Most often, in cascades of volume controls of high-quality sound reproducing equipment, variable resistors are used directly as regulators, allowing the signal gain to be gradually or smoothly changed. However, often in tube LF amplifiers step volume controls are used, made using fixed resistors and switches.
The simplest and most common circuit solution for a tube ULF volume control when choosing smooth control is to introduce a potentiometer with a variable voltage division coefficient into the input circuit, into the interstage circuit or into the negative feedback circuit of the amplifier. By moving the slider of this potentiometer, the volume is directly adjusted. In this case, it is recommended to use variable resistors with a so-called logarithmic characteristic (type B characteristic) as an adjustment potentiometer to ensure a uniform change in the volume of the reproduced signal at different input signal levels.
If desired, the volume control with smooth adjustment can be replaced with a regulator with step adjustment. To do this, it is enough to make an appropriate replacement of the regulating element, that is, instead of a potentiometer, install a chain of series-connected constant resistors, the number of which and the ratio of their values determine the range and law of regulation.
When choosing a volume control circuit, one should not forget that the human ear has different sensitivity to signals of different frequencies and volumes. In practice, this phenomenon manifests itself in the fact that when the volume of the reproduced sound signal decreases, the listener gets the impression of a change in sound timbre, which is expressed in an apparently significantly greater decrease in the relative volume of the components of lower and higher frequencies compared to mid-frequency signals. Therefore, in high-quality sound-reproducing equipment, fine-compensated volume controls are used, in which, when the volume is reduced, the necessary rise in the components of lower and higher frequencies is carried out to ensure equal loudness of perception. As the volume increases, the required rise in the edge frequency components decreases. The basis of fine-tuned volume controls is usually potentiometers with one or two taps, to which the corresponding RC circuits are connected.
Typically, the volume control is used to change the level of the ULF output signal with minimal introduced distortion. In this case, most often a variable resistor is used as such a regulator, connected either at the input of the amplifier or between the preliminary and final stages. Instead of a variable resistor, as already noted, a step regulator can be used, made on the basis of a switch and a cassette of resistors with different resistances. Simplified circuit diagrams of the simplest volume controls are shown in Fig. 1.
Fig.1. Simplified circuit diagrams of volume controls
To prevent the possibility of overloading the first amplifier tube with a large amplitude of the input signal, the volume control connection diagram shown in Fig. 1, a. In this case, the variable resistor is used directly as a load of the previous device. If the maximum amplitude of the input signal is small, a variable volume control resistor can be installed in the control grid circuit of one of the subsequent amplification stages, as shown in Fig. 1, b. The advantage of this connection is that it reduces the impact of external interference, since a useful signal is supplied to the regulator, already amplified to the required level.
The volume level in tube ULFs can also be adjusted using special cascades, which provide a change in the slope of the lamp characteristic. The principle of operation of such volume controls is based on the fact that when a lamp with a high internal resistance is used in the amplifier stage, the gain of such a stage will be proportional to the steepness of its characteristic (S). Therefore, when using a lamp with a variable slope characteristic, to change the gain of the cascade, it is enough to move the operating point to an area with a different slope value. Changing the position of the operating point and, accordingly, the gain can be done in different ways, for example, by changing the value of the bias voltage or the voltage on the lamp screen grid. Simplified circuit diagrams of such volume controls are shown in Fig. 2.
Fig.2. Simplified circuit diagrams of volume controls with changing the slope of the lamp characteristic
It should be noted that the volume controls considered, which use the principle of changing the slope of the lamp characteristic, can only be used in the first stages of the ULF at relatively small amplitudes of the input signal (no more than 200 mV). At higher input signal levels, significant nonlinear distortion may occur due to the curvilinearity of the dynamic response.
To adjust the volume in low-frequency tube amplifiers, regulators are often used that provide compensation for low frequencies at low input signal levels. A schematic diagram of one of these regulators is shown in Fig. 3.
