JPS6119533Y2 - - Google Patents

Description

[Detailed Description of the Invention] This invention relates to a direct-coupled amplifier that is used in equalizer amplifiers, tone control amplifiers, etc., and which aims to reduce output drift.

2. Description of the Related Art Conventionally, equalizer amplifiers, tone control amplifiers, and the like have been provided with a drift detection amplifier in order to reduce their direct current drift. For example, FIG. 1 shows an example of a conventional equalizer amplifier, in which the output of a cartridge 1 is inputted to the non-inverting input terminal of an amplifier 4 via an input terminal 2 and a capacitor 3. After being amplified according to the characteristics it has, it is output from the output terminal 6. On the other hand, the DC component of the output from the output terminal 6 is detected by an integrating circuit (drift detection amplifier) composed of resistors 7 and 8, a capacitor 9, and an inverting amplifier 10.
It is designed to be fed back to the non-inverting input terminal of.

However, in the above circuit, since the output of the integrating circuit is connected to the non-inverting input terminal of the amplifier 4, the cartridge 1 and the amplifier 4 cannot be directly connected, but must be connected through the capacitor 3. Ta. For this reason, there was a problem that the characteristics in the low frequency region deteriorated. Furthermore, in this circuit, if the time constant of the integrating circuit (the time constant of the resistor 8 and capacitor 9) is increased, the discharge time of the capacitor 9 becomes longer, and it takes more time for the feedback circuit to stabilize. Therefore, the values of the resistor 8 and the capacitor 9 are at most 100KΩ for the resistor 8 and 1000μF for the capacitor 9. On the other hand, the value of resistor 7 is set to a certain degree (10KΩ
However, considering the above points, the gain of the integrating circuit cannot be changed significantly, making it difficult to obtain optimal servo characteristics.

This invention was made in view of the above circumstances, and its purpose is to provide a direct-coupled amplifier that can be directly connected to the previous stage and that reduces output drift. The features of this invention include an amplifier to which an input signal is applied to the non-inverting input terminal and the inverting input terminal is grounded via a resistor, and a frequency characteristic imparting element that connects the output terminal of the amplifier and the inverting input terminal. A DC circuit consisting of a first feedback path connected between the two, and an operational amplifier equipped with a phase correction capacitor so that the bare gain characteristic is attenuated above the lower limit of the audible frequency.
a second feedback path having a comparator and connected between the output end of the amplifier and the inverting input end using the signal at the output end of the amplifier as an input signal;
Among the signals obtained at the output terminal of the amplifier, all components to which frequency characteristics are given, a DC component, and an AC component below an audible frequency are fed back to the inverting input terminal of the amplifier.

Embodiments of this invention will be described below with reference to the drawings. FIG. 2 is a circuit diagram showing an embodiment of this invention, in which this invention is applied to both an equalizer amplifier and a tone control amplifier in an audio preamplifier circuit.

In FIG. 2, the output signal of the cartridge 15 is inputted to a non-inverting input terminal of an amplifier 17 via an input terminal 16, and the non-inverting input terminal of this amplifier 17 is connected to ground via a resistor 18. . The output of this amplifier 17 is fed back to the inverting input terminal of this amplifier 17 via the equalizer element frequency characteristic imparting element 19 having predetermined equalizer characteristics, that is, all the components imparted with frequency characteristics are fed back to the inverting input terminal. While being fed back, amplifier 2
0 and the non-inverting input terminal of the operational amplifier 22 (non-inverting amplifier) constituting the DC comparator 46 through the resistor 21,
Further, it is configured to be grounded via a resistor 23. The amplifier 22 is a conventional operational amplifier (e.g.
709 type operational amplifier), and a large-capacity capacitor 24 (for example, 100 μF) is connected to its phase compensation capacitor terminal, so this amplifier 22 outputs only the DC drift component and components below the audible frequency of the output of the amplifier 17. It is becoming more and more amplified.

The inverting input terminal of this amplifier 22 is connected to a resistor 2.
5 to the sliding end of a variable resistor 26, and its output is connected to the amplifier 17 via a resistor 27.
The inverting input terminal of this amplifier 17 is connected to the ground via a resistor 28. On the other hand, one terminal of the variable resistor 26 is connected to a positive power supply (+Vcc) through a resistor 29, and the other terminal is connected to a negative power supply (-Vcc) through a resistor 30. Vessel 2
6 is for compensating for fluctuations in the reference voltage due to the influence of the input offset current of the amplifier 22. The circuit described above is the equalizer amplifier 3.
1.

