Variable slope filter. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Variable slope filter. Encyclopedia of radio electronics and electrical engineering

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Let's admit the incredible - that all frequency response defects are defeated by the selected components and careful installation, and only an indestructible resonance remains. Even when using the highest quality components, this problem is not easy to solve. The first and most obvious way is to separate the crossover frequencies of the subwoofer and the rest of the acoustics. In this case, the subwoofer reproduces frequencies below 120 Hz, and the rest of the acoustics - above 180 Hz. These figures are approximate and depend on the size of the cabin and the features of a particular installation. Since crossovers of at least a second order are used in serious installations, the roll-offs of the frequency response are quite steep, and the band spacing is small. As a result, the subwoofer has to reproduce a wide frequency band and the problem of "rear" bass arises in all its glory. The tail was pulled out, the mane got stuck ...

The way out was found where they were not looking for it, but you will have to start from afar - from circuitry. Budget series amplifiers, in order to keep up with their "big brothers" in terms of equipment, have long switched from a fixed cutoff frequency of the built-in crossover to smooth tuning. The only difference is that they save on variable resistors. To rebuild a second-order filter in a stereo version, you need a four-section variable resistor with good resistance matching, and in some cases also with different section resistances. A thing, if not custom-made, then at least not consumer goods. Therefore, they began to use the usual two-section ones, having accordingly reworked the filter scheme.

Variable Slope Filter
Fig. 1

This is how filters of variable steepness entered the arena, in which only one of its links is rebuilt. Following budget amplifiers, such filters appeared in middle-class models. As an example, a fragment of the built-in crossover circuit of the Hifonics Mercury amplifier (Fig. 1).

The scheme is simple - a passive high-pass filter of the first order, tunable over the range, and in addition to it - an active second-order filter, tuned to the extreme frequency of the range, that is, in total - a third-order filter. But unlike the classical version, when the cutoff frequency changes, the shape of the filter's frequency response also changes. At first glance - a drawback, a deviation from the canons. But if you approach dialectically - dignity is a continuation of shortcomings. Let's look at the frequency response (Fig. 2).

Variable Slope Filter
Fig. 2

The graph in black shows the frequency response of the subwoofer channel with a cutoff frequency of 80 Hz and the frequency response of the HPF in the two extreme positions of the regulator. And in blue - the resulting frequency response in one of its intermediate positions. The result is obvious - in the region of interest to us, about 150 Hz, a dip of the required magnitude appeared. At the same time, the subwoofer frequency band remained within reasonable limits and there is no need to worry about the "rear" bass. And the wolves are full, and the sheep are safe. In addition, in the zone of operation of the first-order link, the phase-frequency characteristic is much more decent than that of a classic third-order filter.

If you look at the test results of various publications, similar frequency response can be found in most modern crossovers and amplifiers in the lower and middle price categories. The conclusion is unambiguous even without opening - filters of variable slope are used in their design. Here is an example of how simplifying a design has improved its quality and expanded its scope. However, the possibilities of correction using this method are limited, so it is too early to talk about the extinction of equalizers.

Author: A. Shikhatov; Publication: bluesmobile.com/shikhman

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