Variable resistor loudness volume control without taps. Loudness volume controllers Calculation of volume elements on a resistor with taps

The peculiarities of our hearing are such that when the volume is lowered, we begin to hear the edges of the audio range worse and worse, i.e. high and low frequencies. If everything is not so bad with high frequencies, then at low frequencies with a decrease in volume, a rather significant increase is required. To solve this problem, a loudness-compensated volume control is used.

To prove this, the following figure shows the curves of equal loudness of the human ear:

The above-mentioned loudness control, simultaneously with changing the volume, also changes the shape of the frequency response so that the timbre of the sound is weakly dependent on the volume level. In order for the loudness to be correct, and the volume change to be uniform, it is necessary that a certain position of the knob create an appropriate volume level at the listening point. So, when the volume control is set to the maximum volume position at the listening point, a volume level of 90 phon should be obtained.

Simple loudness volume controls create a relative boost in the low frequencies, which is greater the lower the volume. There are also more complex circuits, with and without use of active elements(transistors, op amps) that create a relative boost in both low and high audio frequencies.

Loudspeaker volume control on a resistor with additional taps

The simplicity of this circuit is compensated by the problem of finding a variable resistor of group B with two taps.

If you managed to find the required resistor, then based on the value of the resistance of this resistor, you can calculate the remaining elements:

• R3 = R / 1.2
• R1 = R2 = 0.1 R3
• R4 = 0.11 R1
• R5 = 0.125 R1
• C1 = 4 / R1
• C2 = 3.9 / R1
• Where R- resistance of a variable resistor, kOhm
• R1, R2, R3- resistance of sections of a variable resistor, kOhm
• R4, R5- resistance of resistors of correcting chains, kOhm
• C1, C2- capacitance of correcting circuits, μF

This is how one of the variants of a variable resistor with domestically produced taps looks like:

Loudness resistor-based volume control without additional taps

Such a regulator can also be assembled on a variable resistor available to each without additional bends... The diagram of such a regulator is shown in the following figure.

The use of a resistor without taps leads to the need for additional parts, but this does not greatly complicate the circuit.

Both of these schemes realize a relative rise only in the low sound frequency region. It is relative because the absence of active elements does not allow for a rise in excess of the original signal; instead, the rest of the signal is attenuated. This principle is at the heart of any passive audio filter.

The second circuit was assembled and tested. The elements of the correcting circuits were soldered directly to the terminals of the double variable resistor. It is better to install such passive regulators after the preamplifier stage and before the output stages.

Listening in different conditions demonstrated the effectiveness of this scheme, and its application was sufficient for use at home at low volume levels. Loudness volume control maintains the tonal balance of the recording without clipping at low frequencies

Instead of a conclusion ...

I would like to add that the endless debate on audiophile forums about the correctness / incorrectness of the use of fine-correcting circuits often runs counter to the general ideology of Hi-End, the essence of which, first of all, is musical reproduction as close as possible to reality, in which deviations from original.

For the correct perception of the music program, it is necessary to create when playing, which your neighbors will clearly not be happy with. So the loudness control can be perceived as a good compromise for maintaining the correct timbre of music at home.

Regulators are devices for changing a parameter or characteristics of a block, node, device, installation. The regulation process can be carried out either manually by the operator, or automatically according to a predetermined a certain program; accordingly, the regulators are called either manual or automatic.

Regulation can be both smooth, continuous, and abrupt, step, discrete, therefore, all kinds of switches of electrical characteristics should be classified as regulators.

In ULF, the most common are the volume control, treble and bass tone controls, speech-to-music tone switches, as well as multi-range tone registers, which we will talk about more specifically. Stereo amplifiers have an additional stereo balance control.

Regardless of the purpose and functions performed, all regulators are characterized by several indicators common to all of them. The most important of them is the adjustment range, which in various literature has a variety of names: regulation limits, overlap coefficient, range of values ​​and a number of others.

This parameter shows from what minimum and to what maximum value the controlled value changes when you rotate the knob (or press buttons, pedals, etc.) from one fixed end position to another. It makes sense to dwell on this parameter, since for different regulators in Hi-Fi amplifiers, the regulation limits must be selected in different ways.

