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Loudly compensated volume controls
It is known that when the volume level is lowered, a person perceives the low-frequency and high-frequency components of the sound signal worse. For this reason, frequency-dependent (finely compensated) volume controls are installed in modern sound-reproducing devices, which ensure the rise of high and low frequencies at a low volume level in accordance with equal loudness curves. Thus, they improve the subjective perception of the sound picture. The published article talks about the most common thin-compensated volume controls. Coincidence of the loudness curves with the curves of equal loudness, even for an ideally designed thinly compensated volume control (TRG), is possible only with a strictly defined transmission coefficient of the entire signal path, starting from the signal source and ending with the speaker. In other words, the volume level at which timbre balancing was performed during the recording process should be achieved at the same position of the volume control for any signal source. The deviation of the transmission coefficient from the calculated one leads to a violation of the tonal balance. In combined sound reproducing equipment with built-in speakers, all links of the path are matched in terms of signal level, and this condition, although with some reservations, is met. Amplifiers of block devices have to work with signal sources with a sufficiently large range of output voltages (0,25 ... 1,5V) and speakers of unknown sensitivity (84 ... TRGs use a maximum volume control or input sensitivity controls, and more recently, loudness depth controls. Loudness is usually implemented by frequency-dependent dividers (rarely filters) associated with the volume control. The fundamental drawback of most known regulators on variable resistors with taps is the insufficient degree of frequency response correction in the low-frequency region at low volume. For a better approximation to the curves of equal loudness, it is necessary to use variable resistors with several taps [1] or to implement a controller with a distributed frequency correction [2]. However, such control devices are very difficult to implement and therefore are rarely used.
The greatest application in both industrial and amateur designs was received by TEG on a single-tap resistor, the circuit of which is shown in fig.1. (in this and all subsequent figures, next to the TRG diagram, its regulating characteristics are shown). The tap is usually made from 1/10 of the total resistance of the variable resistor (counting from the bottom according to the output circuit), which corresponds to approximately 1/4 ... 1/3 of the angle of rotation of the regulator engine. Connecting to the tap of the RC circuit turns the regulator into a frequency-dependent divider. The R1C1 circuit provides a rise in the frequency response at the highest frequencies of the audio range, and R2C2 - at the lowest. However, such regulators have significant drawbacks. So, the degree of frequency response correction provided by them in the low-frequency region is clearly insufficient (no more than 8 ... 10 dB at a frequency of 50 Hz), and in the process of adjustment, a stepwise nature of the correction is noticeable. As the volume decreases after the passage of the tap, the degree of correction no longer changes, whereas it should be maximum at low volume. Attempts to increase the degree of correction by reducing the resistance of the resistor R2 lead to the appearance of a characteristic dip in the frequency response at medium frequencies at the time of the tap. And yet, despite these shortcomings, many designers of AF amplifiers choose just such a TRG because of its simplicity. The ratings of the elements indicated in Fig. 1 are typical for most designs. Sometimes the resistor R1 may be missing. In this case, the capacitance of the capacitor C1 should be approximately half as much.
A somewhat greater degree of frequency response correction in the low-frequency region is provided by the regulator, the circuit of which is shown in fig.2. Its prototype was used in Philips radio receivers in the 50s [3]. Examples of the use of such regulators in modern industrial designs are unknown to the author. The R2C2R3 circuit forms a low-pass filter, the output signal of which is fed to the regulator tap. This TRG has the same disadvantages as the previous one, although to a lesser extent. The insufficient degree of rise in the frequency response at lower frequencies for the regulators that were discussed is explained by the use of first-order corrective circuits. In the TRG (Fig. 3), the depth of correction at low volume is increased by introducing the R4C3 circuit, which together with the section of the variable resistor from the engine to the tap, forms a second frequency-dependent divider. The use of a two-stage correction allows you to increase the frequency response at a minimum volume of up to 20 ... 26 dB at a frequency of 50 Hz. The reverse side of this advantage is the narrowing of the volume control range to 45-50 dB, which, however, in most cases turns out to be quite sufficient.
In some cases, the use of variable resistors with taps is undesirable. On pic.4 shows a TRG circuit on a variable resistor without taps, using a filter method for correcting the frequency response. The R2R3R4C1C2 filter, which suppresses the middle frequencies of the signal, starts working at low volume levels, due to which the low and high frequencies of the audio range are raised. Variants of such a regulator are widely used in amateur developments. The degree of rise in its frequency response at lower frequencies at minimum volume can be increased by adding a corrective circuit, similar to that shown in Fig. 3.
However, all the schemes considered provide only a fixed and by no means ideal frequency response correction and in some cases require the use of tone controls to adjust the tonal balance. Attempts to create a TRG with adjustable equalization or to combine a TRG with tone controls were made as early as the 50s. Probably, one of the first implementations of this idea was the volume control of the receiver of the German company "Kontinental" [3]. In the circuit, along with a passive TRG on a resistor with two taps, an adjustable frequency-dependent feedback was used, fed to the regulator from the output transformer of the amplifier. The original scheme of the combined passive node for volume and tone controls in a transistor amplifier is shown on pic.5 [4]. Here, the variable resistor R3, together with the circuits R1C1, R2C2, R4C4, forms a correction adjustment circuit at higher frequencies. Chain C5R5, connected to the output of the volume control R7, provides low-frequency correction. A slight rise in the frequency response at lower frequencies in the position of minimum attenuation is created by resistor R2. The depth of the low-frequency correction is regulated by the resistor R6.
Wide ranges for adjusting the frequency response now seem redundant, so it makes sense to exclude capacitor C2, replace capacitor C1 and resistor R1 with a jumper, and reduce the resistance of the variable resistor R6 to 100 kOhm. After such refinement, the decrease in the frequency response in the region of higher frequencies is eliminated, and the frequency response adjustment range at lower frequencies is narrowed to 10 dB. A diagram of a simple TEG developed by the author with adjustable correction based on a resistor with a tap is shown in fig.6. The adjustment of the correction depth simultaneously for the lower and higher sound frequencies is carried out by a variable resistor R1. If adjustment in the higher frequency region is not required, you can eliminate the capacitor C2, and reduce the resistance of the resistor R3 to 10 kOhm. The disadvantage of such a TRG (as, indeed, of all circuits with first-order circuits) is the insufficient correction of low frequencies at the lowest volume. As already noted, by adding a corrective circuit, similar to that shown in Fig. 3, the degree of rise in the frequency response at lower frequencies can be increased. Using the proposed principle, it is easy to introduce a loudness regulator into industrial sound-reproducing equipment.
In the following TRG circuit (Fig. 7), also developed by the author, both the C3R6R7 correction filter and the R2R3C2 frequency-dependent divider are used simultaneously, due to which a wide correction range is achieved. Variable resistor R2 - volume control, R1 - low-frequency correction control, R4 - high-frequency.
Publication: www.bluesmobil.com/shikhman
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