Sound Performance Lab 2595 Equalizer, Passive EQs, The Basics of Frequency Filtering, Passeq

The Basics of Frequency Filtering

Equalizer

In our context here, an EQ is in effect a network of frequency filters which changes its output characteristics depending on frequencies. The common recording studio concept for sound shaping or correction is the amplitude-based EQ. But owing to the large number of filter combinations, there are practically too many existing formulations of this design concept to count. Nonetheless it is sensible to make some general observations that categorize the most important technical differences among these design concepts.

Passive EQs

The filters in a passive network employ no intrinsic amplification elements and therefore need no external power, which means in effect that they can really only cut the energy of a chosen frequency. In order to expand this capability to include boosting the energy of a bandwidth (that is, to be able to boost and cut frequencies), the overall filter input signal level is reduced. Based on this overall reduction and relative to the original input signal, one can achieve a further cut or boost in the form of a return to that original signal level. Thus a passive filter is always followed by an amplifier which is responsible to regain the initial energy reduction and restore level balance between the input and output signal levels.

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Schematic of a passive frequency filter

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Passive filters react differently than active filters, mostly due to the saturation and loading characteristics of its coils and condensers. The characteristics of passive filters often are very advantageous in their ability to create a musically pleasing sonic result: they sound comparably very smooth and harmonious (please see “Introduction—Advantages of Passive Filter s” on page 5).

Block diagram of passive filter set flat

Block diagramm of a passive filter at +18 dB boost

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