For volume controls, it is desirable to have a control range of about 60 dB, however, the design of most conventional potentiometers does not provide such a range. This is explained by the presence of the so-called "zero jump", that is, an abrupt transition of the potentiometer slider from the mastic shoe to the metallized part of the bow. As a result, the volume when the control axis rotates at first monotonously and smoothly decreases, and then at some point the sound immediately disappears.

This does not allow making the volume as low as desired, and sometimes the minimum achievable volume turns out to be too high. The following simple example illustrates what has been said: let the crazy output power of the amplifier P out max = 20 W, and the volume control has a control range of 40 dB. Note that in practice this case is not uncommon and many potentiometers have an even smaller range.

Then this regulator can make it possible to obtain a minimum output voltage 100 times less than the maximum, which corresponds to a decrease in the output power by 100 2 times, i.e., 10 4 times. This means that the minimum achievable loudness will correspond to an input power of 20 W: 10 4 = 2 · 10 -3 W = 2 mW. Let us recall for comparison that the maximum undistorted output power of the industrial transistor receiver "Surprise" is only 50 mW, the receiver "Kosmos" - 30 mW, and such comparatively large ones as "Falcon", "Jupiter", "Signal", "Neiva" 60 mw.

Therefore, in order to ensure a smooth decrease in the volume in Hi-Fi amplifiers to vanishingly small values, it is necessary to select the type and select an instance of a potentiometer that has an adjustment range of at least 60 dB.

Such selection can be done with a multiscale ohmmeter, which allows you to confidently read the ohm units. Select a potentiometer with a minimum value of the jump resistance from the "zero" side, that is, when the axis is rotated counterclockwise.

For tone controls that regulate the characteristic by ± 20 dB, it is quite enough to have a control range of 40 dB at the potentiometer. For a stereo balance regulator, a range of 40 dB turns out to be excessive, therefore limiting resistors are usually provided in circuits.

Next the most important parameter any regulator - the nature or curve of the controlled value change. For potentiometers in household broadcasting equipment, three types (laws) of change in the resistance value during the rotation of the axis are adopted: linear, denoted by the letter "A", exponential (letter "B" on the body) and inverse logarithmic (letter "B").

For volume controllers, only potentiometers with an inverse logarithmic law of resistance change (curve "B") are used, for tone controls - linear and sometimes (in special cases) - logarithmic. In the stereo balance controllers, only linear controllers (with the letter "A") are used.

Variable capacitors are usually produced either direct-capacitive (with a linear character of capacitance change) or direct-frequency. When choosing one or another type of characteristic in each specific case proceed from the purpose of the regulator.

Finally, it is important that the regulating element itself does not introduce nonlinear and frequency distortions, and also has a noise level at least 10-20 dB below the minimum signal level at the point where the regulator is switched on.

The requirements for the mechanical rigidity of the moving system, which exclude the appearance of a microphone effect, and the absence of spark discharges during the rotation of the axis, are imposed on variable capacitors. The last requirement practically eliminates the possibility of using variable capacitors with a solid dielectric in Hi-Fi amplifiers.

Having understood the above, let's move on to considering specific regulator circuits used in ULF.

1. Volume controls. The main difference between the volume controls of Hi-Fi amplifiers from conventional ones is the increased requirements for the character of loudness compensation. We have already agreed in chap. 1 introduce the quantitative characteristics of this parameter. Now let's see how you can ensure that these requirements are met.

In order for the volume control by ear to be not frequency-dependent, that is, so that the listener, when adjusting the volume, does not feel at the same time a change in the timbre of the sound, it is necessary to change the frequency response of the amplifier automatically and in a quite definite way when the volume is changed: when the volume is lowered frequency response at the lowest and highest frequencies, it should acquire a rise relative to the middle frequencies, moreover, the greater, the lower the volume. This is done in order to compensate for the decrease in ear sensitivity at low and high frequencies at low volumes.

All loudness compensation schemes using potentiometers with one or more taps, by their principle, do not allow obtaining the required characteristics, since the method is based on the fact that with decreasing loudness, a progressive attenuation of the higher frequency components occurs, which, as the knob is turned to the left, covers an ever wider part of the spectrum. towards low frequencies.

Adding all kinds of "short-circuiting" and "correcting" capacitors of small capacity to the circuit does not change the position, since the degree of such "short-circuiting" is constant and does not change when the volume control is turned, at the same time reducing the overall efficiency of loudness compensation.

