Hi-Fi and volume control. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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I'll start, perhaps, with a quote: "The task of regulating the signal level - in other words, "loudness" - is one of the most difficult problems in the circuitry of audio equipment" [1]. Here the author, greatly simplifying the problem, equates such concepts as "signal level" and "loudness", and then describes his level control. The signal level is a concept from the field of circuitry for amplifiers of audio (and not only) frequencies. The terms "level control" or "gain control" are used here. And loudness is a concept from the field of physiological acoustics, where "loudness", "loudness level", etc. are in use [2].

The concept of "loudness" is much more complicated than the term "signal level" used by audio engineers and sound engineers and denoting the amount of voltage (in volts or decibels) at different points in the sound amplifying path. Level controls, unlike volume controls, are frequency independent devices. There is even such a thing as a "thinly compensated volume control" (smells like a tautology!), Denoting a control that takes into account the properties of hearing. It is worth mentioning the term "physiological volume control", similar to the one just named. Undoubtedly, the volume controls in Hi-Fi equipment are, as a rule, thinly compensated, or physiological. We will not consider the equipment of the "high end" (Hi-End), since any whims of snobs are fulfilled there for a lot of money. Luxury is a must!

It is known that the sensitivity of the human ear depends on the frequency [3], and therefore the equally perceived sound volume at different frequencies corresponds to different sound pressure levels. Graphically, this dependence is illustrated by "curves of equal loudness" (Fig. 1). To ensure high quality reproduction of a particular sound program, it is necessary, focusing on equal loudness curves, to compensate for the corresponding differences in hearing sensitivity. This task is designed to perform thinly compensated volume controls [2].

However, designing such a regulator is far from easy. The point is that the shape of the curves of equal loudness is ambiguous. It depends on a number of factors, in particular, on the acoustic properties of the listening room, on the presence of masking noises, on the characteristics of the listener's hearing, etc. As a result, the tone of the compensated volume control, which is necessary in one case or another, also turns out to be ambiguous. And yet, according to listeners, good results can be obtained if we use standard curves of equal loudness of pure tones for a plane sound wave. But they need to be adjusted, guided by the considerations below.

When listening to music programs, the volume level usually does not exceed 90 phon and can be reduced by the listener to the threshold of hearing or to the level of noise in the room. For definiteness, we take the range of volume control at frequencies of 1...2 kHz equal to 80 dB. We will assume that the frequency response of the regulator is linear, and the musical program is balanced in terms of timbre in the position of the regulator corresponding to the maximum volume (80 phon). The transition from this volume level to another, for example, 60 phon, requires correction of the frequency response of the regulator.

To obtain from the corrected dependence in Fig. 1, we draw a horizontal line through the division of 80 dB on the L axis (shown by a dotted line). Then we measure the distances from this straight line to several points lying on the curve of equal loudness 80 von. Further, these distances are laid down from the corresponding points on the curve of equal loudness 60 von. Through the new coordinates obtained in this way, we draw a curve that will be the adjusted frequency response of the regulator in a position corresponding to a volume level of 60 phon.

Similarly, relative to the curve of equal loudness 80 phon. the corrected frequency responses are constructed at volume levels of 40 and 20 (0) background, and the family of frequency responses of the volume control required for correct loudness is obtained. In the 3 dB volume range, it is shown in Fig. 80 (solid thick lines).

Now it is necessary to build a thinly compensated volume control whose frequency response family approaches the required one in the best possible way. In the frequency range below 2 kHz, the curve corresponding to the minimum gain can be approximated by the frequency response of an RC circuit. shown in Fig.3a. This characteristic to the left of the inflection frequency f1 (Fig. 3b) has a slope of 6 dB per octave. If the resistor R2 of this circuit is made variable, and its minimum resistance is chosen much less than R1. then when adjusting the resistance R2, along with changing the transmission coefficient of the circuit, the frequency of the inflection of its frequency response will also change. As can be seen from Fig. 2, taking into account the approximation within 3 dB, the inflection frequency must move along the LV line during regulation in order to provide the desired loudness. The range of change of resistance R2 in this case cannot be more than 100, since fa / fv<100. On the other hand, the gain Kp of the regulator at a frequency of 2 kHz, as can be seen from Fig. 2 and as mentioned earlier, should change by 80 dB (by a factor of 10000). The resistance R2 should change by the same amount.

It is quite obvious that by changing the resistance of only one resistor R2, it will not be possible to achieve such a shift in the inflection frequency and change in the transfer coefficient. However, by increasing the number of series-connected RC circuits and at the same time reducing the adjustment limits of the resistor R2 in each of them. this problem can be solved. Already two such RC circuits (the time constant of the second circuit should be 20...40 times greater than the first one) make it possible to obtain a quite acceptable result: the deviation of the curves of the real frequency response family (dashed lines in Fig. 2) from the required one (solid line) does not exceed 3 dB.

At frequencies above 2 kHz, a decrease in volume from 80 to 60 phon is accompanied by the appearance of an inflection on the 60 phon curve at a frequency of 5 kHz with a slope of 3 dB per octave. With a further decrease in volume down to the threshold of auditory sensation (level 3 background), the inflection frequency shifts from 5 to 3 kHz, while the slope of the curves practically does not change. In this frequency range, curve 3 background can be approximated by the frequency response of the RC circuit shown in Fig. 4a. The values ​​of the resistors R1 and R2 are the same as in the RC circuit. shown in Fig.3a. A change in the resistance R2 does not lead to a shift in the inflection frequency f2 (Fig. 4b).

