Tone circuit with variable corner frequency

It is an example of an attenuation-feedback tone control circuit
that can change the corner frequency through a selector switch . RP2 is used for treble control, RP3 is used for bass control, and SA1 and SA2 are used to change the turning frequencies of treble and bass control respectively
.
    In the circuit, VTi acts as common-emitter amplification, VT2 acts as emitter input, and the output signal of the amplifier has the opposite phase
to the input signal . This not only meets the phase requirements of the feedback tone control circuit , but also makes the circuit have low output impedance. Since
the input impedance of VT2 is high, the collector load resistor of VTi can be selected with a higher resistance value (100kf7,), so that the
two-stage amplifier has a high voltage gain, which is very important for the tone control circuit . If there is a margin, this circuit
divides the signal fed back to the tone circuit through the voltage dividing circuit R&, and Rio, so that the tone control circuit obtains about 3 times the mid-frequency
voltage gain.
    The high and low frequency control frequencies of the tone control can be controlled by SAi and Feng also choose different capacitors for adjustment. When SAi
is placed at d, the starting frequency of the treble control is 2.5kHz; when it is placed at b, it becomes 5kHz. When SA2 is placed at c,
the starting frequency of the bass control is 500Hz; when it is placed at d, it becomes 250№.