The author at one time proposed a method for implementing Effective loudness compensation on conventional potentiometers without taps, which gives a very good approximation to curves of equal loudness. Various modifications of such circuits have been used for a number of years in various ULFs and have fully justified themselves. However, over the years, the requirements for the nature of loudness also grew, due to which the schemes were also constantly improved. Today, you can offer radio amateurs two options for such schemes: Fig. 38 for Hi-Fi amplifiers of the "standard" class and fig. 39 - for "extra-class" amplifiers.

Rice. 39. Scheme of a loud-compensated volume control on a double potentiometer for "extra-class" amplifiers

Both of them work on the principle of smooth introduction into the low-frequency signal path in the process of decreasing the volume of an incomplete double T-shaped filter, the frequency response of which is formed by the selection of its elements for the minimum signal level.

With the values ​​of the elements indicated in the diagram, the regulators in " pure form"(ie, not in the amplifier circuit) have the frequency characteristics shown in Fig. 40.

It should be noted that although both circuits have excellent frequency characteristics (especially the second), their inclusion in a specific amplifier with its own negative circuits feedback inevitably somehow changes the character of loudness, and this most often leads to some deficiency in the spectrum of the reproduced signal of the lowest frequencies (moreover, only at the smallest volume levels). Therefore, the author proposes to install a conventional toggle switch directly on the volume control knob that works independently of the axis rotation, for example, by pressing the control knob, or simply install the toggle switch next to the volume control. Electrically, this toggle switch includes an additional large capacitance in the cathode circuit of the lamp of the 1st stage of the ULF, increasing the relative gain at frequencies of 20-60 Hz (Fig. 41).

Let us note in passing that in many of the most expensive models of foreign amplifiers and electrophones there are devices of a similar purpose (firms "Dual", "Ampex", etc.), although they are usually solved differently in schematic form.

Once again, we remind you that regardless of the complexity and nature of the loudness circuit, there should be only one point of attachment to the case (chassis) of all its elements, and moreover only in the place where the grid leakage and automatic bias resistors of the ULF input lamp are connected to the case.

All elements of the loudness compensation circuit must be carefully shielded from electrostatic and electromagnetic interference.

2. The tone controls have achieved significant perfection in recent years, and the circuits of some of them, for example, shown in Fig. 42 have already become "classic". And yet, despite good characteristics regulation and slight mutual influence, these circuits are not entirely suitable for hi-fi amplifiers. The main disadvantage of all common schemes is the low flexibility of regulation.

This term should not be confused with the depth and breadth of regulation. The depth of regulation shows in numbers, that is, quantitatively, within which limits the signal level at the boundary frequencies changes during regulation, the width of regulation is characterized by the range of frequencies captured by this regulation, and the flexibility of regulation characterizes the possibility of a fairly arbitrary change in the shape of the frequency response inside the regulated section, given that the same depth of adjustment. In fig. 43 shows a family of curves of a "classic" tone control according to the diagram in Fig. 42, from the consideration of which it can be seen that in the process of regulation only the angle of inclination of the branches of the curves changes, and the nature of the change in the curve remains the same all the time: either monotonically decreasing, or monotonically increasing from the conditional middle of the curve to its edges. This leads to the fact that the listener cannot arbitrarily emphasize or weaken any specific part of the spectrum, which does not allow obtaining correct reproduction in most cases.

One of the "half-measures", allowing to some extent to reduce the indicated disadvantage, relatively in a simple way, is the author's proposed method of using tap potentiometers for tone controls intended for loudness controls. The diagram for connecting these potentiometers to a "classic" dual-band tone control is shown in Fig. 44, and the family of its frequency characteristics is shown in Fig. 45. From a comparison of these characteristics with the above, it becomes clear how the nature of the regulation changes after the redesign of the circuit.

However, if such a modified tone control circuit can still be used in amplifiers of the "standard Hi-Fi class", then for "extra amplifiers" it is necessary to introduce at least four smooth tone controls in the sections 20-100, 100-1000 Hz, 1 -8 and 8-20 kHz.

Of course, these boundaries are very arbitrary and require clarification in the process of experimenting with high-quality amplifiers.

When dividing a frequency band into several sections, it is not always advisable to apply the same control schemes for all sections. It is more correct to use its own schemes for each section, taking into account the specifics of this frequency range.