The portion following the equalizer amplifier 31, numeral 32, constitutes a tone control amplifier, which has exactly the same configuration as the equalizer amplifier 31. That is, the output of the amplifier 20 is fed back to the inverting input terminal of the amplifier 20 via the tone control element frequency characteristic imparting element 33 having predetermined tone control characteristics, while being fed back to the inverting input terminal of the amplifier 20 via the output terminal 34 and the resistor 36. The signals are respectively supplied to the non-inverting input terminals of the constituent operational amplifiers 37 (non-inverting amplifiers), and are also grounded via a resistor 38. A large capacity capacitor 39 is connected to the phase compensation capacitor terminal of the amplifier 37, its inverting input terminal is connected to the sliding end of a variable resistor 41 via a resistor 40, and its output is connected to the amplifier 20 via a resistor 42.
The inverting input terminal of this amplifier 20 is connected to the ground via a resistor 48. On the other hand, one terminal of the variable resistor 41 is connected to a positive power supply (+Vcc) through a resistor 44, and the other terminal is connected to a negative power supply (-Vcc) through a resistor 45. .

Note that in the above configuration, the resistors 21, 25, 3
The values of 6 and 40 need only be 10 KΩ or more, and the amplifiers 22 and 37 need only have open loop gains of 80 dB or more.

Next, the operation of the circuit configured as above will be explained. The output of the cartridge 15 is amplified by the amplifier 17 of the equalizer amplifier 31 according to a predetermined equalizer characteristic, and then input to the amplifier 20 of the tone control amplifier 32. On the other hand, the DC drift component and components below the audible frequency of the output of the amplifier 17 are transferred to the DC comparator 46.
and is fed back to the inverting input of amplifier 17. In this case, since the signal is amplified and fed back by the DC comparator 46 using the phase correction terminal of the amplifier 22, a sufficient amount of feedback can be obtained. and,
This feedback reduces the output drift of the amplifier 17, and since the feedback signal is input to the inverting input terminal of the amplifier 17, the amplifier 17 and the cartridge 1
5 can be directly coupled (without a capacitor).

Thus, the signal input to the amplifier 20 of the tone control amplifier 32 is amplified by the amplifier 20 and then output from the output terminal 34. This tone control amplifier 32 is the same circuit except that the equalizer element 19 in the equalizer amplifier 31 is replaced with a tone control element 33, and the explanation thereof will be omitted.

In addition, in the above circuit, DC comparator 3
Even if you use a low pass filter instead of 5,46
The same effect as using a DC comparator can be obtained.

As explained above, according to this invention, there is provided an amplifier to which an input signal is applied to the non-inverting input terminal and the inverting input terminal is grounded via a resistor, and a frequency characteristic imparting element, and the output terminal of the amplifier and the inverting input terminal are connected to each other. and a DC comparator consisting of an operational amplifier provided with a phase correction capacitor so that the bare gain characteristic is attenuated above the lower limit of audible frequency; a second feedback path connected between the output end and the inverting input end of the amplifier is provided, and a frequency characteristic is given to the signal obtained at the output end of the amplifier. Direct-coupled amplifiers such as low-drift equalizer amplifiers and tone control amplifiers are very useful in audio circuits because all components, DC components, and AC components below the audio frequency are fed back to the inverting input terminal of the amplifier. can be provided. Furthermore, when feeding back the DC component and the AC component below the audible frequency of the signal obtained at the output terminal of the amplifier, the circuit configuration can be simplified by using a DC comparator that utilizes the phase correction terminal of the operational amplifier. There are benefits to be gained.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an example of a conventional equalizer amplifier, and FIG. 2 is a circuit diagram showing an embodiment of this invention. 17, 20...Amplifier, 19...Equalizer element (frequency characteristic imparting element), 33...Tone control element (frequency characteristic imparting element), 22,3
7...Non-inverting amplifier (amplifier), 35, 46...
DC comparator.

Claims (1)

[Claims for Utility Model Registration] An amplifier to which an input signal is applied to a non-inverting input terminal and whose inverting input terminal is grounded via a resistor, and an amplifier having a frequency characteristic imparting element and connecting an output terminal of the amplifier to an inverting input terminal. a DC comparator consisting of an operational amplifier provided with a phase correction capacitor so that the bare gain characteristic is attenuated above the lower limit of audible frequency; a second feedback path connected between the output end of the amplifier and the inverting input end as an input signal; , a direct-coupled amplifier characterized in that a direct current component and an alternating current component below an audio frequency are fed back to an inverting input terminal of the amplifier.