Modification of 35AC-015 based on ladder filter. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Audio equipment

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The magazine "Radio" has repeatedly described various ways to improve the sound of the 35AC-1 speaker system. The proposal published in [1] can be considered the most successful. However, the speaker, modified in this way, with a good frequency response, also has a very significant drawback - a noticeably degraded directivity characteristic due to an increase in the crossover frequency of the mid-frequency and high-frequency sections of the filters up to 10 kHz. Namely, this parameter, along with the frequency response of acoustic power, is considered the most informative in terms of assessing the sound quality of speakers in real listening rooms.

The directional characteristic of the speaker is measured as the dependence of the sound pressure it develops at a certain frequency or in the frequency band in a dampened chamber on the angle of displacement of the measuring microphone relative to the acoustic axis of the speaker in the horizontal and vertical planes. The frequency response of the acoustic power is the dependence of the emitted acoustic power on the frequency of the signal. The unevenness of the frequency response of acoustic power is considered to be one of the main parameters in assessing the sound quality of AC [2].

Since the axial unevenness of the frequency response in modern speakers does not exceed ±2 dB, the main contribution to the unevenness of the frequency response of acoustic power is made by the change in the width of the directivity characteristic as a function of the signal frequency. The influence of the directivity characteristic on the sound quality of the speakers is manifested in the shift of the stereo image when the spectral composition of the signal changes. In psychophysiological studies of the sound quality of Hi-Fi speakers, it is noted that speakers with a good axial frequency response, but a narrow directivity characteristic (as well as sharp changes in the width of the directivity characteristic with frequency change) sound "hard and tiring" [2].

Crossover filters have a significant impact on the sound quality of multiband speakers, the role of which has been underestimated until recently. In most industrial AC models, crossover filters are used, consisting of passive LC-sections of the "K" type, usually not higher than the third order. In the modified AS "Electronics" 35AC-015 brought to the attention of readers, the so-called sixth-order all-passing type ladder filter is used. Filters of this class were proposed in the second half of the 70s [2].

Unlike other crossover filters used in the AU, they simultaneously satisfy several requirements: they provide a flat total frequency response but voltage, symmetrical directional characteristics of the AU in the region of filter crossover frequencies and a low level of phase distortion. In addition, they are the least sensitive to changes in the values ​​of their constituent elements. A similar filter, but of the fourth order, is used in the industrial system "Orbita" 100AC-003.

When finalizing the acoustic system "Electronics" 35AC-015, the low-frequency section of the separation filter (Fig. 1) was left unchanged. In the mid-frequency link, the ratings of the elements have been changed to match the newly installed head 5GDSH-5-4 (4 Ohm), which is used instead of 15GD-11A. As already noted in the magazine, the latter is not able to provide an acceptable quality of midrange reproduction. The 5GDSh-5-4 head must first be modified by smearing the diffuser on both sides with vibration-absorbing mastic [1]. The windows of the diffuser holder can not be sealed with synthetic felt, but before installing the head in an insulating box, wrap it in foam rubber with a thickness of about 8 mm. The free space of the box must be filled with a small amount of cotton wool.

(click to enlarge)

The mid-frequency link (L4, C5, L5, C6, L6, C7) and high-frequency (C8, L7, C9, L8, C10, L9) are a sixth-order all-passing ladder filter with a crossover frequency of 4500 Hz. As shown by numerous experiments, the choice of such a crossover frequency for a given speaker model is the most appropriate in terms of obtaining the best directivity characteristics.

The choice of a relatively high order of the filter is caused by the desire, when using available dynamic heads, to obtain minimal non-linear distortions at medium and high frequencies and eliminate the unpleasant coloration of the sound of the 10GD-35 high-frequency head, noted by many owners of 35AC-1. It is known that in dynamic loudspeakers, the amplitude of the diffuser displacement with a decrease in the frequency of a constant amplitude sinusoidal signal supplied to the head increases with a steepness of about 12 dB/octave up to the resonant frequency of the head. In this case, the sound pressure generated by the head at the frequencies of the test signal does not increase (and may even fall), and the non-linear distortions increase noticeably. When the resonant frequency is reached, the displacement amplitude of the diffuser no longer depends on the reduced frequency. A first-order high-pass filter with a 6 dB/octave rolloff cannot sufficiently change (reduce) the growth of this dependence.

In order for the slope of the increase in the amplitude of the displacement of the diffuser, starting from the filter crossover frequency and ending with the resonant frequency of the head, to approach zero, the slope of the decay of its frequency response must be at least 12 dB / octave. The slope of the frequency response of the applied high-pass filter of the sixth order exceeds 30 dB/octave. This made it possible to significantly reduce the nonlinear distortions of the 10GD-35 head at frequencies close to its resonant frequency. There is an opinion that high-order filters introduce large transient and phase distortions. However, as shown in [2], such distortions, created by filters of the all-transmitting type of high orders (up to the sixth), measured on multicomponent signals, lie significantly below the subjective thresholds of audibility.

The design of the inductors can be arbitrary. In their manufacture, the author used frames made of organic glass (can be made of textolite or ebonite), a sketch of which is shown in fig. 2.

Coils L4-L6 are wound with PEV-1 0,8 wire and contain 118, 106 and 76 turns, respectively. Coils L7-L9 and L3 are wound with PEV-1 0,5 wire and contain 128, 170, 248 and 236 turns, respectively. Using a suitable inductance meter (the author used E7-9), it is desirable to adjust the inductance of the coils (with an accuracy of the third digit) to the values ​​\u1b\u1,2bspecified on the AC circuit diagram. Filter capacitors - MBGO-5 and MBGP-10 with a deviation from the nominal values ​​of ±XNUMX and ±XNUMX%. Non-standard ratings are dialed by parallel connection of capacitors with standard values. All parts of the filters are fixed on a separate panel installed inside the speaker cabinet under the low-frequency head.

The speakers modified in this way are operated with the UMZCH described in [3]. An amplifier with a passive tone control published in [4] was used as a preamplifier. When operating modified speakers as part of a sound reproducing complex with an active tone control, which provides a depth of frequency response control above ±15 dB, it is advisable to turn on the PEV-7,5 quenching resistors at the inputs of the midrange and HF sections of the filter (indicated by a dashed line in the diagram).

When evaluating the sound quality of the converted speakers, all listeners noted the "lightness and transparency" of the sound, its naturalness, as well as the clear localization of sound sources in space.

References:

1. Zhagirnovsky M., Shorov V. Improving the sound of 35AS-1 and its modifications. - Radio, 1987, No. 8, p. 29-30.
2. Aldoshina I., Voishvillo A. High-quality acoustic systems and emitters. - M .: Radio and communication, 1985, p. 168.
3. Ageev A. UMZCH with small non-linear distortions.- Radio, 1987, No. 2, p. 26-29.
4. Solntsev Yu. High-quality pre-amplifier. - Radio, 1985, No. 4, p. 32-35.

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