Fig.3. Schematic diagram of a volume control with low frequency compensation at low input signal levels
An input signal with a fixed increase in the level of the lower frequencies of the reproduced range is supplied to the input of the cascade. This level is determined by the resistance values of resistors R1, R2 and R3, which form the input divider, as well as the value of the capacitance of capacitor C2. From the output of the regulator, a feedback signal is supplied to the lamp grid circuit through a divider formed by elements R7 and C2. The higher the volume level, the greater the feedback. The resistance value of resistor R7 determines the ratio of the attenuation of low frequencies in the feedback circuit to the rise of these frequencies in the input circuit. Ideally, by selecting the resistance of resistor R7, it should be ensured that the attenuation of low frequencies in the feedback circuit is equal to their increase in the input circuit. In this case, the shape of the frequency response of the signal at the output of the stage will be close to linear. Shown in Fig. 3 element ratings are designed to use one of the triodes of the 6N2P lamp.
When the signal volume is reduced using potentiometer R6, the feedback value also decreases, but the fixed increase in low frequencies remains the same. As a result, the level of low frequencies in the output signal increases. At very low volume values, there is practically no feedback, and the cascade characteristic is determined only by the parameters of the chain R1, R3 and C2. At the same time, the rise in lower frequencies is maximum.
One of the disadvantages of this circuit is that the triode is connected before the volume control, so with a very strong input signal it can be overloaded. However, the signal from the input is fed to the control grid of the lamp through a divider, which, even at a frequency of 50 Hz, provides an attenuation of more than 4 times. As a result, this circuit can operate without distortion at an input signal level of up to 4-5 V. It should also be noted that the circuit in question is sensitive to the level of anode voltage filtering, so the use of the R8C5 filter in the lamp anode power circuit is mandatory.
When designing a tube ULF, radio amateurs often set themselves the task of including a cascade, with which they can adjust the volume remotely. The use of remote consoles with potentiometers placed in them in conventional regulators can hardly be considered a good solution, since most often such consoles are connected to the amplifier using long cables, which leads to very significant distortions. However, there are a variety of circuit solutions that provide volume control at a distance, for example, by changing the DC control voltage, with virtually no distortion. A schematic diagram of one of the options for a volume control with remote control is shown in Fig. 4.
Fig.4. Schematic diagram of a volume control with remote control
A distinctive feature of the regulator in question is the inclusion, instead of the cathode resistor of the amplifier stage triode, of another triode, which acts as a regulating element. When the value of the constant negative voltage supplied to the grid of the second triode changes, the value of its resistance changes. As a result, the depth of negative feedback for the first triode changes. So, for example, as the internal resistance of the second triode increases, the negative coupling increases, and the gain of the first triode decreases. In this circuit, an imported double triode of the ECC82 type can be replaced, for example, with a domestic 6N1P lamp.
In high-quality tube sound-reproducing equipment, volume controls with loudness compensation are widely used. The need to use such volume controls is explained by the fact that the sensitivity of the human ear changes depending on the frequency and volume of the perceived sound signal. For example, better sensitivity corresponds to the perception of mid-frequency components compared to higher and especially lower frequency components. Therefore, when the volume is reduced, the listener has a subjective feeling that the level of the components of the higher and lower frequencies of the reproduced range is simultaneously decreasing. As a result of research carried out in this area, certain dependencies were drawn up, which were called curves of equal loudness.
So that at different volume levels all frequency components of the reproduced signal are perceived equally, high-quality sound-reproducing equipment uses volume controls, in which, as the volume decreases, the necessary rise in the components of lower and higher frequencies is carried out, and with an increase in volume, the rise in the components of the boundary frequencies decreases. Such regulators are called loud-compensated or frequency-dependent. Naturally, developers strive to ensure that the characteristics of thin-compensated volume controls are as close as possible to equal volume curves.
The simplest option for constructing a frequency-dependent volume control is to combine the volume control itself and the tone control using paired variable resistors. Schematic diagrams of such volume controls are shown in Fig. 5, a and 5, b. Often, high-volume volume controls use potentiometers with one or two taps, to which the corresponding RC circuits are connected. A schematic diagram of one of the variants of such a volume control is shown in Fig. 5, c.
Fig.5. Schematic diagrams of simple loudspeaker volume controls
The current-compensated volume control can also have step adjustment. The advantages of such regulators, in addition to the absence of a potentiometer of the appropriate design, include the ability to select a significantly wider adjustment range. A schematic diagram of one of the options for the input stage of a tube ULF with such a regulator is shown in Fig. 6.
Fig.6. Schematic diagram of a thin-compensated volume control with step adjustment
Loudness compensation in volume controls can also be implemented using special filters. The schematic diagram of the regulator with a loudness filter is shown in Fig. 7.
