JP6740561B2 - Preamplifier and transmission circuit using impedance adjusting circuit - Google Patents

Description

The present invention relates to an impedance increasing/decreasing circuit used in a preamplifier or the like for outputting the output of an analog record player as a normal line level signal.

When an MM cartridge is used for an analog record player, the load capacity from the cartridge to the input capacity of the equalizer amplifier has a large effect on the frequency characteristics. The characteristics were shifted to adjust the effect. In order to add capacitance, for example, a capacitor that is provided with several capacitors having a specific capacitance and is configured to be selectively switched by a switch, or a capacitor whose capacitance can be changed electronically is known. There is. It should be noted that when the MC cartridge is used, the number of turns of the coil is smaller than that of the MM cartridge, so that the influence of the inductance is small and the high frequency band is extended, so that it is considered that the influence of the load capacitance is hardly received.

FIG. 4 is an explanatory diagram showing a configuration in which a conventional record player using an MM cartridge and a preamplifier are connected. A record player 1 equipped with an MM cartridge is connected to a preamplifier 3 via a cable 2. The preamplifier 3 is provided with an equalizer amplifier 5 for inputting a signal from the record player 1 to perform RIAA correction and a capacity adjusting circuit 4 for adjusting a load capacity. The capacity adjusting circuit 4 is composed of a plurality of capacitors, and can be switched in the preceding stage of the equalizer amplifier 5 so that a signal from the record player 1 input via the cable 2 can be connected to any of the capacitors. Has become. This makes it possible to adjust the load capacitance and change the peak position of the frequency in the high frequency range. FIG. 5 is a diagram illustrating a change in peak position on the frequency characteristic. The peak is due to the resonance circuit formed by the coil and the capacitor of the cartridge mounted on the record player 1, and occurs at the resonance frequency position determined by the impedance of the coil and the capacitor. The larger the capacity added by this switching, the lower the peak frequency moves. The capacitor 6 is caused by the stray capacitance generated in the cable 2.

In the conventional adjustment method as described above, the user listens to the sound output from the power amplifier (not shown) via the preamplifier 3 while switching the capacitors of the capacity adjustment circuit 4, and performs adjustment so that the optimum sound is obtained. Was there. Therefore, the resonance frequency is changed only by switching the capacitor of the capacitance adjusting circuit 4, and it is not possible to eliminate the resonance itself and obtain a flat frequency characteristic without a peak. In addition, it is difficult to make an accurate adjustment because the user listens to the sound and the adjustment is made by selecting from a limited number of capacitors provided in advance.

Further, as the capacitance adjusting circuit 4, a circuit capable of electronically changing the capacitance as in Non-Patent Document 1 can be used. FIG. 6 is a circuit diagram showing a capacitance increasing/decreasing circuit disclosed in Non-Patent Document 1. According to this document, the circuit is composed of two-stage operational amplifiers, and the variable resistor 8 is connected to the output of the buffer amplifier 7 composed of the first-stage operational amplifiers. The voltage divided by the variable resistor 8 is input to the inverting input terminal of the operational amplifier 9, and the operational amplifier 9 constitutes an inverting amplifier circuit. Assuming that the capacitance of the capacitor 10 connected between the output of the operational amplifier 9 and the input of the buffer amplifier 7 is C2, and the resistance components divided by the variable resistor 8 are Ra and Rb, this circuit is obtained by the following formula (1). It is stated that the total equivalent input capacitance C can be calculated. Here, Ra is a resistance value on the output side of the buffer amplifier 7, and Rb is a resistance value on the output side of the operational amplifier 9.

As a result, the capacity multiplication factor can be continuously changed by the variable resistor 8, and a large-capacity capacitor can be manufactured. Patent Document 1 also discloses a capacitor whose capacitance can be varied by the same calculation formula. However, even if any of the above-mentioned conventional techniques is used, it is not possible to reduce the influence of the resonance circuit generated in the transmission system. That is, it is difficult to obtain a flat frequency characteristic because only the resonance frequency at the peak is changed by increasing the capacitance of the capacitor as described above.

JP-A-53-95554

Inaba Yasushi "Selected Analog Practical Circuits" 413 pages CQ Publishing 1989

The problem to be solved is that the adverse effect on the frequency characteristics due to unnecessary resonance occurring in the transmission signal cannot be improved.

