The Omen is an unusual take on the familiar trope of the passive LPG, as it brings resonance
and a -6dB filter slope to the typical -3dB non-resonant LPG design. You may also meet
people in your musical journey who call your new passive, resonant filter "witchcraft" or
"impossible," but take comfort in the fact that they are sadly misinformed and may soon be
corrected by your superior knowledge of filter topology.
Before transistors, options for designing audio filters were fairly limited, as transistors (and
later, ICs) form the prerequisite for active filter topology. A favored method of creating audio
filters for radio and cinema, however, was using so-called LC filters - filters comprised of a
capacitor and an inductor. LC filters had many desirable characteristics over other passive
filters, among them the fact that they were able to create resonance, dependent on the
relationship between the capacitor and inductor, and they were not as prone to signal loss
as other passive filter topologies were, allowing for steeper filter responses. With the advent
of active filters, however, LC filters quickly fell out of favor, as inductors are expensive,
imprecise and tend to be invariable, compared to continuously sweepable active filters.
A variant of the LC filter, called an RLC filter, allows for variation over the resonant
characteristics of the filter, while the filter cutoff is fixed, determined by the relationship
between the inductor and capacitor. The Omen uses this principle, as the optocoupler inside
forms both the volume response and resonant response of the circuit, while the cutoff can
be varied with a three-way toggle switch choosing different capacitor values. The result is an
LPG that sounds quite unlike anything else, where the resonance brings some character and
bite to the sound, without compromising signal strength and remaining a passive unit.
For the adventurous builder, the included capacitors can be exchanged for other capacitors
to achieve different filter cutoff values. The math involved is slightly beyond the scope of
this build guide, but searching online for LC filter calculators will provide plenty of easy
calculation options.
The Omen was designed entirely on paper, with inspiration from the book Audio Cyclopedia
(Tremaine, 1969) during an unexpected stay at Hvidovre Hospital in 2022 - a stay which led
to the birth of my son, Nemo, to whom I dedicate this design.
