[sdiy] Shruthi 4PM (was re something else...)

Olivier Gillet ol.gillet at gmail.com
Thu Nov 28 17:56:28 CET 2013

Actually, there's one thing for which the behavior of the circuit
deviates from the theory...

Some transfer functions require the first pole to be bypassed. The
schematics show that when the first pole is bypassed, a 10pF cap is
switched between the V- and output port of the first integrator
(instead of 220pF). Actually, this 10pF cap is not really needed
because you already have a handful of pF of capacitance from within
the TL072 (so if you have a 4PM and have used TL072 as recommended,
you can safely the 10pF cap!). Even then, the first pole is never
entirely disabled, merely shifted ~6 octaves higher than if it were
enabled. This is true, to some extent, in every filter design - a
high-pass is never truly a high pass, there's a point at which it has
a roll-off due to op-amp stabilization. But here, this causes the
high-pass modes to have a 1-pole low-pass sidekick ~6 octaves higher,
which can be audible...

This compromise was not there in the original design because it uses
much larger caps for the LP stages 1-2-3. If I had used larger caps in
the filter (say 33nF like with the CEMs), the input resistance of each
2164 cell would have been 220 ohm. Are there any disadvantages from
running the 2164 with that much larger input currents? The 2164
datasheet mentions somewhere that 30k is the 2164's sweet spot (not
sure if it is the 30k value or more the range of currents
corresponding to running line-level audio into 30k). I stayed away
from this...


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