The FIlters are not what they seem

You may think there is nothing special about those Low and High pass filters you use daily in your favorite EQ
I asked Dipl.-Ing. Reimund Dratwa the head of research and development at Black Rooster Audio what makes their VHL-3C so special and charming.
He was kind enough to give me a full and interesting answer and I would like to share it with you.
The peculiarity of passive filter designs is that unlike almost ideally buffered filters,
e.g. op-amp based circuits, adjacent bands will start to interact once their cut-off or
resonant frequencies become close. This will, depending on the kind of circuit and setting,
subtly to drastically bend or skew the filter’s frequency response.
Technically speaking this is an undesired effect if you’re trying to work “surgically” and
be able to adjust each eq band individually. However, it is assigned a certain musicality or
vintage quality, which I guess is one of the reasons why passive eq’s and other vintage
processors have regained so much popularity. Ironically, back in the day, passive designs were
the only way to build equalizer circuits with low THD and at a reasonable cost so frequency
response imperfections were just something to put up with, which doesn’t mean they aren’t
clever and carefully engineered circuits.
Trying to capture all the inter-band interactions in a model also means you will have to
analyze the circuit’s behavior in a high-order transfer function that involves the entire
filter/eq network (6th order in both the VHL-3C’s and VEQ-5’s cases) which of course implies
additional complexity, whereas the bands in modern eq circuits typically can be analyzed
and digitized individually.
The following plot gives a quick example of how this all affects the frequency response of
the VHL-3C’s highpass filter at two settings (500 CPS and 1500 CPS) once you decrease the
lowpass cut-off frequency. See how the high pass edge starts to deviate from the behavior
you would expect with a buffered filter cascade.
Thank you, Reimund. Guys, if you haven’t tried it yet, you definitely should give it a go.
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