In particular, if there are four separate sections in the circuit with the above cutoff frequencies, the author proposes for adjustment in the second and third sections (that is, at frequencies from 100 to 8000 Hz) to use the "classical" circuit on potentiometers with additional taps, similar to the one shown in fig. 44. For the first section, that is, at frequencies where nonlinear distortions are least noticeable by ear, it is easier and best to apply the circuit shown in fig. 46.

The circuit works as follows: in the middle position of the potentiometer R 6, which is a tone regulator, the audio frequency voltage on its engine with respect to the chassis is equal to zero (with complete symmetry of both halves of the secondary winding of the output transformer), therefore the entire tone control circuit does not affect the amplifier stage no influence.

The time constant of the entire circuit C 2, R 4, C 3, R 5, C 4 is chosen so large that at frequencies above 100 Hz signal propagation in the direction shown in Fig. 47 arrow, was not at all.

At lower frequencies, when the axis of the potentiometer R 6 is rotated, an audio frequency voltage will appear on the lower part of the potentiometer R 2, and its amplitude at all frequencies will be proportional to the angle of rotation of the regulator. However, for lower frequencies, the absolute magnitude of the voltage will be greater than for relatively higher frequencies.

In addition (and this is the main thing!), When the regulator passes through the middle zero point at all frequencies, the voltage will change to the reverse phase.

And since the specified circuit is a feedback circuit covering the entire amplifier, then, depending on the position of the regulator slider relative to its middle position, this feedback will be either positive or negative, respectively, increasing or decreasing the gain at frequencies below 100 Hz.

The results of the experiments show that with a two-link filter and a signal supplied to the grid circuit of the first lamp, the depth of adjustment and the cutoff slope at the upper cutoff frequency are quite sufficient, and the efficiency is sufficient. at a frequency of 20 Hz with a maximum rise of the characteristic does not exceed 3.5% in ULF power 20 watts, which is quite acceptable even for Hi-Fi amplifiers.

At frequencies over 40 Hz, c.n.i. does not exceed 2.0% when the characteristic rises, and when the characteristic drops, it drops to values ​​of the order of 0.6% at all frequencies of the section.

True, the circuit is very critical to adjustment in the process of establishing due to the danger of self-excitation at infrasonic (and even at sound) frequencies with positive feedback. However, with enough careful adjustment, the circuit works stably.

The main advantage of the circuit is that it does not require additional amplification, since in the middle position of the tone control slider, the attenuation introduced by the circuit is zero. Potentiometer R 2, brought out "under the slot", serves for the initial adjustment of the amount of feedback or, which is the same, the depth of tone control at the lower cutoff frequency (20 Hz). All values ​​of the filter elements need to be selected in the process of regulating the circuit.

To regulate the timbre in the fourth section, i.e., at frequencies above 8 kHz, the considered scheme is not suitable, since an increase in the c.n.i. more than 1% at higher frequencies in Hi-Fi amplifiers is unacceptable. Therefore, we can offer two other, relatively simple schemes.

The first of them (Fig. 47, a) is assembled on a double potentiometer, one of which R 1 together with the capacitor C 1 regulates the amount of negative current feedback at frequencies above 8-10 kHz. Potentiometer R 2 is part of the output voltage divider, and due to the presence of a small capacitor C 3 at frequencies above 8-10 kHz, this divider is frequency-dependent, since the voltage at its output depends on the position of the potentiometer slider R 2, while at more at low frequencies, the output voltage is practically unchanged for all frequencies at any position of the potentiometer slider.

The potentiometers are turned on in such a way that both sliders move together up or down (according to the diagram). The values ​​of the elements in the diagram are indicated only approximately, since all the same, when adjusting the amplifier, their selection will be required.

Another scheme (Fig. 47, b) is more interesting, although somewhat more complicated. In this circuit, the load of the emitter follower is the circuit L 1 C 2 C 3 C 4, the setting of which can be changed by rotating the regulator axis (variable capacitor C 2) in the range from 8-10 to 18-22 kHz. The exact boundaries of this range and the values ​​of the limiting capacitors C 3 and C 4 are selected when adjusting the amplifier.

The axis of the variable capacitor is rigidly connected to the axis of the potentiometer R 3, from the slider of which the generated signal is removed.