Fig.7. Schematic diagram of a volume control with a loudness filter
In the circuit under consideration, the loudness filter is a double T-bridge, the transmission coefficient of which for the components of the middle frequencies of the reproduced range is less than the transmission coefficient for the components of lower and higher frequencies. In maximum volume mode, the potentiometer R4 slider should be in the upper position in the circuit, while the filter is short-circuited and does not affect the shape of the frequency response. To decrease the volume, the slider of potentiometer R4 should be moved down, which reduces the shunting effect of the upper part of this potentiometer on the filter. As a result, components of certain frequencies begin to pass through the filter in accordance with its frequency response. Since the components of the middle frequencies are attenuated by this filter to a greater extent than the components of the extreme frequencies, the change in the frequency response of the amplifier occurs according to a dependence close to equal volume curves. Potentiometer R4 must have a logarithmic characteristic (type B).
The author has proposed a variant of a thin-compensated volume control using a variable resistor without taps, but with an inductor. The calculated values of the regulator elements for various volume control ranges are given in tabular form.
It is important to note that the frequency response of the regulator transmission at different volume levels must correspond to equal volume curves for a particular listener. This can be achieved with the presence or introduction of a sensitivity regulator into the sound reproduction path, which brings the level of loudness compensation into line with subjective assessments.
In various sound-reproducing equipment, potentiometric, thin-compensated volume controls (VG) on variable resistors with taps and a nonlinear 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 compensation from the curves of equal loudness, which is especially large in the low-frequency and high-frequency regions of the AF spectrum and makes it possible to increase 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. This is also noted in.
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 attenuation of the signal 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 volume.
Rice. 1. Regulator circuit for one channel
Let's take sound pressure levels according to equal loudness curves according to the GOST R ISO 226-2009 standard. For the initial volume level, corresponding to the volume level of 20 von 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, the sound pressure levels (SPL) in the audio frequency band must correspond to those given in table. 1.
Table 1
| F, Hz | |||||||||||
| SPL (dB) |
For measurements, a sinusoidal signal with a peak-to-peak value of 1 V is applied to the input of the controller over the entire audio frequency band. Measurements were carried out when changing the values of elements C1 and R2. Circuit L1C3 is tuned to resonance at a frequency of 20 kHz. A factory dumbbell coil with an inductance of 8.2 mH was used as inductance L1. 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 of standard size K20x12x6. The measurement results are the same. You can use the M2000NM ring of standard size K10x6x3, the estimated number of turns is 115.
The results of measurements of the output voltage swing U2 and the ratio of the output voltage to its value U1 at a frequency of 1 kHz, as well as sound pressure levels at various values of C1 and R2 are given in table. 2-14.
table 2
R1 = 22 kOhm, R2 = 200 Ohm, C1 = 1 µF
| F, GC | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 3
R1 = 22 kOhm, R2 = 100 Ohm, C1 = 1 µF
| F, Hz | |||||||||||
| U2, V | |||||||||||
| U2/U1 | |||||||||||
| DB |
Table 4
R1 = 47 kOhm, R2 = 100 Ohm, C1 = 1 µF
| F, Hz | |||||||||||
| U2, V | |||||||||||
| U2/U1 | |||||||||||
| DB |
Table 5
R1 = 22 kOhm, R2 = 51 Ohm, C1 = 1 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 6
R1 = 22 kOhm, R2 = 27 Ohm, C1 = 1 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 7
R1 = 22 kOhm, R2 = 0 Ohm, C1 = 1 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 8
R1 = 22 kOhm, R2 = 51 Ohm, C1 = 1.5 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 9
R1 = 22 kOhm, R2 = 27 Ohm, C1 = 1.5 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 10
R1 = 22 kOhm, R2 = 0 Ohm, C1 = 1.5 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 11
R1 = 22 kOhm, R2 = 51 Ohm, C1 = 2 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 12
R1 = 22 kOhm, R2 = 27 Ohm, C1 = 2 µF
| F, GC | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 13
R1 = 22 kOhm, R2 = 0 Ohm, C1 = 2 µF
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
Table 14
R1 = 22 kOhm, R2 = 27 Ohm, C1 = 2 µF, middle position of the variable resistor R1 slider
| F, Hz | ||||||||||||
| U2, V | ||||||||||||
| U2/U1 | ||||||||||||
| DB |
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 variable resistor R1 slider. The results of attenuation measurements at various 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 a frequency of 20 Hz and 33 dB at a frequency of 20 kHz. The volume control range at 1 kHz was 46 dB. The corresponding frequency response curves of the RG are shown in the graphs in Fig. 2.