The present invention provides an impedance increasing/decreasing circuit that can substantially cancel the influence of a capacitor, a coil, or a resistor, which is an impedance element forming a resonance circuit. When used in a preamplifier or the like, the main characteristic is to eliminate the influence of resonance on the frequency characteristic and to obtain a flat frequency characteristic.

When the present invention is applied to the preamplifier, the user can add impedance of substantially opposite polarity to the load while monitoring the frequency characteristics, and thus can effectively prevent the generation of the resonance circuit. There is an advantage that a flat frequency characteristic can be obtained. Further, by applying the impedance increasing/decreasing circuit of the present invention to other transmitting/receiving devices, it is possible to prevent the occurrence of a troublesome resonance circuit.

It is a circuit diagram which shows the impedance increasing/decreasing circuit which concerns on 1st Embodiment of this invention. It is a block diagram which connected the record player and preamplifier concerning one Embodiment of this invention. It is an equivalent circuit diagram which connected the record player and preamplifier concerning one embodiment of the present invention. It is a block diagram which connected the conventional record player and the preamplifier. It is a figure explaining the frequency characteristic at the time of connecting a conventional record player and a preamplifier. It is a circuit diagram which shows the conventional capacity|capacitance increase/decrease circuit.

By canceling out substantially unnecessary impedance, the influence on the frequency characteristic due to the formation of the resonance circuit is prevented, and the ideal frequency characteristic can be realized.

FIG. 1 is a circuit diagram showing an impedance increasing/decreasing circuit according to an embodiment of the present invention. Reference numeral 11 denotes a buffer amplifier composed of an operational amplifier, which has the same function as the conventional buffer amplifier 7 of FIG. doing. The output of the buffer amplifier 11 is connected to one terminal of the variable resistor 12, and the other terminal is grounded. The movable contact portion of the variable resistor 12 is connected to the non-inverting input terminal of the operational amplifier 13. The inverting input terminal of the operational amplifier 13 is connected to the output terminal via the feedback resistors 14 and 15 for adjusting the amplification factor. A capacitor 16 is connected between the input terminal of the buffer amplifier 11 and the output terminal of the operational amplifier 13.

Here, assuming that the resistors 14 and 15 have the same resistance value and the amplification factor of the operational amplifier 13 is 2, and the amplification factor of the buffer amplifier 11 is 1, by moving the movable contact of the variable resistor 12, the non-inversion of the operational amplifier 13 is performed. The voltage V applied to the input terminal changes, and the voltage applied to the terminal of the capacitor 16 connected to the output of the operational amplifier 13 changes from 0 to 2V. The input voltage V is always applied to the other terminal of the capacitor 16. As a result, the equivalent input capacitance of the impedance increasing/decreasing circuit changes between -C1 and C1 when the capacitance of the capacitor 16 is C1.

Specifically, when the movable contact of the variable resistor 12 is moved to set the resistance value between the output of the buffer amplifier 11 and the non-inverting input of the operational amplifier 13 to 0, the equivalent input capacitance becomes −C1. With a resistance value of /2, the equivalent input capacitance is 0, and when the resistance value is maximum (when the non-inverting input is grounded), it becomes C1. The capacitance of the capacitor 16 connected between the output of the operational amplifier 13 and the input of the buffer amplifier 11 is C1, the resistance divided by the movable contact of the variable resistor 12 is r1, r2, the operational amplifier 13 and the resistors 14 and 15 are connected. Assuming that the amplification factor of the circuit constituted by is G, the equivalent input capacitance C of the entire circuit can be calculated by the following equation (2). Here, r1 is a resistance value between the output of the buffer amplifier 11 and the movable contact, and r2 is a resistance value between the movable contact and the ground.

Generally, when the amplification factor of the circuit formed by the operational amplifier 13 is N, the equivalent input capacitance can be changed from C1 to C1(1-N). Further, instead of the capacitor 16, an impedance element such as a coil or a resistor can be used. In this case, assuming that the impedance of the impedance element is Z, the impedance can be similarly changed from Z to Z(1-N) by using the above-mentioned mathematical expression 2.

