JPS631514Y2 - - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a preamplifier circuit for a moving coil type cartridge.

In general, a moving coil (hereinafter referred to as MC) type cartridge has an output voltage of 0.1 to 0.5 mV, which is lower than the output voltage of a moving magnet (hereinafter referred to as MM) type cartridge, which is 1 to 5 mV. It is. Therefore, if an MC type cartridge is used by directly connecting it to a preamplifier for an MM type cartridge, the signal-to-noise ratio will be poor and it will be impractical, so a step-up transformer as in the conventional example shown in Fig.
I was using _T1 . In the figure, MC indicates an MC type cartridge, and _A1 indicates a preamplifier for an MM type cartridge.

However, in this case, the audio frequency band becomes narrow and the sound quality deteriorates, and the transformer is expensive.

The present invention has been made in view of these drawbacks, and it is an object of the present invention to provide a preamplifier circuit for MC type cartridges that uses a transistor in place of the above-mentioned transformer.

Figure 2 shows an example of the circuit that is the basis of the present invention. In the figure, Q ₁ is an NPN type transistor;
R ₁ is the load resistance, R ₂ is the feedback resistance, and R ₃ is the input resistance (load resistance for MC type cartridges).

In this way, the preamplifier circuit for the MC type cartridge is composed of R ₁ , R ₂ , R ₃ and Q ₁ , and within the audio frequency band, the reactance due to the internal inductance of the MC type cartridge is small compared to the internal resistance and can be ignored. Therefore, the voltage amplification degree A _v of this circuit within the above band is MC type cartridge MC
If the internal resistance of is R ₄ , then A _v = R ₂ / (R ₃ + R ₄ )
becomes.

The equivalent circuit on the input side of this circuit is shown in FIG. 3, where R _bb is the base spread resistance of the transistor Q ₁ and R _ee is the emitter junction resistance.

Here, the internal resistance of the MC type cartridge MC is R ₄
is typically 10-50Ω, and the load resistance _R3 is typically 47
Ω is used. Voltage amplification degree A _v of this circuit
may be about 10, so if R ₄ =30Ω and R ₃ =47Ω, the feedback resistance R ₂ will be about 770Ω.

Also, according to the thermal noise theorem, when considering the noise voltage of this circuit, with the above calculated value, R ₂ ≫ R ₃ + R ₄
Therefore, R ₂ can be ignored and the noise voltage is √4
(R ₄ +R ₃ +R _bb +R _ee ) (k is Boltzmann's constant, T is absolute temperature, and f is bandwidth). Here, R ₄ and R ₃ are fixed constants, and in order to reduce the noise voltage of this circuit, R _bb and R _ee must be made small. The emitter junction resistance R _ee is expressed as 26 mV/emitter current, and the resistance value can be reduced by increasing the emitter current, R ₄ +
Since it is small compared to R ₃ , it can be ignored.

However, the base spread resistance R _bb is an inherent value of the transistor, and in order to keep the increase in noise due to the influence of R _bb within the negligible 10% range, √ ₄ + ₃ + _bb /√ ₄ derived from the thermal noise theorem is used. ₊₃ <
Conditional expression 1.1 must be satisfied. Substituting the above value into this equation results in approximately R _bb <20Ω.

Therefore, the base spread resistance R _bb of the transistor Q ₁ to which the output of the MC type cartridge is first input is
It is sufficient to suppress the resistance to within 20Ω, and this value can be achieved by using medium-power or higher-power transistors.

Further, the cartridge MC is generally replaced with another type of MC cartridge depending on the user's hobby or deterioration over time. In such a case, the internal resistance value of the new MC cartridge will change to about 10 to 50Ω as described above, and in extreme cases, the internal resistance _R4 will change by about 1/5 or 5 times. On the other hand, since the voltage amplification degree A _v of the preamplifier circuit is given by A _v = R ₂ / (R ₃ + R ₄ ) as mentioned above, if there is no load resistance R ₃ of the MC cartridge, it will be 5 times However, by providing a load resistor R ₃ , it will change to about 1/5 from that of other models.
Changing to an MC cartridge has the effect of suppressing changes in the voltage amplification degree _Av of the amplifier circuit to a small extent.