Rice. 2. Frequency response curves of the RG
Table 15
| F, Hz | ||||||||||||
| K 2, dB | ||||||||||||
| K 4, dB | ||||||||||||
As a result of considering the data obtained, the following conclusions can be drawn. The resulting shapes of the frequency response of the RG are close to the curves of equal loudness. Lower values of resistor R2 shift the treble boost towards higher frequencies and are more consistent with equal volume curves. In addition, larger values of the capacitance of capacitor C1 (1.5 and 2 μF) and lower values of the resistance of resistor R2 (27 Ohms and 0 Ohms - jumper) increase the frequency correction and expand the range of volume control. 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-SZ).
Some disadvantage of this type of regulators is the reduction in the range of volume control.
This RG can be built into a device (UMZCH and AC), ensuring that the sound pressure corresponds to curves of equal loudness. If this is not ensured, then in addition to the RG, you should include in the path a sensitivity regulator that brings the signal level to the nominal level so that the loudness corresponds to equal loudness curves at the corresponding sound pressure (volume level). Volume control, the frequency response of which is shown in Fig. 2, was built into the active speaker. Thanks to sufficient loudness, low and high frequencies are clearly audible even at minimum volume.
Literature
1. Fedichkin S. Thin-compensated volume control. - Radio, 1984, No. 9, p. 43, 44.
2. GOST R ISO 226-2009. Acoustics. Standard equal volume curves. - URL: http://protect.gost.ru/document.aspx? control=7&baseC=6&page=2&month=8& year=2010&search=&id= 175579 (04/13/15).
Radio No. 6 2003 A Pakhomov
It is known that as the average volume level decreases, the sensitivity of the human ear decreases most to the lowest frequencies (LF) of the sound spectrum. To compensate for this physiological feature of hearing, sound-reproducing equipment requires a corrective increase in the bass: at minimum volume (depending on the noise level in the room) it should reach 25...40 dB at a frequency of 50 Hz relative to a frequency of 2 kHz. Moreover, according to equal loudness curves, the slope of the rise should increase as frequency decreases: 6 dB per octave starting at 250 Hz, and 12 dB per octave below 100 Hz.
Most of the known circuits of thin-compensated volume controllers (TCVR), with the possible exception of the most complex ones that have not found widespread use, do not provide the required law and depth of correction. In the most common TCRGs with a tapped variable resistor (or without taps), the depth of low-frequency correction is no more than 15 dB, and its slope decreases at frequencies below 100 Hz.
For example in Fig. Figure 1 shows the typical frequency response of a passive TCRG using a variable resistor without taps. It can be seen that the corrective rise at a frequency of 50 Hz with a regulator gain of -40 dB is equal to 13 dB, the slope below 100 Hz does not exceed 3 dB per octave, which is completely insufficient. The TCRG on a resistor with one tap also has similar characteristics.
During operation, such regulators create an unpleasant effect: when the volume is reduced, the depth of the sound is lost and a tendency to “mumble” appears. Attempts to increase the degree of correction at the lowest frequencies by adding an RC circuit to the common wire of a variable resistor lead to a narrowing of the volume control range. In this case, the volume is not reduced to zero, which is very inconvenient in practice.
Another disadvantage of the mentioned devices is the incorrect change in correction as the volume is adjusted. A noticeable correction of the frequency response often occurs when the control is in the middle position, when the actual volume (sensitivity) is still high. As a result, the tonal balance is disrupted in the most frequently used area of average sound volume.
Unfortunately, all of the listed disadvantages are also characteristic of electronic TCRGs made on specialized microcircuits. In Fig. Figure 2 shows the frequency response of a very complex regulator TC9235 from Toshiba, which has a low level of noise (less than 2 μV) and nonlinear distortion (less than 0.01%), multi-stage digital volume control, convenient push-button control, etc. With all this, the regulator provides fine correction no better than those already considered by the TCRGs. In household sound reproduction devices, the frequency range below 100 Hz is considered “problematic” for the final links of the path. Thus, a small-sized acoustic system rarely has a lower limit frequency of less than 50...60 Hz at a level of -3 dB. Typically, the sound pressure drop begins at a frequency of 100 Hz. Sometimes high-quality equalizers or special bass correctors based on high-order filters are used to compensate for it. But in this case, it is necessary to take into account the limited overload capacity of the UM3CH at low frequencies and reduce the degree of correction simultaneously with increasing the volume. Applying signals below the resonant frequency to the dynamic heads only leads to an increase in distortion.
