Dome Filter Question

[jhaible]

> I hope you don't mind me emailing you personally, but I posted this
question
> to the DIY list without any useful answers and am sure that you are the
best
> person to ask.
> 
> Question: Is it possible to replace the presets in the Dome Filter with
fixed
> value resistors? If so, how do I calculate their values?

Ok, I have seen the question on diy when I was at the office and had my
FS shifter documents not in front of me. So now I will answer with a cc
to synth-diy, as I have the EN article in front of me now.

I used two branches of 6 opamp based all pass filters in series. The
RC products for each stage are:

Branch A:

Stage 1: RC = 29.4ms
Stage 2: RC = 3.87ms
Stage 3: RC = 951us
Stage 4: RC = 237us
Stage 5: RC = 59.1us
Stage 6: RC = 13.3us ( NOT 1.33us, as in EN Preferred Circuits Collection
!)

Branch B:

Stage 1: RC = 8.47ms
Stage 2: RC = 1.91ms
Stage 3: RC = 475us
Stage 4: RC = 118us
Stage 5: RC = 29.1us
Stage 6: RC = 3.83us

If you build opamp based filters, you don't need trimmers.
Whar you do instead is this:

(1) Choose a reasonably sized capacitor
(2) Measure this capacitor
(3) calculate the resistor
(4) choose a standard resistor (E24) that is slightly smaller than the
required value
(5) measure this resistor
(6) select a small series resistor to get near the exact value.

Example, for Branch A, Stage 2:

(1) chosen 220nF
(2) measured 229nF
(3) calculated 16.89kOhm
(4) chosen 16kOhm
(5) measured 15.99kOhm
(6) selected 910 Ohm

With this method, you will get a very good approximation without trimmers.
Make sure that your opamps are good enough - LF353 or TL072 are fine.

Hope this helps (and hope it doesn't violate copyrights - that's the only
part
of my FS that's taken directly from El*ctr*n*tes).

JH.