Loudly compensated volume control-2. Encyclopedia of radio electronics and electrical engineering

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

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In the process of development by the author of a volume control for a high-quality stereo amplifier and acquaintance with a number of publications on this topic, it turned out that some of the provisions of the articles published in the Radio magazine and other amateur radio literature are quite controversial. So, for example, it is widely believed that a high-quality volume control cannot be built on variable resistors, since their characteristics vary widely from instance to instance and the resistances change during operation. However, these shortcomings can be eliminated by weakening the influence of non-identity and instability of the parameters of variable resistors on the characteristics of the volume control by shunting each section of the resistor between adjacent taps with a constant resistor with a significantly lower resistance.

There is also a point of view that the volume control range of a high-quality amplifier should be 60...70 phon. But in this case, the volume level at the minimum position of the control is about 30 phon and most of the dynamic range of a high-quality music program is in the inaudible region (we mean signal sources with a dynamic range of at least 80 dB, for example, a CD player or Hi-Fi record player).

Based on his experience, the author considers the optimal volume control range to be 50 phon, i.e. at the maximum volume level of 95 ... 100 phon, the minimum should be 45 ... 50 phon, which is convenient for listening at night in the absence of domestic noise and does not interfere with those sleeping in the next room.

Further, all published control circuits have a horizontal part of the frequency response only at the maximum volume position. But for correct sound reproduction, it is necessary that the frequency response be horizontal at the volume level at which the sound engineer balances the program musically. And this level does not necessarily have to be the maximum.

And finally, the statement contained in the article by P. Zuev "Volume control with distributed frequency correction" (see "Radio", 1986, No. 8, p. 49-51) that the required rise in the frequency response in the region of lower sound frequencies is directly proportional to the the signal introduced by the volume control at medium frequencies, according to the author, is incorrect. The control, assembled in accordance with the recommendations of this article, gives a feeling of an excess of low frequencies at almost all but the highest volume levels. If we analyze the shape of the curves of equal loudness, we can see that at high levels, the rise in the frequency response at lower frequencies with a decrease in the gain of the controller should be less than indicated in the named article.

Taking into account the above considerations, the author calculated, built and tested the volume control for a high-quality stereo amplifier. A schematic diagram of one of the channels of the regulator is shown in fig. 1, and its frequency response - in Fig. 2.

The calculation and adjustment of the values ​​of the elements were carried out on a personal computer. The volume control was calculated for a sound reproducing complex that provides a maximum sound pressure in the listening room of 96 ... 100 dB. It was assumed that the level of musical balancing lies within 86...90 von. The peculiarity of this regulator is the attenuation of the signal at the maximum volume position by 12 dB (in the author's version from 775 to 200 mV). The input impedance module is at least 11 kΩ, the permissible load resistance is at least 1 MΩ. The deviation of the actual frequency response of the regulator from the equal loudness curves increases as the signal is attenuated. The maximum deviation is observed at a frequency of 31,5 Hz at volume levels close to the minimum and is equal to 4 dB.

When assembling the regulator, it is necessary to select resistors R7, R8 and R9 of such resistance that when they are connected in parallel to the corresponding sections of the variable resistor R12, their total resistances are equal to those indicated in brackets in the diagram.

The author used single variable resistors SPZ-23 (engine stroke 60 mm) with taps located at a distance of 25 and 40 mm from the minimum volume position. The resistance of the variable resistor can be in the range of 82 ... 120 kOhm, group A or B. As high-capacity capacitors (C1, C5, C7), you can use, for example, capacitors K73 or K76 with a maximum allowable voltage of 25 ... 50 V , capacitors of small capacity, any paper or ceramic (thermostable groups) or, in extreme cases, NZO groups.

The resistances of the resistors and capacitances of the capacitors used in different channels of the volume control must differ from each other by no more than 2%, and from the ratings indicated in the diagram, by no more than 5%. Subjective tests of the volume control have shown that the timbre coloring of the sound of various musical programs is preserved when the volume level is changed throughout the entire range without the use of any tone control.

Publication: cxem.net

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