Currently, there are special auto-bass correctors (X-Bass, etc.) that dynamically form the frequency response, taking into account all the listed factors. But they most often represent closed “proprietary” developments, made on specialized microcircuits without markings.
The proposed device solves these problems in a simpler way. During its development, new circuit solutions were used, obtained by computer modeling in Micro-Cap 7.1.0, followed by testing on a breadboard. As a result, it was possible to create a simple device that successfully combines the TCRG itself with a bass corrector, which “completes” the frequency response in the frequency range of less than 100 Hz and regulates its course depending on the position of the volume control.
The schematic diagram of the device (one channel) is shown in Fig. 3. It consists of a passive TKRG and an active bass corrector assembled on the DA1 chip. Both parts are combined into a single whole so that the disadvantages of the passive regulator are eliminated by the active part of the device.
Rice. 3
Passive TCRG is made on elements R1-R4, C1, C2 according to the well-known scheme (see Fig. 1) in a simplified version. Filter R3R4C1C2 lowers mid frequencies depending on the position of the R2 control slider. The filter parameters are selected to provide the maximum possible low-frequency boost. HF correction does not present any problems and is set by the capacitance of capacitor C1.
From the output of the passive TCRG, through circuit C3R6, the signal is supplied to the inverting input of op-amp DA1.1, which amplifies the signal (up to 14 dB) and forms the frequency response with two OOS circuits. The first is through resistor R5, TKRG elements including the volume control R2, and the input circuit C3, R6; the second - through the T-shaped link R7-R10 and the DA1.2 microcircuit with related elements.
The DA1.2 chip contains a gyrator that simulates an inductor. Together with capacitor C5, it forms an oscillatory circuit with a resonance frequency of 45...50 Hz. At this frequency, the OOS signal is weakened to the maximum extent and a hump in the frequency response of op-amp DA1.1 is formed. In this case, the slope of the frequency response below 100 Hz reaches 10 dB per octave, and the overall rise (adjustable) at a frequency of 45 Hz is +27 dB relative to the frequency of 2 kHz with the volume control position -41 dB (Fig. 4). These parameters are close to the required values of characteristics of equal loudness.
The limitation of the amplitude of signals with frequencies below the resonant speaker is formed in the device due to the natural slope of the resonant curve of the analogue of the LC circuit on DA1.2 and two high-pass filters: C3R6 and C6Rin, where Rin is the input resistance of the cascade following the regulator. For this regulator, the equivalent load resistance is taken to be 100 kOhm; for another input resistance, capacitance C6 should be recalculated so that the time constant C6Rin does not change.
The second OOS - through resistor R5 - is also frequency-dependent, since it includes a filter formed by resistors R3, R5 and capacitor C2. Such a compensating environmental protection system was proposed by the author in an article where the principle of its operation is described in detail. The result comes down to additional straightening of the low-frequency branch of the frequency response as the volume increases. Thus, the required correction is achieved when moving from low to medium volume (Fig. 4), and not from medium to high (see Fig. 1, 2). Moreover, by choosing the appropriate OOS depth, you can eliminate the overload of the UMZCH at volume levels close to maximum, similar to dynamic bass correctors.
The effectiveness of the feedback loop through resistor R5 is illustrated by simulated frequency response (Fig. 5). The curves are calculated for the version with OOS (R5 = 12 kOhm) and without it (R5 - 1 MOhm). As can be seen from the graphs, the OOS acts selectively and only low frequencies are weakened. When the volume control is set to -20 dB, the attenuation is small - about 7 dB, and at maximum gain it reaches 26 dB. At the same time, the OOS completely smoothes out the peak of the bass correction, leveling the frequency response. Without this, the UMZCH would be overloaded already at the middle position of the TKRG and it would be necessary to perform manual manipulations with the bass tone control.
