Loosely compensated volume control with a variable resistor without taps. Encyclopedia of radio electronics and electrical engineering

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

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The author proposed a variant of a thinly compensated volume control on a variable resistor without taps, but with an inductor. The calculated values ​​of the regulator elements for different ranges of volume control are given in tabular form.

It is important to note that the frequency response of the regulator's transmission at different values ​​of the volume level must correspond to the curves of equal loudness for a particular listener. This can be achieved if there is or if a sensitivity regulator is introduced into the sound reproduction path, which brings the loudness level in line with subjective estimates.

In various sound-reproducing equipment, potentiometric thin-compensated volume controls (RG) on variable resistors with taps and a non-linear dependence of resistance on the angle of rotation (group B) are widely used. One of the disadvantages of using such resistors is their scarcity. Another drawback is the deviation of the actual frequency response of the loudness from the curves of equal loudness, which is especially large in the low-frequency and high-frequency regions of the AF spectrum and allows you to raise the relative levels in these regions by no more than 15 ... 20 dB. And the third drawback is the distortion of the shape of the frequency response, namely, the shift of the corrective rise towards the middle frequencies. The same is noted in [1].

The thinly compensated RG considered here on a variable resistor of group B without taps (the regulator circuit for one channel is shown in Fig. 1), with a significant signal attenuation in level, allows you to raise the extreme low and high frequencies by 30 ... 40 dB and bring the shape of the frequency response of the regulator closer to the curve equal loudness.

Rice. 1. Regulator circuit for one channel

Let's take sound pressure levels according to equal loudness curves according to GOST R ISO 226-2009 [2]. For the initial volume level, corresponding to the volume level of 20 phon at a frequency of 1 kHz and the lower position of the variable resistor R1 slider, set the value to 0 dB. Then, according to GOST, sound pressure levels (SPL) in the audio frequency band should correspond to those given in Table. 1.

Table 1

For measurements, a sinusoidal signal with a swing of 1 V was applied to the input of the regulator in the entire audio frequency band. Measurements were taken when changing the values ​​of the elements C1 and R2. The L1C3 circuit is tuned to resonance at a frequency of 20 kHz. As the inductance L1, a factory dumbbell coil with an inductance of 8,2 mH was used. The regulator was also tested with a coil of 80 turns of winding wire with a diameter of 0,25-0,41 mm, wound on a ferrite ring M2000NM, size K20x12x6. The measurement results are the same. You can use the M2000NM ring of size K10x6x3, the estimated number of turns is 115.

The results of measurements of the output voltage range U2 and the ratio of the output voltage to its value U1 at a frequency of 1 kHz, as well as the sound pressure levels at various values ​​of C1 and R2 are given in Table. 2-14.

Table 2

R1 \u22d 2 kOhm, R200 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 3

R1 \u22d 2 kOhm, R100 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 4

R1 \u47d 2 kOhm, R100 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 5

R1 \u22d 2 kOhm, R51 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 6

R1 \u22d 2 kOhm, R27 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMXuF

Table 7

R1 \u22d 2 kOhm, R0 \u1d 1 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 8

R1 \u22d 2 kOhm, R51 \u1d 1,5 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 9

R1 \u22d 2 kOhm, R27 \u1d 1,5 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 10

R1 \u22d 2 kOhm, R0 \u1d 1,5 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 11

R1 \u22d 2 kOhm, R51 \u1d 2 Ohm, CXNUMX \uXNUMXd XNUMX uF        

Table 12

R1 \u22d 2 kOhm, R27 \u1d 2 Ohm, CXNUMX \uXNUMXd XNUMX uF 

Table 13

R1 \u22d 2 kOhm, R0 \u1d 2 Ohm, CXNUMX \uXNUMXd XNUMX uF

Table 14

R1 \u22d 2 kOhm, R27 \u1d 2 Ohm, C1 \uXNUMXd XNUMX uF, the middle position of the variable resistor RXNUMX slider

For one of the RG variants with element ratings R1=22 kOhm, R2=0, C1=2 µF, the frequency response of the transmission was measured for different attenuation levels. The attenuation step of 10 dB at a frequency f = 1 kHz was determined by the position of the slider of the variable resistor R1. The results of attenuation measurements at different frequencies of the audio spectrum relative to the input signal are given in Table. 15. In this combination of elements, the rise at minimum volume was 40 dB at 20 Hz and 33 dB at 20 kHz. The volume control range at a frequency of 1 kHz was 46 dB. The corresponding frequency response curves of the RG are shown in the graphs of fig. 2.

Rice. 2. Frequency response curves of RG

Table 15

As a result of the analysis of the obtained data, the following conclusions can be drawn. The obtained forms of the frequency response of the RG are close to the curves of equal loudness. Smaller values ​​of resistor R2 shift the rise of high frequencies towards high frequencies and are more in line with equal loudness curves. In addition, large capacitance values ​​of the capacitor C1 (1,5 and 2 microfarads) and smaller values ​​​​of the resistance of the resistor R2 (27 ohms and 0 ohms - jumper) increase the frequency correction and expand the volume control range. In the volume control, you can use a variable resistor R1 of group B, for example, SPZ-12 or SPZ-Zob, and capacitors K73-17 (C1-C3).

Some disadvantage of this type of regulators is the reduction of the volume control range.

This RG can be built into a device (UMZCH and AC) that ensures that the sound pressure corresponds to equal loudness curves. If this is not provided, then, in addition to the RG, a sensitivity regulator should be included in the path, bringing the signal level to the nominal value so that the loudness corresponds to equal loudness curves at the appropriate sound pressure (loudness level). The volume control, the frequency response of which is shown in fig. 2 was built into an active speaker. Thanks to sufficient loudness, low and high frequencies are clearly audible even at minimum volume.

Literature

1. Fedichkin S. Loosely compensated volume control. - Radio, 1984, No. 9, p. 43, 44.
2. GOST R ISO 226-2009. Acoustics. Standard Equal Loudness Curves. - URL: protect.gost.ru/document.aspx?control=7&baseC=6&page=2&month=8& year=2010&search=&id=175579.

Author: B. Demchenko

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