Loudly compensated volume control. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Loudly compensated volume control. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Tone, volume controls

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The article proposes a description of a simple device capable of correcting the amplitude-frequency characteristic of signal transmission in accordance with the characteristics of human hearing at different volumes of sound reproduction. Recall that the human ear has unequal sensitivity when listening to different frequencies of the sound range - higher at medium frequencies (500 ... 7000 Hz) compared to high (10000 ... 18000 Hz) and especially low (20 ... 100 Hz) , therefore, the same level of acoustic impact at different frequencies is provided by different signal voltage levels. This physical phenomenon is reflected in the so-called Fletcher and Munson Equal Loudness Curves.

Modern high-fidelity audio devices of industrial and amateur manufacture usually strive to do with a frequency response that is linear in a wide frequency band, which does not fully correspond to the physiological foundations of hearing, but allows you to design a universal device in which each individual listener can set the required ratio of playback levels at different frequencies . Such a setting can be made with the volume control and frequency response correctors (tone controls or equalizer). There is, however, a certain inconvenience in this - not everyone will be able to do it correctly, since you have to manipulate several controls at the same time (depending on the design of the amplifier - from two to twenty), when you change the volume level, you again have to select the desired ratio.

Loudly compensated volume control
Fig. 1

A more convenient way out is to create a thinly compensated volume control, in which the necessary levels of frequency response correction depending on the playback volume are determined by the circuit design. A diagram of such a regulator, which has proven itself in operation, is shown in the figure. Compensation elements are included between two operational amplifiers (op-amps) connected as non-inverting followers. Adjustment is made by a variable resistor R4. When its engine is in the leftmost position according to the diagram, the signal from the output of the op-amp DA1.1 is transmitted directly to the input of the op-amp DA1.2. In this case, the gain is equal to 1 and the frequency response is uniform in the range from 10 Hz to 100 kHz.

With the extreme right position of the variable resistor slider, the transfer coefficient at the lowest frequencies is determined by the series-connected resistors R4 - R6. As the frequency increases, capacitors C3 - C5 come into operation, and then the R3 C2 chain. The minimum transmission coefficient of the device is in the range of 500...1500 Hz. When operating at frequencies above 5000 Hz, the action of the R2 C1 circuit begins to affect and the transmission coefficient increases with increasing frequency.

The depth of suppression of frequency components depends on the position of the variable resistor R4 - it is maximum in the extreme right position. In a real working device, the following values were obtained (the frequency response level at a frequency of 1500 Hz was taken as 0 dB): 20 Hz - +24 dB, 100 Hz - + 17 dB, 200 Hz - + 12 dB, 500 Hz - + 4 dB, 5 kHz - + 8 dB , 10 kHz - + 10dB.

When performing a design, it is necessary to take a resistor with an inverse logarithmic characteristic of resistance change (group B) as a variable resistor, and variable resistors with the smallest possible spread of resistance for stereo designs. Capacitors C3 - C5 are best taken non-polar. If this is not possible, then oxide ones can be used, but types of capacitors with minimal leakage currents should be chosen.

The device is powered from a bipolar current source with a voltage of +15 and -15 V (the minimum allowable is +12 and -12 V). The current consumption does not exceed 50 mA for a stereo design. The conclusions of the microcircuits connected to the power circuits should be decoupled with capacitors with a capacitance of at least 0,1 μF.

The device operates in a wide range of input signal voltages - from a few millivolts to 2 V.

According to the materials of the magazine "Radioelektronik", 12/1993, p.3

PS Instead of the microcircuits recommended in the circuit, you can use almost any domestic operational amplifiers. Capacitors C1 and C2 are ceramic KM-4, as C3 and C4 it is better to use polyethylene terephthalate capacitors K73-11, K73-16, lacquer film K76 of any variety, polycarbonate K77-1, C5 - any non-polar oxide. All fixed resistors can be selected with a power dissipation of 0,125W.

Publication: cxem.net

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