Fig. 4 shows another circuit example that is the basis of the present invention, in which an emitter follower circuit consisting of a transistor _Q2 and a resistor _R5 is added to lower the output impedance of the circuit, and matching is performed. The base spread resistance of transistor _Q1 is within 20Ω, as in Fig. 2.

Based on the circuit described above, an embodiment of the present invention is shown in FIG. 5 and will be described. In FIG. 5, the same reference numerals as in FIG. 2 indicate the same or corresponding parts. Q ₃ is a second PNP transistor, and like the first NPN transistor Q ₁ , the base spreading resistance is within 20Ω, and the emitters of transistors Q ₁ and Q ₂ are connected to ground, so that a complementary Configure a push-pull type preamplifier. Capacitors C ₁ and C ₁ are output coupling capacitors connected to the collectors of each transistor Q ₁ and Q ₂ , respectively, and capacitors C ₂ and C ₂ are connected to the bases of each transistor Q ₁ and Q ₂ , respectively. This is an input coupling capacitor. In addition, R ₆ ,
R ₇ is a bias resistor for transistors Q ₁ and Q ₃ , respectively.

Next, the operation of the device shown in FIG. 5 will be explained. The signal picked up by the cartridge MC passes through the load resistor R ₃ and is applied to the base of the transistor Q 1 via the input capacitor C ₁ , while _{the signal is passed through the input capacitor C 1 to} the base of the transistor Q _2.
given on the basis of. These signals are applied to the bases _of the transistors Q ₁ and _{Q 3} via their respective capacitors C ₂ , and are amplified symmetrically by the transistors Q ₁ and Q ₃ . The signals are mixed and output via each capacitor _C1 .

When dealing with extremely small signals, such as in an amplifier for MC cartridges, if a current is passed through the ground, there will be problems such as hum noise mixed in due to the ground impedance, but this circuit uses a circuit with positive and negative symmetry, so that the voltage can be reduced from _Vcc . The current flowing through transistor Q ₁ almost passes through transistor Q ₃ and flows to −V _cc ,
The current flowing through the ground can be significantly reduced, thus preventing the occurrence of power supply hum.

As explained above, this invention has the features of high S/N ratio, low distortion, and wide band by using a transistor whose base spread resistance is within 20Ω of the transistor to which the output from the MC type cartridge is first input. It is possible to provide an inexpensive preamplifier circuit for MC type cartridges.

In addition, since a load resistor is provided for the MC cartridge, even if the MC cartridge is replaced with another model of MC cartridge, the preamplifier's voltage amplification A _v
changes can be suppressed to a small extent.

Furthermore, since the preamplifier is configured as a push-pull type, the influence of power supply hum can be prevented.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram of a conventional preamplifier circuit for a moving coil type cartridge using a step-up transformer _T1 , and Fig. 2 is a circuit diagram showing an example of a preamplifier circuit for a moving coil type cartridge, which is the basis of the present invention. Figure 3 is an equivalent circuit diagram of the input side in Figure 2, Figure 4 is a circuit diagram that is another basis of the present invention,
FIG. 5 shows an embodiment of the preamplifier circuit according to the present invention. MC……Moving coil type cartridge, A ₁
……Preamplifier for moving magnet type cartridge, Q ₁ to _{Q 3} ……Transistor, R _bb ……Base spread resistance of transistor, R _ee ……Emitter junction resistance of transistor, R ₁ to _{R 6} ……Resistance, C ₁
~C ₂ ... Capacitor.

Claims (1)

[Claims for Utility Model Registration] (1) A preamplifier circuit for a moving coil type cartridge having a feedback resistor, comprising first and second transistors constituting a push-pull amplifier circuit, is input to the base of each of the transistors, and the base spread resistance of each transistor is within 20Ω, and a load resistor having a low resistance value that serves as a load of the cartridge is provided between the output of the cartridge and the base input of the transistor. A preamplifier circuit for a moving coil type cartridge, characterized in that it is provided with a. (2) Utility model registration claim 1, characterized in that the load resistance is configured with a resistance value approximately equal to the internal resistance value of the moving coil type cartridge.
A preamplifier circuit for a moving coil type cartridge as described in 2.