In the right position according to the diagram, the slider of resistor R9 and the upper resistor R13, with the values indicated in the diagram, has the characteristics shown in Fig. 4. However, a wide variation in the type of frequency response is possible: using trimming resistor R9 you can adjust the depth of bass correction in the range of 0...+6 dB (Fig. 6). The range is indicated at average sound volume; when it decreases, it increases, when it increases, it decreases, i.e. the device adaptively adjusts the depth of adjustment in accordance with equal volume curves and overload capabilities of the UMZCH.
If desired, variable resistor R9 can be displayed on the front panel and used as a bass tone control. Its advantage is that, unlike bridge and other RC perylators, it regulates the bass, and not the entire frequency band up to 1000 Hz. To smoothly change the timbre, you need a variable resistor with a type B control curve.
The high quality of the regulator as a whole is due to deep OOS, the absence of oxide capacitors and the use of the TL074 microcircuit. Its four op-amps are characterized by extremely low harmonic distortion (Kh ~ 0.003%) and good noise characteristics (e = 15 nV/√Hz). Thanks to this, the device can be used as a preamplifier with a gain of up to 14 dB, sufficient, for example, to compensate for losses in a passive tone control. Otherwise, the gain can be reduced to unity and less by trimming resistor R13, which will proportionally reduce the noise level. As with all TCRGs, the accuracy of loudness compensation depends on the transmission coefficient of the audio path. It can be adjusted by the mentioned trimming resistor R13 or another one available in the path. You only need to take into account the distribution of the gain and noise properties of the path links. By changing the signal level and selecting resistor R5, we achieve preservation of tonal balance throughout the entire volume control range. If the UMZCH is overloaded at maximum volume, you should reduce the value of resistor R5 based on the subjective feeling of the bass content and its distortion. Other adjustment options include shifting the resonant peak of the bass correction by selecting resistors R11, R12 for a specific speaker. The bass depth is adjusted with resistor R9, as described above.
In the highest quality paths, replacing the TL074 op amp with the NE5534A is possible. However, in simpler cases it is quite possible to use the K157UD2A op-amp with the appropriate correction circuits. In this case, the harmonic coefficient increases by approximately an order of magnitude, and the noise level at a unit transmission coefficient will be no worse than -80 dB.
Otherwise, the regulator is assembled using ordinary parts: MLT-0.125 resistors, small-sized KM capacitors. An imported small-sized dual variable resistor with a nominal value of 50 kOhm (type B regulation characteristic) is used as regulator R2. The presence in the device of resistors R3, R4, connected in parallel to the upper section R2 according to the diagram, allows the use of a variable resistor with a linear control characteristic (type A), however, in this case, an initial jump in volume is inevitable with further smooth control.
Experimental testing and subjective listening confirmed the high quality of the regulator. The deviation of the real frequency response from the simulated ones did not exceed several decibels. The level of the regulator's own noise at unity gain was below the audibility limit. The operation of the regulator is characterized by correct tonal balance at any volume, preservation of “deep” bass at minimum volume and no overload of the UMZCH at volume levels close to maximum. In many cases, it is possible to avoid using a conventional tone control altogether and use only the bass corrector.
LITERATURE
Tikhonov A. Acoustics within us. - STEREO&VIDEO, 1999. No. 4, p. 18.
Shikhatov A. Thin-compensated volume controls. - Radio, 2000, No. 10, pp. 12, 13.
http://chipinfo.ru/docs/TOS/001456.pctf
Shikhatov A. Circuit design of automobile power amplifiers. - Radio, 2002, No. 1.С14, 15.
PakhomovA. Adjustment unit for wearable radio. - Radio, 2002, No. 9, p. 16, 17.
From myself: testing, printed circuit board
Testing:
Stabilized bipolar power supply from +15V and -15V banks (7815 and 7819) connected to a 25V power supply;
Power amplifier 20W on IC LM1875;
Acoustic system 15W Vega 15AC-109;
The regulator worked immediately and did not require any settings. There were no complaints about the operation of the loudness compensation, it worked properly, the degree of its adjustment is changed by the tuning resistance R9, R9’, but low frequencies began to predominate in the sound and, what is most distressing, a dip appeared at mid frequencies. Therefore, I had to abandon the use of this block and exclude it, because... I didn't like the sound of it.
