Theory: volume and tone control. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur
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The volume control in the UZCH, as a rule, is a conventional variable resistor. However, in high-quality equipment, it is desirable to take into account the psychophysical characteristics of human hearing - its sensitivity drops at lower and higher frequencies at low volume. This is represented by "equal loudness curves" (Fig. 40), showing what signal level is needed at different frequencies for the same subjective sensation of loudness. They correspond to the frequency response of a "thinly compensated" volume control, which does not reduce the "juiciness" of the sound when the volume is reduced.
A diagram of a simple regulator, which was used in the VEF electro-acoustic unit, is shown in fig. 41.
The input signal from various sources is fed through the X1-X1 connectors to dividers from resistors R5-R8, R0, providing approximately the same maximum amplitudes on the resistor R3 (this is the volume control). From the engine of this resistor, the signal is fed to the XNUMXH preamplifier.
The variable resistor has two taps, to which the correcting chains R6C2 and R7C3 are connected. In the upper position of the resistor slider according to the scheme (maximum volume), they have almost no effect, the capacitor C1 is closed, the frequency response is even. As the slider is moved down, the influence of the chains increases: capacitors C2 and C3 shunt the higher and middle frequencies of the audio spectrum, creating a relative boost in the lower frequencies. At the same time, the role of capacitor C1 is also enhanced, creating a "bypass" for higher frequencies and their corresponding rise. As a result, the frequency response of the regulator approximately corresponds to equal loudness curves.
Tone controls usually come down to attenuating or raising the higher and lower frequencies of the sound spectrum, in accordance with the nature of the transmitted program and the tastes of the listener. The controller circuit (of the same "VEF" unit) is shown in fig. 42, and its frequency response - in fig. 43.
The variable resistor R2 regulates the level of low frequencies (LF), while the middle and high frequencies are closed by capacitors C2, C3 and their level remains approximately constant. The C4R5C5 chain regulates the higher frequencies (HF), while the currents of medium and lower frequencies almost do not flow through this chain due to the capacitance of capacitors C4 and C5 increasing at these frequencies. Resistor R4 weakens the mutual influence of the regulators. The ratings of the parts depend on the output impedance of the preamplifier and the input impedance of the terminal.
It must be kept in mind that the described passive tone control introduces a noticeable overall signal attenuation of about 10 times, which is compensated by a corresponding increase in the UZCH gain. A regulator that practically does not attenuate the signal is obtained by including adjustment elements in the OOS circuit of the amplifier stage, which is quite common in ultrasonic circuitry.
Author: V.Polyakov, Moscow
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