JPH0115199B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a voltage-current feedback amplifier used in audio power amplifiers, etc., in which voltage feedback is applied to the high range and current feedback is applied to the low range. This system compensates for the lack of sound pressure in the speaker, suppresses an increase in frequency characteristics in the high range, and prevents the amount of compensation from becoming too large near the low resonance frequency of the speaker.

Audio speakers are generally made so that the frequency characteristics of the output sound pressure are as flat as possible for a constant voltage input. For this reason, conventional power amplifiers that drive speakers also generally use voltage feedback, which maintains a constant voltage gain, as shown in FIG. In Fig. 1, 1 is the input terminal;
2 is an amplifying section, 3 is an output terminal on the hot side thereof, 4 is a load speaker, and 5 and 6 are voltage feedback resistors.

In the configuration shown in FIG. 1, the voltage gain of the amplifier is determined by the division ratio of voltage feedback resistors 5 and 6, so its frequency characteristics become flat as shown in FIG. 2a.

However, as shown in Figure 3c, the frequency characteristics of the speaker's sound pressure for a constant voltage input range from around the low-frequency resonance frequency _p (hereinafter simply referred to as _p ).
Because the attenuation is rapid at 12 dB/octave, especially when the volume of the speaker box is small, _p becomes high, resulting in a lack of low frequencies.

Therefore, in order to increase the sound pressure in the low range, especially near _p , by applying current feedback, taking advantage of the fact that the impedance of the speaker near _p increases as shown in Figure 4d, Conventionally, methods have been proposed to compensate for the lack of low frequencies.

An example is shown in FIG. In Figure 5, 7
8 is a cold side output terminal of the amplifier, and 8 is a load current detection resistor inserted between the cold side output terminal 7 and ground.

If the value of the load current detection resistor 8 is set sufficiently small with respect to the speaker load resistance value, a signal inversely proportional to the speaker impedance as shown in FIG. 4d is fed back to the inverting input terminal of the amplifier 2. Therefore, the voltage gain of the amplifier has a frequency characteristic as shown in FIG. 2b, and the low frequency range is enhanced.

However, in general, the speaker impedance is
Since the gain increases as the frequency goes up, there is a problem in that the gain of such a current feedback amplifier increases in the high frequency range, as shown in FIG. 2b.

Also, the impedance of the speaker near _p is
Because it is several times the impedance of the midrange,
Another problem was that the increase in gain near _p was too large.

The present invention solves these problems and provides an amplifier that compensates for the lack of sound pressure in the low range of the speaker, suppresses the rise in frequency characteristics in the high range, and further suppresses the amount of compensation near _p from becoming too large. It provides:

In other words, the present invention uses both voltage feedback and current feedback, and combines both feedback signals using a resistor and a capacitor, so that voltage feedback is applied to high frequencies and current feedback is applied to low frequencies. be.

FIG. 6 shows a first embodiment of the invention. 6th
In the figure, numerals 9 and 10 are a feedback signal synthesis capacitor and a resistor for synthesizing a voltage feedback signal and a current feedback signal and feeding them back to the inverting input of the amplifier, respectively.

Here, if the value of the feedback signal synthesis resistor 10 is set sufficiently large compared to the parallel value of the voltage dividing resistors 5 and 6 and the value of the load current detection resistor 8 (this condition is fully realized), ) capacitor 9 and resistor 1
Voltage feedback is applied at frequencies higher than frequency _c determined by a time constant of 0, and current feedback is applied at frequencies lower than _c . Therefore, if the ratio of the dividing resistors 5 and 6 is set close to the ratio between the resistance value of the speaker 4 and the load current detection resistor, the frequency characteristics due to the voltage feedback shown in Fig. 6 will be as shown in Fig. 7 e. The frequency characteristics due to current feedback are as shown in FIG. The frequency characteristic obtained by combining both feedbacks becomes as shown in Fig. 7g.

Furthermore, the peak near _p is also limited by the frequency characteristic e due to voltage feedback, so it is suppressed lower than in the case of only voltage feedback. and _c
The higher c becomes, the higher the level of the frequency characteristic e due to voltage feedback at _p becomes, so the peak of the synthesized frequency characteristic g becomes higher, and conversely, the lower _c becomes, the lower the peak of the synthesized frequency characteristic g becomes. I understand.

Therefore, by making the feedback signal synthesis resistor 10 a variable resistor, the degree of the peak at _p can be set according to preference by changing the resistance value. Note that, according to the present invention, even when the feedback signal synthesis resistor is made variable in this way, the ratio of voltage feedback and current feedback before and after _c does not change.

