Preamplifier with audio controls

In Figure 3-7, Vr1 and VTz form a differential input circuit, VT3.VT4 form a composite tube voltage amplification stage, R
(300kfr,) is a negative feedback resistor, VTs~VT7 and the RC network form a feedback tone control circuit. The function of the control circuit
is rather special. The potentiometer RPz can only improve
or attenuate signals in the frequency range of 500 to 2000 Hz by 6 to 8 dB, because increasing the midrange component of this part can make the playback produce a sense of depth of the sound image, enhancing the Presence, so it can be
called a presence controller. Especially when listening to singing programs, it feels like you are immersed in the scene oRP3 is a subwoofer control potentiometer.
Different from ordinary bass control circuits, its starting control turning frequency is very low, about 100Hz. It is mainly used to compensate for
most non- sounding frequencies. The lack of bass in professional speakers makes the speakers with inherent deficiencies appear powerful, and it also makes
the bass of high-end speakers elastic.
    Behind the feedback tone control circuit is a set of universal RC attenuation tone control circuits, of which RP4 is
the bass control potentiometer. It cooperates with the subwoofer control potentiometer RP3 to well control the frequencies in the low and mid-frequency areas.
Characteristics: RPs is a treble control potentiometer. The usual treble and bass control circuit has insufficient uniformity of control characteristics, especially when
close to both ends of the potential. For this reason, this circuit is connected in parallel between the boom and the lower end of RP4 and the deputy respectively. A fixed resistor
to improve its shortcomings.
    RPt is the input volume control potentiometer. There is an output control potentiometer at the output end of the circuit, which is a
volume control potentiometer with equal loudness compensation. Setting the volume control potentiometer at the signal input terminal t
has a very good effect on improving the overload capability of the input stage circuit and improving the signal-to-noise ratio of the entire circuit output.剉, is the balance control potentiometer
    between the two channels when outputting two channels .     This circuit uses a positive and negative symmetrical power supply, which is beneficial to improving the circuit gain and stability, and also plays a certain role in improving the dynamic range of the circuit and reducing distortion.