RackPack: Vox Ranger
17
Bell Filters, Passive Filters, Advantages of Passive Filters
Technology
Bell Filters
A bell filter boosts or cuts a chosen frequency‘s energy with a maxi-
mum amplitude and a definable frequency range around this fre-
quency with a fall off of up to 3 dB to both sides. The chosen frequency
with the maximum amplitude is called center frequency—it takes
place in the middle at the peak of the response curve. The response
curve forms a bell, but bell filters are also often referred to as peak
filters (also refer to page 13, “Diagram Of Filter Curves”).
Passive Filters
The filters in a passive network employ no intrinsic amplification ele-
ments 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 band-
width (that is, to be able to boost and cut frequencies), the overall fil-
ter 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 pas-
sive filter is always followed by an amplifier which is responsible to
regain the initial energy reduction.
Advantages of Passive Filters
• Typically coil inductance in virtually all active filters is achieved
through simulation. True passive coil filters, on the contrary, can
only deliver the genuine, characteristic sound associated with
inductive components.
• Inherent distortion elements of active filters are ruled out by pas-
sive filter design.
• For any number of reasons stemming from design and component
advantages over active filters, passive filters achieve a very natural
aural quality and through their harmonic treatment (THD, distor-
tion, phase response, etc.), offering at the very least, a clear sonic
alternative—in our ears often an extremely attractive one.
• All passive filter components (variable resistor, capacitor and coil)
work in concert to produce this beautiful sonic result. An important
part of this process is played by coil and condenser loading and
saturation characteristics. The resulting difference in latency from
characteristically extremely fast reaction of active filters provides
for more pleasant, musical sonic qualities. We tend to perceive
these attributes in terms of an increased suppleness and transpar-
ency, with perceptibly improved, silky highs and robust basses.