The potentiometer must necessarily be of type "A" and its extreme outputs are included in the circuit in such a way that a lower resonant frequency of the circuit corresponds to a decrease in the output signal. The variable capacitor C 2 is necessarily direct-frequency. With the correct adjustment of the circuit and the appropriate selection of its elements, the nature of the change in the control curves will be the same as shown in Fig. 48.

It can be seen from these curves that the second circuit not only regulates the level of higher frequencies, but also noticeably changes the character of the curves, providing a rather sharp drop above the cutoff frequency. This is the main advantage of the scheme, which compensates for its relative complexity.

3. Content switches and tone registers. Hi-Fi amplifiers have two completely mutually exclusive requirements for tone control. On the one hand, the amplifier should have as many smooth controls as possible, allowing the musically educated listener to adjust the frequency response in any way desired. On the other hand, the amplifier must provide a sufficiently accurate sound reproduction of broadcasts of various genres when used by a listener without special technical and musical education. This contradiction can be eliminated only in one way: by introducing a push-button tone switch - the so-called tone register - into the amplifier.

The tone register is a device that has several buttons for hopping the tone and 4-6 smooth tone controls. One of the buttons has the inscription "timbre smoothly", the rest have inscriptions corresponding to certain genres of musical programs (for example, "Jazz", "Solo", "Symphony", "Speech", etc.).

When you press the "soft tone" button, the fixed frequency shaping circuits are turned off, and the listener can manually adjust the frequency response using the smooth tone controls. When you press any other register button, on the contrary, all smooth regulators tone, and regardless of their position, the frequency response becomes fixed, properly corresponding to the transmission genre indicated on the button.

The tone registers are thus the most successful combination of flexibility and ease of tone control.

All tone registers are fairly complex devices, sometimes more complex than the rest of the amplifier. No completely complete schemes of tone registers for their exact copying cannot be cited, since each specific amplifier has its own individual, unique features, which determine the parameters and values ​​of the circuit elements of the tone register. Therefore, we will restrict ourselves to giving as an example one relatively simple circuit (Fig. 49), which experienced radio amateurs can repeat, remembering that some of the circuit elements will have to be selected empirically in the process of setting up the amplifier.

4. Stereo balance controls (RB) are the simplest controls in Hi-Fi amplifiers and, in fact, do not require a separate description. Therefore, we will give only a few of the most common control schemes (Fig. 50) and point out that if the regulator is included in a section of an amplifier with a high signal level, for example, in front of a pre-amplifier or phase inverter, then circuits with a common ground point can be used. If the regulator is turned on at the input of the amplifier or in circuits subject to the influence of interference and especially stray currents of the chassis, then it is better to use a circuit with two independent regulators on one common axis, and in this case, disconnect the connection points with the case, using the connection point with each channel. the housing of the leakage resistor of the grid of the lamp of the adjustable channel. Once again, we remind you that potentiometers for all types of RSB must be linear, with the letter "A" on the case cover.

A useful, although not necessary, addition to the stereo balance control is the balance indicator, which allows you to accurately mark the position of the PCB corresponding to the same gain of the stereo amplifier channels. There are many methods and indication schemes. We'll look at a few simple but effective ones.

The left side of Fig. 51, a, common to all indicators, represents the outputs of both amplifier channels. Using the Kn button, the outputs are connected to the indicator with a comparison circuit. In the diagram in Fig. 51, b, the voltages from inputs A and B are supplied in antiphase to the halves of the primary winding having the same number of turns. The magnetic fluxes of the semi-windings, with their complete identity and equality of voltages A and B, are the same and directed towards each other. Therefore, the total magnetic flux is zero, there is no voltage on the secondary winding, and the "magic eye" of the indicator is completely closed. In case of unbalance in any direction, the voltage on the secondary winding will be proportional to the value of the unbalance and will cause the expansion of the darkened sector of the indicator.

The diagram in Fig. 51, c works on the principle of a photometer, that is, a device that compares the brightness of two light sources. Incandescent lamps (6.3 V, 0.28 A) are placed in an opaque case with a partition in the middle. One of the walls of the case is frosted or milky light-scattering glass. When the channels are out of balance, the border of two different brightness is clearly visible; with full balance, the glass glows evenly. The brightness of the glow of the lamps depends on the magnitude of the output voltage of the amplifiers and can be changed by the volume control.

