Crossover filters for three-way loudspeakers. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Speakers

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In order to reduce intermodulation distortion during sound reproduction, the loudspeakers of Hi-Fi systems are composed of low-frequency, mid-frequency and high-frequency dynamic heads. They are connected to the outputs of amplifiers through crossover filters, which are combinations of LC filters of low and high frequencies.

Below is a method for calculating a three-band crossover filter according to the most common scheme.

The frequency response of the crossover filter of a three-way loudspeaker is generally shown in fig. 1. Here: N is the relative voltage level on the voice coils of the heads: fn and fv are the lower and upper boundary frequencies of the band reproduced by the loudspeaker; fр1 and fр2 - section frequencies.

Ideally, the output power at the crossover frequencies should be distributed equally between the two drivers. This condition is met if, at the crossover frequency, the relative voltage level supplied to the corresponding head is reduced by 3 dB compared to the level in the middle part of its operating frequency band.

The crossover frequencies should be chosen outside the region of the greatest sensitivity of the ear (1...3 kHz). If this condition is not met, due to the difference in the phases of the oscillations emitted by the two heads at the crossover frequency at the same time, a "bifurcation" of the sound may be noticeable. The first crossover frequency usually lies in the frequency range 400 ... 800 Hz, and the second - 4 ... 6 kHz. In this case, the low-frequency head will reproduce frequencies in the range fn ... fp1. mid-frequency - in the range fp1 ... fp2 and high-frequency - in the range fp2 ... fv.

One of the common options for the electrical circuit diagram of a three-way loudspeaker is shown in fig. 2. Here: B1 - low-frequency dynamic head connected to the output of the amplifier through the low-pass filter L1C1; B2 - mid-range head connected to the amplifier output through a bandpass filter formed by high-pass filters C2L3 and low-pass filters L2C3. The signal is fed to the high-frequency head B3 through the high-pass filters C2L3 and C4L4.

The calculation of capacitances of capacitors and inductances of coils is carried out on the basis of the nominal resistance of the loudspeaker heads. Since the nominal resistances of the heads and the nominal capacitances of the capacitors form series of discrete values, and the crossover frequencies can vary over a wide range, it is convenient to calculate in this sequence. Given the nominal resistance of the heads, the capacitances of the capacitors are selected from a series of nominal capacitances (or the total capacitance of several capacitors from this series) so that the resulting crossover frequency falls within the above frequency intervals.

Separation filters usually use metal-paper capacitors of the MBGO, MBGP and MBM types with a permissible deviation from the nominal capacity of not more than ± 10%. The most suitable capacitor ratings for use in filters are given in Table. 1.

The capacitances of the filter capacitors C1 ... C4 for various head resistances and the corresponding values ​​of the section frequencies are given in Table. 2.

It is easy to see that all capacitance values ​​can either be taken directly from the nominal range of capacitances. or obtained by parallel connection of no more than two capacitors (see table. 1).

After the capacitances of the capacitors are selected, the inductances of the coils are determined in millihenries according to the formulas:

In both formulas: Zg-in ohms; fp1, fp2 - in hertz.

Since the impedance of the head is a frequency-dependent quantity, the nominal resistance Zg indicated in the head passport is usually taken for calculation, it corresponds to the minimum value of the head impedance in the frequency range above the main resonance frequency to the upper cutoff frequency of the operating band. At the same time, it should be borne in mind that the actual nominal resistance of various samples of heads of the same type may differ from the passport value by ± 20%.

In some cases, radio amateurs have to use existing dynamic heads with a different nominal impedance from the nominal impedances of the low-frequency and high-frequency heads as high-frequency heads. In this case, resistance matching is carried out by connecting the high-frequency head B3 and capacitor C4 to different terminals of the L4 coil (Fig. 2), i.e. this filter coil simultaneously plays the role of a matching autotransformer. Coils can be wound on round wooden, plastic or cardboard frames with getinaks cheeks. The lower cheek should be made square; so it is convenient to attach it to the base - a getinax board, on which capacitors and coils are attached. The board is fixed with screws to the bottom of the loudspeaker box. To avoid additional non-linear distortions, the coils must be made without cores made of magnetic materials.

Filter calculation example

As a low-frequency loudspeaker head, a 6GD-2 dynamic head is used, the nominal resistance of which is Zg \u8d 4 Ohm. as a mid-frequency one - 4GD-15 with the same value of Zg and as a high-frequency one - ZGD-6,5, for which Zg = 2 Ohm. According to Table. 8 at Zg=1 Ohm and capacitance C2=C20=1 μF fp700=3 Hz, and for capacitance C4=C3=2 μF fp4,8=3 kHz. In the filter, MBGO capacitors with standard capacitances can be used (C4 and CXNUMX are made up of two capacitors).

According to the above formulas, we find: L1=L3=2,56 mg; L2=L4=0,375mH (for an autotransformer, L4 is the value of the inductance between terminals 1-3).

Autotransformer transformation ratio

On fig. 3 shows the dependence of the voltage level on the voice coils of the heads on the frequency for a three-way system corresponding to the calculation example. The amplitude-frequency characteristics of the low-frequency, mid-frequency and high-frequency regions of the filter are designated LF, MF and HF, respectively. At crossover frequencies, the filter attenuation is 3,5 dB (with a recommended attenuation of 3 dB).

Fig. 3

The deviation is explained by the difference between the total resistances of the heads and capacitances of capacitors from the given (nominal) values ​​and the inductances of the coils from those obtained by calculation. The steepness of the decline of the bass and midrange curves is 9 dB per octave and the high frequency curve is 11 dB per octave. The HF curve corresponds to the uncoordinated inclusion of the loudspeaker 1 GD-3 (at points 1-3). As you can see, in this case the filter introduces additional frequency distortions.

In the given calculation method, it is assumed that the average sound pressure at the same input electric power for all heads has approximately the same value. If the sound pressure generated by any head is noticeably greater, then in order to equalize the frequency response of the loudspeaker in terms of sound pressure, it is recommended to connect this head to the filter through a voltage divider, the input impedance of which should be equal to the nominal impedance of the heads adopted in the calculation.

Author: E. Frolov, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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