SPL Passeq, Manual

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7
The Basics of Frequency Filtering
Passive EQs
A filter in a passive network got no amplification elements and therefore does not need
any external power. That is why you can only attenuate the energy of the frequencies.
However, to change the energy of the frequencies of a passive filter network in both ways
(attenuate and increase), the signal level of the filter input signal gets reduced by a certain
value. Proceeding from this value, an additional attenuation (cut) or an increase (boost)
of the reduced condition can be put into effect. A passive filter is always followed by an
amplifier which regains the initial energy of the attenuated signal level – though it is not
part of the actual filter circuit.
Passive filters react differently than active filters, mostly due to the saturation and load-
ing 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.
Schematic of a passive frequency filter
Block diagram of passive filter set flat
Blockdiagrammofapassivef18dBboost
dB
0
-2
-20
-36
-50 Hz 20 kHz
Input signal level
Filter output level
(when set for flat response)
Bass cut at 50 Hz by -18 dB
Bass boost at 50 Hz by +18 dB
Input Output
0 dBu 0 dBu -20 dBu Filter (flat):
-20 dBu
Amp
+20 dBu
Input Output
0 dBu +18 dBu -2 dBu 18 dB
boost
Amp
+20 dBu