[sdiy] Oberheim Xpander Multimode VCF Question

[David G. Dixon]

Steven,

In order to get the 4P AP response, you would need to add the following gain
factors for {A,B,C,D,E} where A = the input, B = stage 1, etc, and the
opposite sign on every other stage is assumed:

{A,B,C,D,E} = {1,8,24,32,16}

In my version of the interface (which I have only simulated, but have as yet
to build), I am using R-2R ladders on each stage except A, and the maximum
gain multipliers of these ladders are:

{A,B,C,D,E}_max = {0,7,15,15,7}

I have also summed in one more gain factor for each stage through an
attenuating potentiometer, including the input A, which allows me to dial in
the gain factor to any real number between 0 and N+1.  Hence, if I crank the
all of the pots up to full on, I can get the following:

{A,B,C,D,E}_max = {1,8,16,16,8}

This obviously will not allow me to get the 4P AP response.  However, I can
get the response at an overall gain of 1/2 if I set the input pot A to a
setting of 0.5 thus:

{A,B,C,D,E} = {0.5,4,12,16,8}

This would require ladders for D and E to be fully switched on, including
the pots.  By amplifying the filter output by a factor of 2, the 4P AP
response at unity gain is obtained.

Clear as mud?

> Is it possible to get a -24dB/oct allpass response out of an Oberheim
> Xpander type of 4-pole filter arrangement? I've got the following for
> an -18dB allpass, from a post by David G. Dixon...
> 
> input + pole1*3 - pole2*6 + pole3*4
> 
> Which seems to work fine (in software).