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Passive tone controls
In this article, readers are offered a number of different tone controls in terms of circuitry and functionality, which can be used by radio amateurs in the development and modernization of sound reproducing equipment. The main disadvantage of the recently popular active tone controls is the use of deep frequency-dependent feedback and large additional distortions that they introduce into the regulated signal. That is why it is desirable to use passive regulators in high-quality equipment. True, they are not without flaws. The largest of them is a significant signal attenuation corresponding to the control range. But since the depth of tone control in modern sound reproducing equipment is small (no more than 8 ... 10 dB), in most cases it is not required to introduce additional amplification stages into the signal path. Another, not so significant, disadvantage of such regulators is the need to use variable resistors with an exponential dependence of resistance on the angle of rotation of the engine (group "B"), which provide smooth control. However, the simplicity of design and high quality indicators still incline designers to use passive tone controls. It should be noted that these regulators require a low output impedance of the preceding stage and a high input impedance of the subsequent one.
Developed by the English engineer Baksandal in 1952, the tone control [1] has become, perhaps, the most common frequency corrector in electroacoustics. The classical version of the circuit consists of two first-order links forming a bridge - low-frequency R1C1R3C2R2 and high-frequency C3R5C4R6R7 (Fig. 1a). The approximated logarithmic amplitude-frequency characteristics (LAFC) of such a controller are shown in Fig. 1b. The calculated dependences for determining the time constants of the inflection points of the AFC are also given there. Theoretically, the maximum achievable frequency response slope for first-order links is 6 dB per octave, but with practically implemented characteristics, due to a slight difference in the inflection frequencies (no more than a decade) and the influence of previous and subsequent stages, it does not exceed 4 ... 5 dB per octave. When adjusting the tone, the Baksandal filter only changes the slope of the frequency response without changing the inflection frequencies. The attenuation introduced by the regulator at medium frequencies is determined by the ratio n=R1/R3. The frequency response control range in this case depends not only on the attenuation value n, but also on the choice of inflection frequencies of the frequency response, therefore, to increase it, the inflection frequency is set in the mid-frequency region, which, in turn, is fraught with the mutual influence of adjustments. In the traditional version of the considered controller R1/R3=C2/C1=C4/C3=R5/R6=n, R2=R7=n*R1. In this case, an approximate coincidence of the frequencies of the inflection of the frequency response in the region of its rise and fall is achieved (in the general case, they are different), which ensures a relatively symmetrical regulation of the frequency response (the fall, even in this case, inevitably turns out to be steeper and more extended). With the commonly used n = 10 (for this case, the minimum values of the element ratings are indicated in Fig. 1, a-3, a) and the choice of crossover frequencies near 1 kHz, the tone control at frequencies of 100 Hz and 10 kHz relative to the frequency of 1 kHz is + - 14 ...18 dB. As noted above, to ensure smooth control, the variable resistors R2, R7 must have an exponential control characteristic (group "B") and, in addition, to obtain a linear frequency response in the middle position of the regulators, the ratio of the resistances of the upper and lower (according to the circuit) sections of the variable resistors at this must also be equal to n. With "Hyend" n=2...3, which corresponds to a control range of +-4...8 dB, it is quite acceptable to use variable resistors with a linear dependence of resistance on the angle of rotation of the engine (group "A"), but at the same time it is somewhat coarsened adjustment in the frequency response decline area and stretches in the rise area, and a flat frequency response is obtained by no means in the middle position of the regulator engines. On the other hand, the resistance of the dual variable resistor sections with linear dependence is better matched, which reduces the frequency response mismatch of the channels of the stereo amplifier, so that uneven regulation in this case can be considered acceptable. The presence of the resistor R4 is not important, its purpose is to reduce the mutual influence of the links and bring together the frequency response inflection frequencies in the high frequency region. As a rule, R4=(0,3...1,2)*R1. As shown below, in some cases it can be completely abandoned. The above "basic" version of the regulator is usually used in high-end radio equipment. In household appliances, a somewhat simplified version is used (Fig. 2). The approximated logarithmic amplitude-frequency characteristics (LAFC) of such a controller are shown in Fig. 2b. The simplification of its high-frequency link led to some vagueness of the regulation in the region of higher frequencies and to a more noticeable influence of the