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4.5 Parallel Passive EQ
Parallel EQ exhibits quite different sonic properties to the
familiar serial parametric EQ. We are generally used to
hearing the effect of one EQ band superimposed on another,
as opposed to the band interaction inherent to a parallel EQ.
Because the bands are placed in a parallel configuration, phase
cancellations and re-enforcements happen which is not always
obvious when first encountered.
Passive EQ is something that is found in old equaliser units
and is generally known for its transparent and natural sound,
but has some problems associated with it. However, in the
digital domain these shortcomings do not have such an
influence.
A passive EQ does not have any gain elements, but can still
have controls to seemingly boost frequencies as well as cut.
What actually happens is that the entire signal is cut by an
amount, but the frequencies which are apparently ‘boosted’
are simply not cut as much. Therefore the unit must attenuate
either the input, the output, or both to allow enough headroom.
Unfortunately in the analogue domain, a 20dB reduction in
signal level produces a 20dB increase in the noise floor.
Luckily, in the digital domain with a 40-bit floating point DSP,
these issues do not remain.
In X-EQ – when the ‘parallel’ button is engaged – you are
presented with a parallel passive EQ model which the original
designers of these devices could only have dreamed of. The
noise floor can be disregarded due to the huge resolution
allowed by Duende.
You may find yourself entering this mode more and more as
you become familiar with the sonic signature. Larger gain
changes are possible without colouration, and boost starts to
become something that is useable to a significant degree in a
digital EQ!
Parallel EQ does however exhibit asymmetry in its boost and
cut characteristics. But this is not such a bad thing as most
engineers would agree that boost is best done with low (wide)
Q values and cut with a higher (narrower) Q.