediately after the power is turned on, the first relay contact (12
) is connected to the a contact. Therefore, even if a noise signal is output from the power amplifier (7) due to a transient state,
The noise signal is not supplied to the speaker (10), and in a state where the first relay contact (12) is connected to the a contact, the speaker (10) connected to the speaker terminal (9) is connected to the reference resistance. (11). For that reason,
At voltage division point A, the reference resistor (11) and the speaker (
10), a voltage obtained by dividing the operating voltage (+vceI) is generated.

Here, the speaker terminal (9) has a nominal impedance of 8Ω.
If the speaker is fully connected, a voltage higher than the Zener voltage of the Zener diode (13) is generated at the voltage dividing point A, so the Zener diode (13) is turned on. When the Zener diode (13) turns on, the base voltage of the first transistor (15) becomes higher than the conduction voltage between the base and emitter of the first transistor (15) due to the voltage drop across the pull-down resistor (16). Therefore, the first transformer (15) is turned on. Therefore, current flows through the relay coil (19) and the second relay contacts (17) and (18) are energized.
The relay contacts (17) and (18) are each switched to a contact. Therefore, the AC voltage generated from the end of the secondary winding of the power transformer (2) is transferred to the rectifier circuit (4).
Since the voltage is rectified by the power amplifier (7), a higher second level power supply voltage is applied to the power amplifier (7).

By the way, the collector of the first transistor (15) has
Second transistor (24) via diode (23)
Since the bases of the second transistor (15) are in the on state, the second transistor (15)
24) is in the off state. Therefore, in this state,
Turning on the second transistor (24) does not turn off the first transistor (15).

After a predetermined period of time has passed after the power is turned on, the first relay contact (
12) is switched to a b contact. Then, the reference resistance (11)
Since the speaker (10) is simply disconnected from the zener diode (13), a voltage higher than the zener voltage is still applied to the zener diode (13), so the first transistor (15) continues to be turned on. Therefore, the speaker terminal (9
) is connected to a speaker with a nominal impedance of 8Ω, the higher second level power supply voltage is applied to the power amplifier (7), so it drives the speaker (10) at the predetermined rated output as designed. I can do it.

On the other hand, when a speaker with a nominal impedance of 4Ω is connected to the speaker terminal (9), the first relay contact (12)
When three contacts are made, a Zener diode (
Since a heavy pressure less than the Zener voltage of 13) is generated, the Zener diode (13) is turned off. Therefore, the first transistor (15) is turned off, so no current flows through the relay coil (19), and the second relay contact (15) is turned off.
7) and (18) are each switched to a b contact. Therefore, since the AC voltage generated from the center tap of the secondary winding of the power transformer (2) is rectified by the rectifier circuit (4), the power amplifier (7) is supplied with the lower level power supply. Voltage will now be applied.

Furthermore, in the off state of the first transistor (15),
The base of the second transistor (24) has an operating voltage (+
V ce* ) is the resistance (25) and the relay fill (19)
), the voltage is applied via the diode (23) and the time constant circuit (20), so the second transistor (24) is turned on after a predetermined time according to the charging of the capacitor (22) of the time constant circuit (20). . Here, the capacitor (22
) is mainly set by the capacitance of the capacitor (22) and the resistance value of the resistor (21). The second transistor (24) is set to be controlled during the muting operation before the contact piece (12) switches to the b contact. Therefore, the second transistor (24) is turned on after the first transistor (15) is completely controlled and during the muting operation. When the second transistor (24) is turned on, the cathode of the Zener diode (13) is grounded through the collector-emitter of the second transistor (24).

Therefore, even if the first relay contact (12) is switched to the b contact after a predetermined period of time has passed after the power is turned on, the off state of the Zener diode (13) continues. Therefore, when a speaker with a nominal impedance of 4 Ω is connected to the speaker terminal (9), the lower tfj voltage of the 1st level is applied to the power amplifier (7), so the speaker (7) is operated at a predetermined rated output as designed. 10> can be driven.

(G) Effects of the Invention As described in 2., the present invention allows the voltage level of the source voltage applied to the power amplifier to be changed according to the nominal impedance of the speaker connected to the power amplifier. Since the speaker can be driven at a predetermined rated output, there is an advantage that thermal damage to the power transistors constituting the output stage of the power amplifier can be prevented. Further, in the present invention, the control for switching the power supply voltage level applied to the power amplifier is performed within a predetermined time after the power is turned on, and the circuit for controlling the switching of the power supply voltage level is then separated from the speaker. This has the advantage that no adverse effects are caused on the drive of the speaker.

[Brief explanation of the drawing]

The figure is a circuit diagram showing one embodiment of the present invention. Explanation of main drawing numbers (7>...Power amplifier, (9)...Speaker terminal, (10)...Speaker, (11)...Reference resistor, (12)...First Relay contact piece, (13)...
Zener diode, (15)...first transistor, (17)(1g)...second relay contact, (19)
)...Relay fill, (rejection)...Time constant circuit, (
24)...Second transistor.

Claims (1)

[Claims] (1) It has a speaker terminal to which a speaker is connected, a power amplifier that supplies an output signal to the speaker terminal, and a reference resistor that is connected in series with the speaker connected to the speaker terminal and to which an operating voltage is applied. and a voltage divider circuit that divides the operating voltage by the reference resistor and the speaker, a transistor that is controlled according to a voltage generated at a voltage division point of the voltage divider circuit, and a transistor that is controlled depending on whether the transistor is turned on or off. and a switch for selectively connecting the speaker terminal to the output terminal of the power amplifier and the reference resistor, and a switch for selectively connecting the speaker terminal to the output terminal of the power amplifier and the reference resistor. By connecting the resistor to the speaker terminal, the level of the power supply voltage generated from the power supply circuit is switched according to the pure resistance component of the speaker connected to the speaker terminal, and the power supply voltage of the speaker terminal is switched after a predetermined time has elapsed since the power is turned on. An acoustic amplifier characterized in that it is connected to the output end of an amplifier.