In fig. 51, d shows a bridge comparison circuit with diodes. The indicator is a dial gauge, the zero of which is in the middle of the scale (you can use an ammeter from any car with a shunt).

The first system can be very elegantly designed constructively, especially when using finger indicators such as 6E3P or 6E1P, it allows you to adjust the sensitivity of the indicator over a wide range, but it cannot be used to determine the direction of unbalance. The other two circuits are free from this drawback, but they are more difficult to design nicely enough on the front panel of the amplifier.

In all cases, the reference signal is a voltage with a frequency of 50 Hz, supplied from that filament winding of the power transformer, one of the ends of which (or the middle point) is connected to the chassis. This voltage is supplied to the input jacks of the amplifier through the contacts of the Kn button.

There are other indication systems, for example, using relaxation generators on neon lamps, but they do not have any advantages over those described.

In conclusion, we can give one more practical advice: all potentiometers before setting them to Hi-Fi amplifier useful to lubricate to prevent rustling and crackling during rotation and to increase service life. To this end, you need to carefully remove the protective cover and carefully lubricate the entire shoe with a very small amount of pure petroleum jelly, and drop 1-2 drops of any liquid mineral oil between the axle and the bushing.

"Radio" No. 8, 1986

Distributed EQ volume control

P. Zuev

One of the main requirements for loudness controllers (TKRG) of high-quality stereo AF amplifiers is high accuracy of loudness in a wide range of signal level control with a small mismatch of amplitude-frequency characteristics (AFC) and transmission coefficients.

The most commonly used TKRGs based on double variable resistors with taps for loudness circuits do not provide the identity of the gains of the stereo channels. The accuracy of their loudness is also insufficient, as a result of which the audible low-frequency components of musical programs are weakened at low volume levels. Described in a double step TCRG has almost identical frequency response and a small mismatch of the transmission coefficients of the channels, however, the range of its regulation (40 dB) is small for high-quality equipment, and the frequency response in the region of the lowest sound frequencies is quite different from those recommended in.

The analysis of the latter made it possible to establish that the required rise in the frequency response in the region of the lowest sound frequencies (20 ... 1000 Hz) is directly proportional to the attenuation of the signal introduced by the TCRG at medium frequencies. In other words, with a decrease in the TCRG transmission coefficient at medium frequencies, the necessary increase in the frequency response at each of the lowest frequencies depends practically not on the initial value of the controller transfer coefficient, but on the frequency itself and on the change in the volume level relative to the initial value. So, when changing the transmission coefficient of TKRG at a frequency of 1 kHz by 10 dB, the required change in the transmission coefficient at frequencies of 31.5; 63, 125 and 250 Hz were 3, 4.5, 6 and 7.5 dB, respectively. Moreover, these ratios practically did not depend on the initial value of the transmission coefficient.

Two very important conclusions follow from the above. Firstly, if the frequency response of the TCRG corresponds to those recommended in, then it will equally well carry out the frequency compensation of the low-frequency components of a musical program, regardless of the level of its musical balancing (usually 70 ... 90 dB). Just enough to First level loudness (corresponding to the maximum transmission coefficient TKRG) was close to the level of musical balancing of the program being played. This level should be set by another, frequency-independent volume control (the so-called maximum volume control - RMG), the frequency response of which is horizontal and does not depend on its transmission coefficient.

Secondly, to implement the required law of changing the frequency response depending on the transmission coefficient of the TCRG, it is not enough to introduce a one or two-link correcting circuit, as is done in most cases, and a distributed frequency correction is necessary using multi-link correcting circuits, the number of which should be the greater. the greater the signal attenuation introduced by the regulator.

Two versions of such TKRGs are offered to the attention of readers.

Main technical characteristics
Volume control range, dB ............................. 70
Control step, dB ...................................... ........ ........... 3 1/3
Input impedance modulus in the strip
frequencies 20 ... 20 000 Hz, kOhm, not less ................................. 20
Permissible load resistance kOhm, not less ... .... 330
Frequency response mismatch of stereophonic TKRG
in the operating range of regulation, dB, no more ......... 1
The level of intrinsic noise at the TKRG output
in the frequency band 20 ... 20,000 Hz, μV, not more ... ............... 3

The first regulator (Fig. 1) is made on the basis of a switch for 23 positions and consists of seven identical correcting circuits A1 - A7, each of which is a combination of low (R1 - R4C1) and high (R1 - R4C2) frequency filters. The resistors and capacitors are selected in such a way that the signal attenuation created by each of the circuits at medium frequencies is 10 dB , the control step is 3 1/3 dB, and the frequency response of the TKRG as a whole is as close as possible to the required in the entire operating range of regulation. The elements R5, R6, SZ connected to the output of the last correcting circuit A7 perform the functions of its load, ensuring the identity of the frequency response of all correcting circuits.