previous and subsequent cascades on the frequency response in this region. A similar corrector for n = 2 (with variable resistors of group "A") was especially popular in simple amateur amplifiers [2] of the late 60s ... early 70s (mainly due to low attenuation), but soon the value of n has increased to the usual values today. Everything said above regarding the range of regulation, matching and choice of regulators is also true for a simplified version of the corrector. If we abandon the requirement of symmetrical regulation of the frequency response in the areas of their rise and fall in the frequency response (by the way, there is practically no need for a decline), then the circuit can be further simplified (Fig. 3, a). The LAFC of the regulator shown in Fig. 3b correspond to the extreme positions of the sliders of the resistors R2, R4. The advantage of such a regulator is simplicity, but since all its characteristics are interconnected, it is advisable to choose n=3...10 for the convenience of regulation. It should be noted that as n increases, the degree of rise increases, and the degree of decline decreases. Everything said above about the traditional versions of the Baksandal corrector fully applies to this extremely simplified version. However, the Baksandal circuit and its variants are by no means the only possible implementation of a passive two-band tone control. The second group of tone controls is not made on the basis of bridges, but on the basis of a frequency-dependent voltage divider. As an example of an elegant circuitry solution for a tone control, we can cite a tone block used in various variations in tube electric guitar amplifiers. The "highlight" of this regulator is the change in the inflection frequencies of the frequency response in the process of regulation, which leads to interesting effects in the sound of a "classical" electric guitar. Its basic scheme is shown in Fig. 4, a, and the approximated LAFC - in Fig. 4, b. The calculated dependences for determining the time constants of the inflection points are also given there. It is easy to see that adjustment in the region of lower audio frequencies changes the inflection frequencies without changing the slope of the frequency response. When the slider of the variable resistor R4 is in the lower (according to the scheme) position, the frequency response at lower frequencies is linear. When the engine is moved upwards, a rise appears on it, and the inflection point in the process of regulation shifts to the region of lower frequencies. With further movement of the slider, the upper (according to the circuit) section of resistor R4 begins to shunt resistor R2, which causes a shift in the high-frequency inflection point to higher frequencies. Thus, when adjusting, the rise of low frequencies is complemented by the fall of the middle ones. The high-frequency regulator is a simple first-order filter and has no special features. On the basis of this scheme, you can build several options for timbre blocks that allow you to adjust the frequency response in the low and high frequencies. Moreover, in the region of lower frequencies, both an increase and a decrease in the frequency response are possible, and in the region of higher frequencies, only an increase. A variant of the tone block with frequency response control of the AFC inflection in the low-frequency region is shown in Fig. 5, a, and its LAF - in Fig. 5 B. Resistor R2 controls the inflection frequency of the frequency response, and R3 controls its slope. The combined action of regulators allows you to get significant limits and greater control flexibility. A diagram of a simplified version of the timbre block is shown in Fig. 6, a, its LFC - in Fig. 6b. It is, in essence, a hybrid of the low-frequency link shown in Fig. 3a and the high-frequency link shown in Fig. 4a. By combining the frequency response control functions in the low-frequency and high-frequency regions, you can get a simple combined tone control with one control, very convenient for use in radio and car equipment. Its schematic diagram is shown in Fig. 7, a, and LACHH - in Fig. 7b. In the lower position according to the scheme of the engine of the variable resistor R1, the frequency response is close to linear over the entire frequency range. When moving it up, a rise in the frequency response appears at lower frequencies, and the low-frequency inflection point in the process of regulation shifts to lower frequencies. With further movement of the engine, the upper (according to the scheme) section of the resistor R1 turns on the capacitor C1, which leads to a rise in higher frequencies. When replacing the variable resistor R1 with a switch (see Fig. 8, a and 8, b), the considered regulator turns into the simplest tone register (position 1-classic, 2-jazz, 3-rock), popular in the 50s-60s and again used in the equalizers of radio tape recorders and music centers of the 90s. Despite the fact that in the field of tone control, it would seem, everything has long been said, the variety of passive corrective circuits is not limited to the proposed options. Many forgotten circuit solutions are now experiencing a rebirth at a new qualitative level. Very promising, for example, a volume control with separate loudness control for low and high frequencies [3]. Literature
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