So whether to use a loudspeaker or not is up to you. In my opinion, such devices can be used in the absence of a subwoofer, although of course the use of loudness compensation, tone blocks, etc. is purely a matter of taste and also depends on the sound source.
At low volume levels, the sound of low-class sound reinforcement equipment, as a rule, does not provide high-quality reproduction. This is due to the fact that at low volumes 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 response (AFC) at low sound volumes, that is, the upper and lower frequencies are further amplified, as a result the AFC is leveled out and the sound quality does not change by ear at any volume level. The easiest way to achieve this effect is to use volume controls with loudness compensation. The schemes are quite simple and do not require the use of scarce parts or any configuration.
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 upper and especially lower frequencies at certain positions of the variable resistor slider with one or two taps.
Below are diagrams of fine-compensated volume controls on resistors of group “B” without taps (ordinary variable resistors, widely used in various radio equipment. The resistor group determines the dependence of the input resistance when turning the engine and is designated by a letter, for example, “A”, “B”, “C” " in its marking, before or after the designation of its nominal resistance)
Figure 2 shows a circuit where high-frequency (HF) correction is carried out by circuit R1C1, and low-frequency (LF) correction is carried out by 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 this scheme is the slight steepness of the rise in the frequency response in the regions of lower and higher frequencies, as well as the use of a high-resistance variable resistor (2 MOhm), which are not very easy to find at the present time.
Rice. 2
Improved loudness can be achieved by connecting additional RC circuits, as in Fig. 3. In addition, a variable resistor of a widely used value is used here (you can put 47 ... 68 kOhm). In this case, the function of a low-frequency corrector will be performed not only by the T-shaped filter R2C3R3, but also by the introduced additional circuit R7C4. In fact, this will already be a low-pass filter (LPF) of the second order, 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 introducing a high-pass filter C2R5R6C5R7 in addition to the traditional circuit R1C1.
It should be noted that in this circuit, the loudness compensation in the high frequency region is slightly higher than necessary. This was done deliberately and was due to the purely subjective perception of musical soundtracks at home. A slight dip in the frequency response at a frequency of 3.5 kHz in the lower position of the slider of resistor R4 is due to the phase shift between the signals of this frequency that passed through the high-pass filter and resistor R4. When elements C2, R5, R6, C5 are excluded, this dip disappears, and the additional rise in frequency response at higher frequencies also disappears, which brings the corrector parameters to the standard ones recommended for such loudspeakers 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.
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 resistor R7 in the control circuit. But in practice, such a small decrease in the adjustment range is, as a rule, uncritical.
Rice. 3
A circuit of such loudness can be used in the development and manufacture of new sound amplification equipment, as well as for modifying existing amplifiers, radio tape recorders, and receivers. If such devices use conventional volume controls, that is, simply a variable resistor of appropriate resistance that is not included in the feedback circuit of the amplifier nodes, then you can turn on this circuit instead. But in this case, it is necessary to take into account the output resistance of the previous stage (before the volume control) - it should be significantly less than the resistance of resistor R5, and the input resistance of the next stage after the control, which should be greater than the resistance of resistor R3. The greater the difference between these resistances, the better the load matching will be ensured and the equipment as a whole will work better. As a last resort, 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 cascades (Fig. 4).
Rice. 4
The circuits represent additional amplification stages on the K157UD2 microcircuit (two amplifiers in one housing, the location of the pins of both channels is shown) and a transistor. Any operational amplifier can be used as DA1, for example K140UD6, UD7, K153 UD1, UD2 and others, taking into account the pinout of their outputs and correction circuits (here these are capacitors C2). The feedback coefficient depends on the value of resistor R2. The lower the value of this resistor, the lower the cascade gain and the less nonlinear distortion. Therefore, the resistor should be set as low as possible!
The transistor in the second circuit can be replaced with KT315, KT342, KT306. The resistance of resistor R2 here depends on the supply voltage (the lower the supply voltage, the lower the resistance), and resistor R1 sets the direct current operating mode of the transistor. By selecting this resistor, in rest mode (without an input signal) you need to set the output (transistor collector) to a voltage equal to half the supply voltage.
I am attaching drawings of printed circuit boards:
- pl1 – matching stage board on a transistor;
- pl2 – matching stage board on MS K157UD2 (two channels);
- pl3 – board of a loud-compensated volume control according to the diagram in Fig. 3.