The TKRG works like this: as the input signal weakens (Fig. 2), everything turns on more correcting circuits, which leads to an increase in the rise of the frequency response at the lower and higher sound frequencies relative to the average (since the transmission coefficients of all previous correcting circuits are multiplied). In the last, 23rd position of the switch, there is no signal at the TKRG output (infinite attenuation). The maximum deviation of the actual frequency response of the regulator in the lower frequency range from the recommended frequency response is observed at a frequency of 250 Hz and as the signal attenuates from 0 to -70 dB, it increases from 0 to 5 dB.

The second TKRG (Fig. 3) is implemented on the basis of the 11-position switch, which is more accessible to radio amateurs. Unlike the first, the number of correcting circuits is reduced to three, which narrowed the control range of this regulator to 33 1/3 dB. The expansion of the regulation range up to 70 dB is achieved by switching on one more correcting circuit R5 - R7C3C4, which attenuates the signal by 37 dB (it is turned on by pressing the SB1 "Quiet" button). The frequency response of this TKRG (Fig. 4) is closer to the required ones (the deviation at the lowest frequencies does not exceed 2 dB in the entire control range).

It should be noted that the increase in frequency response in the region of higher sound frequencies in the proposed TKRGs is greater than recommended in. This had to be done, because listening to music programs at low volume in living quarters showed a subjectively perceived lack of higher frequencies if the frequency response of this area corresponded to the recommendations.

The proposed TKRG should be used in conjunction with the RMG and the output signal level indicator.

In the TKRG according to the scheme in Fig. 1, you can use the MP1-2 rocker switch in two directions and 24 positions with continuous switching of contacts, in the TKRG according to the diagram in Fig. 3 switch PGK or PGG in two directions and 11 positions. It is recommended to adjust the elements of the switch position detent to a lower, but sufficient for precise operation, fixing torque. So that the mismatch of the frequency response of the channels of stereo amplifiers does not exceed 1 dB, the resistances of the corresponding resistors and the capacitances of the capacitors used in different TKRG channels should not differ by more than 2%.

It is recommended to solder the elements of the correcting circuits R2, R3, R4, C2 directly to the switch terminals, and place R1, C1 on two printed circuit boards ah, installed between the biscuits on its tie rods. It is recommended to mount the elements of the additional correcting circuit (see Fig. 3) on the terminals of the push-button switch SB1 (P2K), placing it in the immediate vicinity of the wafer-type switch.

Unlike the known ones, the considered TKRGs have a significantly higher output impedance, which depends little on the output signal, therefore, to reduce external interference, all their elements should be placed in a metal screen, and the input and output circuits should be made with shielded wires.

Subjective tests of the TKRG showed a high accuracy of loudness: up to the smallest volume levels, the timbre of the sound was kept balanced in terms of the highest and lowest frequencies, which practically eliminated the need to use the timbre controls when adjusting the volume.

At low volume levels, the sound of low-class sound-amplifying equipment does not, as a rule, provide high-quality reproduction. This is due to the fact that at low volume, the human ear becomes less sensitive to the frequencies of the lower and upper spectrum. To eliminate this drawback, high-quality equipment provides various compensation schemes for the amplitude-frequency characteristic (AFC) at low sound volumes, that is, the upper and lower frequencies are additionally amplified, as a result, the frequency response is leveled and the sound quality does not change by ear at any volume level. The easiest way to achieve this effect is to use loudness controls. The circuits are quite simple and do not require the use of scarce parts and any settings.

The vast majority of such circuits were previously built on the basis of special variable resistors with additional taps, as shown in Fig. 1. The main disadvantage of such circuits is the use of special resistors and a small depth of loudness compensation. They are also characterized by a certain nonlinearity, stepwise reproduction of high and especially low frequencies at certain positions of the engine of a variable resistor with one or two taps.

