Preamplifier with four inputs

Figure 3-25 is a preamplifier with four-way mixed input function. In the circuit, cZ1, cz, C23 and
cz4 can input microphone, line , crystal phono and magnetic head signals respectively. The four channels can be input individually, or the four channels can be
mixed input at the same time. Editing processing such as program dubbing, interjection, and commentary can also be performed as needed. Regardless of whether they are input individually or mixed
, they will not have any influence on each other.
    This preamplifier applies the principle of the inverting input proportional adder in the operational amplifier. When several signals are input at the same time
, the output voltage is equal to the sum of the input voltages. At the same time, when the inverting input is used, the summing point of the operational amplifier is virtual ground,
so the input signals are isolated from each other. When the input signal of a certain channel is 0, the voltage across the input resistor
is also 0, so it It will not affect the work of other channels.
    In order to adapt to the different input voltages of each signal source, the four input interfaces adopt different circuit structures and component values
. At the same time, more input terminals can be added as needed. The component values ​​in the circuit input interface are
calculated according to the input signal sensitivity marked in the figure. If the actual input signal is different from the value marked in the figure,
the resistance values ​​of R2, R, R6 or where can be appropriately adjusted. When C24 is used as the magnetic head input, Ra takes 1kfl, thus attenuating the treble and playing a frequency
balancing role at the treble end.
    The volume control potentiometer RPi, Rl1, and C form an automatic treble compensation circuit; RPi, Rt2, c9, and Rl3 form a
bass automatic compensation circuit. . When RPl adjusts the volume smaller, the compensation amount of the compensation circuit is larger. Therefore, when adjusting the volume
, there is no need to readjust the tone controller. ． SA is the compensation circuit control switch.
    The RC network forms a five-stage equalization circuit with an adjustment range of ±12dB.
    The sensitivity of czJ and C24 is extremely high. Do not mistakenly connect large amplitude signals such as lines to avoid damage to the amplifier due to overload
. When C24 is not in use, the debugging point J should be disconnected.
    In order to reduce induced hum and noise, (:zl~C24 and its input components should be shielded with metal covers and well
grounded. All input signal lines should be metal shielded wires, and their ends should be well grounded.
    Circuit It uses four operational amplifiers TL084 with good performance and good frequency response. For line input, the passband
bandwidth is about 20Hz ~ 20kHz, and the distortion is <0.5%o.