List of radioelements
| Designation | Type | Denomination | Quantity | Note | Shop | My notepad | |
|---|---|---|---|---|---|---|---|
| Figure 2 | |||||||
| C1 | Capacitor | 51 pF | 1 | To notepad | |||
| C2 | Capacitor | 6800 pF | 1 | To notepad | |||
| R1, R3 | Resistor | 220 kOhm | 2 | To notepad | |||
| R2 | Resistor | 820 kOhm | 1 | To notepad | |||
| R4 | Variable resistor | 2 MOhm | 1 | To notepad | |||
| Figure 3 | |||||||
| C1 | Capacitor | 680 pF | 1 | To notepad | |||
| C2 | Capacitor | 0.01 µF | 1 | To notepad | |||
| C3, C4 | Capacitor | 1 µF | 2 | To notepad | |||
| C5 | Capacitor | 0.047 µF | 1 | To notepad | |||
| R1 | Resistor | 6.8 kOhm | 1 | To notepad | |||
| R2 | Resistor | 3.3 kOhm | 1 | To notepad | |||
| R3 | Resistor | 12 kOhm | 1 | To notepad | |||
| R4 | Variable resistor | 68 kOhm | 1 | To notepad | |||
| R5 | Resistor | 910 Ohm | 1 | To notepad | |||
| R6 | Resistor | 47 Ohm | 1 | To notepad | |||
| R7 | Resistor | 200 Ohm | 1 | To notepad | |||
| Figure 4. Op-amp matching stage | |||||||
| DA1 | Chip | K157UD2 | 1 | To notepad | |||
| C1 | Capacitor | 1 µF | 1 | To notepad | |||
| C2 | Capacitor | 15 pF | 1 | ||||
HIGH COMPENSATION VOLUME CONTROL
first order, ensuring the steepness of the rise in the frequency response of the regulator in the low frequency region of 12 dB per octave. High-frequency correction was achieved by introducing a second-order high-pass filter (HPF) C2R5R6C5R7 in a traditional R1CI circuit
The vast majority of loudness-compensated volume controls are built according to the scheme shown in Fig. I. The regulator itself is a variable resistor with two taps, to the motor of which a high-frequency correction circuit (RICI) is connected. and to the taps - low-frequency (R3C2 and R4C3).
The main disadvantage of such volume controls is the small depth of sound in the region of lower audio frequencies. So. in ||] it is noted that all finely compensated volume controls using variable resistors with one or more taps do not allow obtaining the required characteristics, since with this method of adjustment, reducing the volume causes a progressive weakening of the components of the middle and higher audio frequencies, which, as the regulator slide moves downwards (according to the scheme) captures an increasingly wider part of the spectrum of the signal reproduced by the amplifier. In confirmation of what was said in Fig. Figure 2 shows the frequency response of the TBN-compensation circuits of the volume control using a variable resistor with two taps (solid lines) and equal volume curves (dashed lines). A comparison of these curves shows that the deviation of the actual frequency response of tonkompensacin from the curves of equal loudness is especially large in the low-frequency region at low volume levels.
For radio amateurs who do not have the opportunity to purchase variable resistors with taps, in the 60s they proposed (4) a circuit-compensated volume control circuit using a conventional resistor of group B without taps (Fig. 3). High-frequency correction is carried out here by the RICI target, low-frequency correction is carried out by the T-shaped filter R2C2R3. highlighting the low-frequency components of the signal and transmitting them to the output with a weakened resistor R4 depending on the position of the slider. The frequency response of tonkompensacin of this regulator is approximately the same. as well as devices using a variable resistor with two taps.
Improvement of tonkompensacin can be achieved by connecting additional R&uencA (see Fig. 4). In this case, the functions of a low-frequency corrector will be performed not only by the T-shaped filter R2C3R3, but also by the additionally introduced target R7C4. In fact, we are already dealing with a low-pass filter (LPF) of the second
It should be noted that in this regulator the loudness compensation in the high frequency region is slightly higher than necessary. This was done deliberately, since subjective tests ■ at home have so far revealed the advisability of a greater increase in the frequency response at higher frequencies at low volume levels compared to the value recommended in (3). If necessary, it is not difficult to bring the loudness of the high-frequency region to standard: for this it is enough
exclude elements C2. R5, R6. C5.
