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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Loudly compensated volume control with active bass correction. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment The article describes a volume control with loudness and active bass correction. The device allows you to select the required frequency response correction depth in accordance with the acoustic conditions of the room and the sensitivity of a particular speaker system. It is known that with a decrease in the average volume level, the sensitivity of the human ear to the greatest extent falls to the lowest frequencies (LF) of the sound spectrum. To compensate for this physiological feature of hearing, sound reproducing equipment requires a corrective bass boost: 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 the equal loudness curves, the slope of the rise should increase as the frequency decreases: 6 dB per octave starting at 250 Hz and 12 dB per octave below 100 Hz [1]. Most of the well-known circuits of thinly compensated volume controls (TKRG), with the possible exception of the most complex ones that have not found wide application, do not provide the required law and depth of correction. In the most common TKRG with a variable resistor having a tap (or without taps) [2], the depth of LF correction is not more than 15 dB, and its steepness decreases at frequencies below 100 Hz. For an example in fig. 1 shows the typical frequency response of a passive TKRG on a variable resistor without taps [2]. It can be seen that the corrective rise at a frequency of 50 Hz with a regulator gain of -40 dB is 13 dB, the slope below 100 Hz does not exceed 3 dB per octave, which is completely insufficient. Similar characteristics have also TKRG on a resistor with one tap.
In operation, such controls create an unpleasant effect: when the volume is reduced, the depth of the sound is lost and a tendency to "mumbling" appears. Attempts to increase the degree of correction at the lowest frequencies by adding an RC circuit to the break in the common wire of a variable resistor lead to a narrowing of the volume control range. The volume in this case does not decrease to zero, which is very inconvenient in practice. Another disadvantage of the mentioned devices is the incorrect change in the correction as the volume is adjusted. A noticeable correction of the frequency response often occurs at the middle position of the regulator, when the actual volume (sensitivity) is still high. As a result, the tonal balance is disturbed in the most frequently used area of average sound volume. Unfortunately, all the listed shortcomings are also characteristic of electronic TKRG, made on specialized microcircuits. On fig. Figure 2 shows the frequency response of a very complex Toshiba TC9235 regulator, which has a low noise level (less than 2 μV) and non-linear distortion (less than 0,01%), multi-stage digital volume control, convenient push-button control, etc. [3]. With all this, the regulator provides fine correction no better than the already considered TKRG.
In consumer sound reproduction devices, the frequency range below 100 Hz is considered "problematic" for the end links of the path as well. So, a small-sized acoustic system rarely has a lower cut-off frequency of less than 50...60 Hz at a level of -3 dB. Typically, the sound pressure drop begins already at a frequency of 100 Hz. Sometimes high-quality equalizers or special bass equalizers based on high-order filters are used to compensate for it. But at the same time, one has to take into account the limited overload capacity of the UMZCH at low frequencies and reduce the degree of correction at the same time as increasing the volume. Applying signals below the resonant frequency to the dynamic heads only leads to an increase in distortion. Currently, there are special bass auto-correctors (X-Bass, etc.), which dynamically form the frequency response, taking into account all of the above factors. But they are most often closed "proprietary" developments made on specialized microcircuits without marking [4]. The proposed device solves these problems in a simpler way. During its development, new circuit solutions were used, obtained by computer simulation 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 TKRG itself with a bass corrector, which "finishes" the frequency response in the frequency range of less than 100 Hz and regulates its course depending on the position of the volume control. 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 a 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.
Passive TKRG is made on the elements R1-R4, C1, C2 according to the well-known scheme (see Fig. 1) in a simplified version. The R3R4C1C2 filter lowers the mids depending on the position of the R2 slider. The filter parameters are chosen to provide the maximum possible bass boost. RF correction does not present any problems and is set by the capacitance of capacitor C1. From the output of the passive TKRG, through the C3R6 circuit, the signal is fed to the inverting input of the op-amp DA1.1, which amplifies the signal (up to 14 dB) and forms the frequency response by two OOS circuits. The first - through the resistor R5, the elements of the TKRG, including the volume control R2, and the input circuit C3.R6; the second - through the T-shaped link R7 - R10 and the DA1.2 chip with related elements. A gyrator is assembled on the DA1.2 chip, imitating 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 attenuated to the maximum extent and a hump of the frequency response of the OA DA1.1 is formed. In this case, the slope of the frequency response below 100 Hz reaches 10 dB per octave, and the total rise (adjustable) at a frequency of 45 Hz is +27 dB relative to a frequency of 2 kHz with the volume control set to 41 dB (Fig. 4). These parameters are close to the required values of equal loudness characteristics.