In order to increase the volume from 60 to 80 phon is not accompanied by a rise in higher audio frequencies, the RC circuit must provide frequency compensation at the maximum transmission coefficient, which can be achieved by shunting the resistor R2 with a capacitor C2 of such a capacity that the time constants T2 = R1C1 and x3 would be equal =R2-C2. In this case, the decrease in resistance R2, necessary for volume control, will be accompanied by a decrease in the time constant T3 and a shift in the cutoff frequency of the RC circuit (f3=1/2nR2-C2) to a higher frequency region, while the inflection frequency f2 will remain unchanged, which will ensure the required correspondence Frequency response of the RC circuit with equal loudness curves in the frequency range above 2 kHz.

An example of the practical implementation of a thinly compensated volume control is shown in Fig. 5 [4, 5]. The resistances of the resistors and capacitors included in it can be calculated using the following relationships:

• R1=R3=R:
• R4min=R5min=0.01R;
• R4max=R5max=10R;
• R1C2=R3C3=20MKC;
• R4minC4=4000 µs;
• R5minC5=100 µs;
• R5maxC6=20 µs. Resistance R can be selected within 103..106 Ohm. In Fig. 5 R=510 kOhm. R5minC5=2000 µs (4000); R4minC4=100 µs.

To avoid shunting the R5-C5 circuit. the AF amplifier connected to the output of the regulator must have a large input impedance and a small input capacitance. It, in particular, can be performed according to the voltage follower circuit on the op-amp with field-effect transistors at the input. The output impedance of the amplifier connected before the regulator must be 20 times less than the resistance R2. The variable resistors of the thinly compensated volume control must be doubled. In our case, their functions are performed by photoresistors R4, R5, and the resistor R10 serves as an adjustment organ. changing current through an incandescent lamp HL1. The photoresistors SFZ-1 used in the volume control have high speed (time constant - less than 0,06 s) and the necessary range of resistance change. Incandescent lamp (subminiature) - NSM (6,3 Vx20 mA). the current through it varies within 6 ... 18 mA. Photoresistors are placed close to the incandescent lamp, and the entire regulator is placed in an opaque metal screen.

Figure 5 shows a two-channel control for a stereo amplifier. In it, it is necessary to select photoresistors in pairs in different channels so that when changing in the range from 104 to 106 Ohms, their resistances differ by no more than 20%. Otherwise, channel imbalance will be noticeable when the volume is changed.

Stereo balance is adjusted by resistor R9 within ±6 dB. Capacitors C7, CB eliminate rustles and crackles created by variable resistors.

The variable resistor R10 must have a linear regulation characteristic. Fixed resistors - with a resistance deviation from the nominal value of not more than ± 5%. Capacitors C1. C4, C5 - paper MBM, the rest - ceramic. The capacitance of capacitor C6 depends on the capacitance of the installation and the input capacitance of the amplifier connected to the output of the volume control. Incandescent lamps must be powered by a stabilized power source.

Adjusting the regulator comes down to ensuring the linearity of the frequency response at Kn = 0 dB (by selecting C6) and checking the identity of its frequency response family in different channels of the stereo amplifier at different volume levels.

Another example of a regulator is shown in Figure 6. It uses dual variable resistors with a linear dependence of resistance on the angle of rotation of the axis (group "A"). For a stereo regulator, you need to use two dual variable resistors. Such a solution does not cause any particular problems with adjusting the balance, if volume level scales are applied to the panel where both resistors are installed.

The attempt to use a quad resistor runs into great difficulties; firstly, it is a very rare "bird" in our area, secondly, its resistors have large variations in resistance, and thirdly, a balance regulator is additionally required, which does not simplify the entire design. The spreads in the resistances of the dual resistors are quite acceptable for this circuit. If the dual resistors have a different resistance, then the capacitances of the capacitors must be recalculated according to the given ratios. Resistors R3 and R5 serve to stop the rise of low frequencies outside the audio range.

With the sliders of variable resistors in the upper position, the gain of the regulator is -6 dB. The adjustment range at a frequency of 2 kHz is 80 ... 85 dB. Deviation from the required AMX - no more than ±2 dB. if the load resistance of the regulator is greater than 1 MΩ, and the load capacitance is less than 50 pF. Capacitors C1. C3. C5 - film, the rest - mica. Adjustment of the regulator - yes, no adjustment!

And finally, I will say that if you listen only to loud music, then it is enough to have a level control with a control range of 10 ... 15 dB. But if you want to feel the charm of quiet music, as if coming from the nearest park, then build this volume control, you will not regret it!

Literature

1. A. Nikitin. Volume control in Hi-Fi equipment. - Radiohobby, 2002. No. 2, p.63.
2. Terekhov P. About volume control. - Radio, 1982, No. 9, S. 42.
3. Zwicker E. Feldkeller R. Ear as a receiver of information. - M.: Communication. 1971.
4. I. Pugachev. Loudly compensated volume control. - Radio, 1988. N911.C.35.
5. Author's certificate of the USSR No. 1390776. - Bulletin "Discoveries, inventions...". 1988, No. 15.

Author: I. Pugachev, Minsk

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