Combined frequency response control unit. The art of audio

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Combined frequency response control unit

The art of audio

The technique of high-quality sound reproduction is being improved every year, but designers have not yet given unambiguous answers to a number of questions. This applies, in particular, to the problems of constructing volume and tone controls.

So, at the turn of the 80s, it was proposed to completely abandon the tone controls (RT) and replace them with carefully designed thinly compensated volume controls (TRG). However, the experience of operating such TRGs with the use of variable resistors with taps has shown that their frequency response differs significantly from equal loudness curves, especially at loudness levels of -35 ... -50 dB, which means that the need for RT remains. The same can be said about the TRGs described in recent years in the journal "Radio" [1-3].

As for the RT, there are also disagreements regarding their technical characteristics. In particular, the requirement of symmetry of regulation does not seem indisputable, when the RT must necessarily provide the same rise and blockage of the frequency response. Practice, in any case, does not confirm this. For example, when operating small-sized speakers in ordinary residential premises, the need for a decrease in frequency response at lower, and even more so at higher sound frequencies, practically does not arise. At the same time, when using passive RT, it is necessary to compensate for the attenuation introduced by them at medium audio frequencies and reaching 20 dB in other cases.

According to [1], due to the insufficient efficiency of the low-frequency drivers of small-sized speakers and the increased attenuation of the high-frequency components of the audio signal in residential premises, the frequency response of the TRG at the edges of the operating range should pass above the curves of equal loudness. Moreover, the required level of overcompensation depends on the acoustic properties of the premises, the power of the UMZCH and the characteristics of the speakers.

In view of the foregoing, readers are offered a frequency response control, in which there is no traditional separation of the volume and tone control functions, and the features of sound perception by the human ear are taken into account to a much greater extent.

The combined frequency response control unit is based on a thinly compensated volume control published in [4]. It additionally includes elements for adjusting the degree of loudness and a maximum volume control, which makes it possible to achieve a more accurate correspondence between the frequency response of the TRG and the characteristics of the listening room, UMZCH and speakers.

Combined frequency response control unit

The variable resistor R1 regulates the frequency response in the region of higher audio frequencies, R4 - in the region of the lower ones. In the upper position of the resistor R4 engine according to the scheme, the frequency response has a rise, and in the lower position - a blockage in the region of higher sound frequencies. When installed in the upper position of the resistor R4 engine, the frequency response has a rise in the lower audio frequencies. In the lower position of the slider of this resistor, the frequency response is horizontal. Resistors R3, R5, R6 perform the functions of volume, maximum volume and balance controls, respectively.

The nature of the frequency response correction and the range of regulation depend on the position of the volume control slider R3. In the upper (according to the scheme) position of its slider (maximum volume level), the frequency response will have the form shown in Fig. 2, a. This volume level is taken as 0 dB. Frequency response at loudness levels of -20 and -40 dB are shown in Fig. 2, b and 2, c, respectively.

The following ratios were used to calculate the ratings of the regulator elements: R1=R3=R4=R6=R, R5=5*R, R2=0,4*R, R7=0,2*R; C1(nF)=100/R(kΩ), C(nF)=10000/R(kΩ).

The calculated coefficients in the formulas are empirical and are advisory in nature. In the instance of the regulator implemented by the author, R is assumed to be 100 kOhm. The standard values of resistors and capacitors corresponding to this value can have deviations of up to 30%. For example, R1=R3=R4=R6=100 kΩ; R%=470 kΩ; R2=39...43 kOhm; R7=10...22 kOhm; 1=750...1200 pF; C2=0,1uF.

Combined frequency response control unit

Variable resistors R1, R3, R4 must have adjustment characteristics B, R4, R6 - A or M. It is permissible to use all variable resistors with characteristic M. The output resistance of the amplifier stage connected before the regulator must be no more than 0,1 * R, and the input impedance of the stage following it is at least R. The presence of the maximum volume control R5 is not necessary, its functions can be successfully performed by the input sensitivity controls (if any).

The RMG engine should be installed as close as possible to the natural sound of phonograms in a particular room and when using specific speakers, and it is not necessary to display the RMG axis on the front UMZCH panel.

The regulator was tested in conjunction with a stereo UMZCH with a nominal output power of 10 W per channel (A2030V microcircuits - analogue of K174UN19) and 15AC315 acoustic systems, the Vega PKD 122S PKD served as a signal source. Subjective examination confirmed the above characteristics.

An important advantage of the described regulator is that in the position of minimum attenuation, the relative rise in the frequency response does not exceed 3 dB, which makes it possible to avoid overloading the sound reproducing path even when using UMZCH and speakers with a small power margin. In addition, the low attenuation introduced by the regulator at medium frequencies reduces the required gain of the AF path.

The disadvantage of the regulator can be considered a narrowing of the volume control range (at the maximum depth of loudness, the attenuation does not exceed 40 dB). However, this drawback is not so significant, since, firstly, the attenuation can be increased by the maximum volume control, and secondly, when using sound reproducing equipment with a rated output power of up to 20 W per channel in modern residential premises, the volume control range is expanded over 40 ...45 dB is hardly appropriate.

Literature

S. Fedichkin. Loud volume control - Radio No. 9/1984 p.43,44;
P. Zuev. Volume control with distributed frequency correction - Radio No. 8/1986 p.49-51;
I. Pugachev. Loud volume control - Radio No. 11/1988 p.35,36;
A. Shikhatov. Loudly compensated volume control in a tape recorder. - Radio No. 6/1992 p.47

Publication: www.bluesmobil.com/shikhman

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