Crossover LC filters in multiband UMZCH. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Transistor power amplifiers

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It is known that multiband amplifiers significantly improve the sound quality of sound reproducing equipment. To separate frequencies in AF amplifiers, RC filters or complex active filters are most often used. According to the author, in terms of circuitry, especially when forming several frequency bands, it is much easier to use LC filters. A variant of the AF amplifier, in which just such a filter is applied, is described in the article published below.

The scheme for including an LC filter in the amplifying path is shown in fig. 1. The composition of the filter includes the inductance of the coil Lf, the capacitance of the capacitor Cf, the output impedance of the cascade on the transistor VT1 and the input impedance of the cascade on the transistors VT2, VT3. The crossover frequency provided by such a filter coincides with the resonant frequency of the circuit

For effective frequency separation, it is important that the quality factor of the circuit has a fairly noticeable value, for example, it would be at least 5. To comply with this condition, the cascades on transistors VT2, VT3 are made according to a common base circuit that provides low input resistance Rin. Naturally, the filter itself should not have a pronounced resonance. The attenuation required for this is introduced into it from the side of the cascade on the transistor VT1, the output resistance Rout of which is approximately equal to the resistance of the resistor R3. The value of the output impedance is selected depending on the acoustic design of the loudspeaker heads.

Let us assume that during the reproduction of an audio signal, the summation of the sound power of various frequency channels is provided. This can happen if the loudspeaker heads reproducing different frequencies are spaced apart in space and their radiation axes are turned relative to each other at an angle close to 90°. Then, as follows from the theory of calculation of crossover filters, to equalize the dependence of the total power on the frequency, the output impedance Rout should be equal to: - characteristic resistance of the circuit LfSf.

It is also possible to ensure the addition of the amplitudes of sound vibrations, for example, by placing sound heads reproducing different frequencies next to each other on the same reflective board. Moreover, the heads should be turned on in antiphase, since at the separation frequency fo the currents flowing through the coil with inductance Lf and the capacitor with capacitance Cf are phase-shifted by 180 °. Equalization of the total frequency response is provided in this case with the output resistance Rout=0,5p.

On fig. 2 shows the logarithmic frequency response of the filter channels. As can be seen from the figure, the slope of the frequency response out of the passbands reaches -40 dB / dec, that is, it has the same value as the slope of the decays of the two-section RC filter. The influence of the input resistance of the cascades on transistors VT2, VT3, Rin, as well as the active resistance (rL) of the inductor Lf causes a decrease in the steepness of the declines at some distance from the crossover frequency fо to -20 dB/dec.

According to the author, it is desirable that the length of the frequency response decay sections with a steepness of -40 dB / dec should be at least 10 ... 15 dB along the axis of the transmission coefficients Uout / Uin. In this case, the main sound power will be divided between the frequency channels and the course of the further decline in the frequency response is less significant. The required shape of the frequency response is achieved with the quality factor of the circuit LfSfQ=p/(2Rin+rL)>5, as indicated at the beginning of the article.

You can adjust the crossover filter using a sound generator and an AC voltmeter. Before setting the resistor R3 in place, you should install a resistor whose resistance would be greater than the characteristic resistance of the circuit p. During the tuning process, it is necessary to ensure that the supply voltage on the transistor VT1 is sufficient for its normal operation. Now, having applied voltage from the sound generator to the input of the transistor VT1 and measuring the voltage on its collector with a voltmeter, they make sure that the quality factor of the oscillatory circuit is greater than 5, then determine the resonance frequency, that is, the separation frequency fo, and, if necessary, select the capacitance of the capacitor Cf. After that, the characteristic impedance p of the circuit is calculated from the values ​​of fo and Сf and the required output impedance Rout of the first stage is calculated. In conclusion, a resistor with a resistance equal to the calculated value of Rout is soldered in place of the resistor R3.

At first glance, it may seem that in practice it is possible to create an LC filter only with a sufficiently high crossover frequency, since a coil with high inductance and low losses is needed. This, however, is not the case. Let's explain this situation with an example.

The author assembled a four-band amplifier with similar filters (Fig. 3).

The entire audio frequency band is first separated at 850 Hz using the L1C4 filter. Then, frequencies below 850 Hz are divided by an L3C8 filter at 220 Hz, and frequencies above 850 Hz are divided by an L2C7 filter at 3,2 kHz.

Inductors require magnetic circuits with suitable magnetic properties and possibly a large ratio of the cross-sectional area of ​​the magnetic circuit to the length of the field line. These requirements are met by ferrite rings of small diameter, and if necessary, the magnetic circuit can be made up of several rings. All coils are wound with PEV-2 0,12 wire. K12x5x5,5 rings made of 1000 NN ferrite were used as a magnetic circuit. Coil L1 is wound on two rings glued to each other and contains 520 turns, L2 - on one ring and contains 400 turns, and L3 - on four rings and consists of 520 turns.

The inductor of the lowest-pass filter L3 was made as follows. The author glued the ferrite rings together so that two cylinders of equal height were obtained, touching along the generatrix. I insulated the inner surface of the rings with paper. The winding was carried out with a bundle of ten wires, which were then soldered into a single winding. All solder joints for isolation and fixation of the relative position were clamped between two strips of adhesive tape.

The filters used capacitors KM, KLS. For resistors and capacitors selected during setup, the printed circuit board provides mounting locations for two parts.

Coil L3 is slightly affected by pickups from the side of the mains transformer. Its installation location and orientation had to be chosen to a minimum of interference.

The device worked normally at a signal voltage of 0,1 V. There were no signs of nonlinear distortion due to saturation of the magnetic circuits of the inductors. In conclusion, a few words about the entire sound reproducing system. The Vega-206 stereo electric player was used as a soundtrack source. In the four-band amplifier, power amplifiers from the Accord-stereo electrophone are used, and in the loudspeakers, dynamic heads 4GD-28, 10GD-35, 10GD-36 spaced apart in space in various acoustic design are used. The sound of the installation was distinguished by high purity and "transparency".

It is advisable to install metal-film capacitors in crossover filters, for example, K73-16 or K73-17, since ceramic capacitors have too large TKE for the required capacitance values.

Author: N. Boyko, Voronezh

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