The limitation of the amplitude of signals with frequencies below the resonant AC is formed in the device due to the natural slope of the resonant curve of the analog of the LC circuit on DA1.2 and two HPFs: C3R6 and C6Rin, where Rbx is the input impedance of the cascade following the regulator. For this regulator, the equivalent load resistance is assumed to be 100 kOhm, for another input resistance, the capacitance C6 should be recalculated so that the time constant C6Rbx 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 FOS was proposed by the author in the article [5], where the principle of its operation is described in detail. The result is reduced to an 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 depth of feedback, you can eliminate the overload of the UMZCH at volume levels close to the maximum, similar to dynamic bass equalizers. The effectiveness of the feedback through the resistor R5 is illustrated by the simulated frequency response (Fig. 5).
The curves are calculated for the version with negative feedback (R5 = 12 kΩ) and without it (R5 = 1 MΩ). As can be seen from the graphs, the OOS acts selectively and only low frequencies are weakened. With the volume control set to -20 dB, the attenuation is small - about 7 dB, and at the maximum gain it reaches 26 dB. At the same time, OOS completely smooths out the bass correction peak, aligning the frequency response. Without this, the UMZCH would have been overloaded already at the middle position of the TKRG and it would have been necessary to perform manual manipulations with the bass tone control. In the right position according to the diagram of the slider of the resistor R9 and the upper resistor R13, the regulator, with the ratings indicated on the diagram, has the characteristics shown in fig. 4. However, a wide variation in the type of frequency response is possible: with a tuning resistor R9, you can adjust the depth of bass correction in the range of 0 ... + 6 dB (Fig. 6).
The range is indicated at an average sound volume; when it decreases, it increases, when it increases, it decreases, i.e. the device adaptively adjusts the adjustment depth in accordance with equal loudness curves and UMZCH overload capabilities. If desired, the variable resistor R9 can be brought to the front panel and used as a bass tone control. Its advantage lies in the fact that, unlike bridges and other RC controls, it regulates the bass, and not the entire frequency band up to 1000 Hz. For smooth tone changes, 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 harmonics (Kg - 0,003%) and good noise performance. 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 one or less with a tuning resistor R13, which will proportionally reduce the noise level. As with all TKRGs, the loudness accuracy depends on the gain of the audio path. It can be adjusted by the mentioned tuning resistor R13 or others available in the path. It is only necessary to take into account the distribution of the gain and the noise properties of the path links. By changing the signal level, by selecting the resistor R5, the tonal balance is maintained throughout the entire range of volume control. If the UMZCH is overloaded at maximum volume, the value of the resistor R5 should be reduced according to the subjective feeling of the bass content and their distortion. Other tuning options include shifting the resonant bass correction peak by selecting resistors R11, R12 for a specific speaker. Bass depth is controlled by resistor R9 as described above. In the highest quality paths, the replacement of the TL074 op amp is possible with the NE5534A. However, in simpler cases, it is quite possible to use the K157UD2A OU with the appropriate correction circuits. In this case, the harmonic coefficient increases by about an order of magnitude, and the level of intrinsic noise at a unit transmission coefficient will be no worse than -80 dB. Otherwise, the regulator is assembled on ordinary parts: MLT-0,125 resistors, small-sized KM capacitors. As a regulator R2, an imported small-sized dual variable resistor with a nominal value of 50 kOhm (type B regulation characteristic) is used. The presence in the device of resistors R3, R4, connected in parallel with the upper section R2 according to the scheme, allows the use of a variable resistor with a linear control characteristic (type A), however, in this case, the initial jump in volume is inevitable with further smooth regulation. Experimental verification and subjective listening confirmed the high quality of the regulator. The deviation of the real AFC from the simulated ones did not exceed a few decibels. The regulator's own noise level at unity gain turned out to be below the audibility limit. The operation of the regulator is characterized by the correct tonal balance at any volume, the preservation of "deep" bass at minimum volume and the absence of UMZCH overload at volume levels close to maximum. In many cases it is possible to forego the use of the conventional tone control altogether and use only the bass equalizer. Literature
Author: A.Pakhomov, Zernograd, Rostov region
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