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

[Tom Wiltshire]

I've never gone as low as 220R, but I did an 2164 SVF design with 15K instead of 30K, and doubled the cap values. I found running the filter with more juice made the resonance perform better. As you know, the problem with SVFs is getting them to resonate at the bass end, without going totally nuts at the top end. More current seemed to help push the resonance more at the bottom. It was that or start fitting hand-picked 5pF and 10pF caps all over the place...

T.

On 28 Nov 2013, at 16:56, Olivier Gillet <ol.gillet at gmail.com> wrote:

> 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...
> 
> Olivier
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