Below are the diagrams of loudness-compensated volume controls on resistors of group "B" without taps (ordinary variable resistors, widely used in various radio equipment. "In its marking, before or after the designation of its nominal resistance)

Figure 2 shows a circuit where high-frequency (HF) correction is performed by the R1C1 circuit, and low-frequency (LF) correction is performed by a T-shaped filter R2C2R3. The frequency response of the loudness compensation of this regulator is approximately the same as that of devices using a regulator with two taps. The disadvantage of such a scheme is the small steepness of the rise in the frequency response in the regions of lower and higher frequencies, as well as the use of a variable resistor of high resistance (2 MΩ), which is not very easy to find at the present time.

Improvements in loudness can be achieved by connecting additional RC circuits, as in Figure 3. In addition, a variable resistor of a widespread rating is used here (you can put 47 ... 68 kOhm). In this case, the function of the low-frequency corrector will be performed not only by the T-shaped filter R2C3R3, but also by the introduced additional circuit R7C4. In fact, it will already be a second-order low-pass filter (LPF), providing a steepness of the rise in the frequency response of the regulator in the low-frequency region of 12 dB per octave. High-frequency correction is achieved by the introduction of a C2R5R6C5R7 high-pass filter in addition to the traditional R1C1 circuit.

It should be noted that in this scheme the loudness compensation in the higher frequency region is slightly higher than the required one. This was done deliberately and is due to a purely subjective perception of musical phonograms at home. A small dip in the frequency response at a frequency of 3.5 kHz in the lower position of the slider of the resistor R4 is due to the phase shift between the signals of this frequency that have passed through the HPF and resistor R4. When the elements C2, R5, R6, C5 are excluded, this dip disappears, and the additional rise in the frequency response at higher frequencies also disappears, which brings the equalizer parameters to the standard ones recommended for such loudness compensators in various technical literature on acoustics. Therefore, these elements can be excluded, it all depends on the specific features of the equipment and personal auditory perception.

The minor disadvantages of this circuit include a slight decrease (up to 48 dB) in the volume control range, which is due to the presence of a resistor R7 in the control circuit. But in practice, such a small decrease in the adjustment range is usually not critical.

The scheme of such a subtlety can be used in the development and manufacture of new sound-amplifying equipment, as well as to refine existing amplifiers, radio tape recorders, and receivers. If conventional volume controls are used in such devices, that is, just a variable resistor of the corresponding resistance, not included in the feedback circuit of the amplifying nodes, then you can turn it on instead this scheme... But at the same time it is necessary to take into account the output resistance of the previous stage (before the volume control) - it must be much less than the resistance of the resistor R5, and the input resistance of the stage following the control, which must be greater than the resistance of the resistor R3. The greater the difference in these resistances, the better the load matching will be ensured and the equipment as a whole will perform better. In an extreme case, you can turn on additional matching stages on transistors or microcircuits before and after the regulator and thereby also compensate for a possible slight decrease in the maximum volume of the entire audio path. In my personal practice, such a need did not arise, but below I will give a couple of diagrams of such additional matching stages (Fig. 4).

The circuits are additional amplification stages on the K157UD2 microcircuit (two amplifiers in one package, the arrangement of the terminals of both channels is shown) and a transistor. As DA1, you can use any operational amplifier, for example K140UD6, UD7, K153 UD1, UD2 and others, taking into account the pinout of their outputs and correcting circuits (here these are capacitors C2). The feedback factor depends on the value of the resistor R2. The lower the value of this resistor, the lower the gain of the stage and the less harmonic distortion. Therefore, the resistor should be set with as little resistance as possible!

The transistor in the second circuit can be replaced with KT315, KT342, KT306. The resistance of the resistor R2 here depends on the supply voltage (the lower the supply voltage, the lower the resistance), and the resistor R1 sets the operating mode of the transistor according to direct current... By selecting this resistor, in rest mode (without an input signal), set the output (collector of the transistor) a voltage equal to half the supply voltage.

I am attaching pictures of printed circuit boards (download):

- pl1 - board of the matching stage on the transistor;

- pl2 - board of the matching cascade on the MS K157UD2 (two channels);

- pl3 - board for a loudness-compensated volume control according to the diagram in Fig. 3.