FIG. 2 is a configuration diagram in which a record player and a preamplifier according to an embodiment of the present invention are connected, and reference numerals 1, 2, 5, and 6 in the figure are the same as those in FIG. Reference numeral 17 denotes the impedance increasing/decreasing circuit according to the present invention described with reference to FIG. 1, which is built in the preamplifier 19 and connected to the front stage of the equalizer amplifier 5. A display device 18 is connected to the equalizer amplifier 5 and built in the preamplifier 19. The display device 18 is configured to measure the frequency characteristic of the signal output from the equalizer amplifier 5 and display the measurement result.

FIG. 3 shows an equivalent circuit of the entire structure shown in FIG. 2. Reference numeral 20 denotes a load of the MM cartridge provided in the player 1, which is composed of a coil and a resistor. The load of the equalizer amplifier 5 is composed of a resistor and a capacitor. All capacitors are connected in parallel, and the total capacitor capacity is the sum of the capacitor capacities. By adjusting the variable resistor 12 of the impedance increasing/decreasing circuit 17, it is possible to add a substantially negative (reverse polarity) capacitor capacity, which was not possible in the prior art, so that the entire capacity of the capacitor can be canceled. .. The variable resistor 12 can be adjusted by the user while observing the frequency characteristic output from the equalizer amplifier 5 displayed on the display device 18. Thereby, it is possible to easily adjust to a flat frequency characteristic in which the resonance circuit is not formed. The same effect can be obtained by adjusting the impedance increasing/decreasing circuit formed of a coil (inductance) instead of the capacitor.

In addition, the test player can reproduce the test record, display the left and right output levels of the preamplifier 19 on the display device 18, and the user can adjust the volume balance, or can adjust the inclination of the cartridge while observing this display. .. The distortion component can be measured by mounting the spectrum analyzer function on the display device 18.

The application example to the MM cartridge of the record player has been described above, but it goes without saying that the invention can also be applied to general transmitters and receivers connected by a cable or the like. Further, although the variable capacitance and cancel circuit has been described as an example, it is clear that the impedance such as the resistance component and the inductance component can be increased/decreased and canceled by using a similar circuit other than the capacitance.

Industrial applications

The impedance increasing/decreasing circuit of the present invention can be applied not only to those that transmit a signal relating to voice between audio devices, but also to devices in which it is essential to remove unnecessary impedance components.

1 record player 2 cable 3 preamplifier 4 capacitance adjustment circuit 5 equalizer amplifier 6 capacitor 7 buffer amplifier 8 variable resistor 9 operational amplifier 10 capacitor 11 buffer amplifier 12 variable resistor 13 operational amplifier 14 resistor 15 resistor 16 capacitor 17 impedance increasing/decreasing circuit 18 display Device 19 Preamplifier 20 Cartridge

Claims (3)

A preamplifier having an equalizer amplifier for performing RIAA correction on an output signal from a record player, wherein the output signal from the record player includes a first operational amplifier and a second operational amplifier, and the output of the first operational amplifier is a variable resistor. Connected to the non-inverting input terminal of the second operational amplifier, and the inverting input terminal of the second operational amplifier has a resistor having the other end connected to the output of the second operational amplifier and the other end grounded. A resistor is connected, and an impedance element is connected between the input of the first operational amplifier and the output of the second operational amplifier, so that the impedance of the impedance element can be increased or decreased by the variable resistor. A preamplifier connected to the equalizer amplifier via. The preamplifier according to claim 1, further comprising a display device that is connected to an output of the equalizer amplifier and displays a frequency characteristic whose impedance is adjusted by the variable resistor. A first operational amplifier and a second operational amplifier are provided between the transmitter and the receiver connected by a cable and on the side of the receiver, and the output of the first operational amplifier is the second resistor via a variable resistor. A resistor whose other end is connected to the output of the second operational amplifier and a resistor whose other end is grounded are connected to the non-inverting input terminal of the operational amplifier, and the inverting input terminal of the second operational amplifier is connected to the inverting input terminal of the second operational amplifier. A transmission circuit comprising an impedance adjusting circuit, wherein an impedance element is connected between the input of the first operational amplifier and the output of the second operational amplifier, and the impedance of the impedance element can be increased or decreased by the variable resistor.