By the way, depending on the usage situation of the amplifier, the type of speakers to be combined, the user's preference, etc., it may be more convenient to use only voltage feedback. Furthermore, since the load current detection resistor 8 also causes a loss of output power, it is desirable to short it when current feedback is not used.

FIG. 8 shows a second embodiment of the present invention that achieves the above requirements in a very simple manner. This embodiment uses switch 11, and when used as a voltage-current feedback amplifier, switch 11 is
Switch to the side of the feedback signal synthesis resistor 10 shown in the figure,
When using only voltage feedback, it is designed to switch to the opposite side. In this way, the voltage feedback amplifier and the voltage/current feedback amplifier can be switched simply by switching the switch 11.

Note that if a switch 11 as shown in FIG. 8 is used, when the switch 11 is switched to the ground side,
Since the load on the feedback signal synthesis capacitor 9 is open, the feedback signal synthesis capacitor 9 and the resistor 1
This has the advantage that the influence of 0 is eliminated.

In the above embodiment, the synthesized feedback signal is directly fed back to the inverting input of the amplifier 2, but even if it is fed back through the sound quality adjustment circuit 12 as shown in FIG. The present invention is effective.

In addition, when the amplifier output becomes open, or when switch 11 is switched to the ground side,
Even when using it as a normal voltage/current feedback amplifier, there is a problem that the DC component is no longer fed back.
The present invention is effective even when DC component feedback circuits 13 to 15 are added, as shown in FIG. 9, in order to solve problems such as insufficient feedback of the DC component.

Furthermore, in all of the above embodiments, the current feedback signal is supplied from the load current detection resistor 8 directly or via a switch to the feedback signal synthesis resistor 10. The present invention is also effective even when the signal is further divided by resistance and supplied.

As described above, the present invention applies voltage feedback to the high range and current feedback to the low range, so when driving a speaker, the lack of sound pressure in the low range can be avoided. It is possible to suppress an increase in the high frequency characteristic in the case of only current feedback, and it is also possible to suppress the amount of compensation from becoming too large near the low frequency resonance frequency of the speaker.

In addition, by adding a switch, it is possible to easily switch between voltage and current feedback and voltage feedback, and when switching only to voltage feedback, it is not affected by the feedback signal synthesis capacitor and resistor, and load current detection This provides an excellent effect in that there is no loss of output power due to external resistance.

[Brief explanation of drawings]

Figure 1 is a circuit diagram of a conventional voltage feedback amplifier, Figure 2 is a circuit diagram of a conventional voltage feedback amplifier.
The figure shows a frequency characteristic diagram of the amplifiers shown in Figures 1 and 5, and Figure 3 shows a frequency characteristic diagram of a general speaker.
FIG. 4 is a frequency characteristic diagram of the impedance of a general speaker, FIG. 5 is a circuit diagram of a conventional current feedback amplifier, and FIG. 6 is a circuit diagram of the first embodiment of the present invention.
FIG. 7 is a frequency characteristic diagram of the embodiment of FIG. 6, FIG. 8 is a circuit diagram of a second embodiment of the invention, and FIG. 9 is a circuit diagram of a third embodiment of the invention. 1... Input end, 2... Amplifying section, 3... Hot side output terminal of the amplifying section, 4... Speaker, 5, 6... Resistance divider circuit, 7... Cold side output terminal of the amplifying section,
8... Resistor for load current detection, 9... Capacitor for feedback signal synthesis, 10... Resistor for feedback signal synthesis, 1
1...Switch, 12...Sound quality adjustment circuit, 13~
15...DC component feedback circuit.

Claims (1)

[Scope of Claims] 1: an amplifier section, a resistance divider circuit that divides the output voltage of the amplifier section, a load current detection resistor inserted between the cold side output terminal of the amplifier section and the ground, one end of which is connected to the Consisting of a feedback signal synthesis capacitor connected to the resistance divider circuit side, and a feedback signal synthesis resistor whose one end is connected to the cold side output end side and the other end is connected to the other end of the feedback signal synthesis capacitor. and a voltage/current feedback amplifier configured to feed back a signal at a connection point between the feedback signal synthesis capacitor and the feedback signal synthesis resistor to the input side of the amplifier section. 2. The voltage-current feedback amplifier according to claim 1, characterized in that a switch is provided for switching and connecting the cold side output end to the ground side and the feedback signal synthesis resistor side. 3. The voltage-current feedback amplifier according to claim 1, characterized in that the division ratio of the resistance division circuit is set to a value close to the ratio of the load resistance to the load current detection resistance. 4. The voltage-current feedback amplifier according to claim 1, characterized in that the feedback signal synthesis resistor is made variable.