[sdiy] Oberheim Xpander VCF

[Dave Manley]

Digging thru the files I found some LTspice simulations of the Expander 
filter circuit I ran back in Feb 2007.  I was all set to build a circuit 
that could sweep (not step as in the Expander/Extrapole) between 
different modes, and then ran some simulations looking for sensitivity 
to component variation.

The link below is an AC transfer plot for the "4 pole BP" mode.  In this 
mode three sections of the filter are summed with ratio 100:50:100.  
There are 27 curves in the plot representing sweeping the 3 summing 
resistors plus/minus one percent. Solid lines are voltage, dotted are 
phase.  Note how the low pass half of the bandpass
varies more than 40dB, with a resistor tolerance of 1%.

http://www.sonic.net/~dlmanley/ExtraPole4PBP_1_PerCent.png

Here's the "3 pole HP" mode, with 1% and 5% tolerance on 3 of the 4 
summing resistors (this mode needs 4 resistors, but LTspice only allows 
3 varying components per sim).

http://www.sonic.net/~dlmanley/ExtraPole3PHP_1_PerCent.png
http://www.sonic.net/~dlmanley/ExtraPole3PHP_5_PerCent.png

Clearly the circuit is extremely sensitive to gain error.  Switches and 
precision resistors work, but will a VCA based circuit?  How tightly 
controllable are any of the proposed gain circuits?  DAC controlling 
VCA?  PWM controlling VCA?  Programmable Potentiometer?  Vactrol?!!!  I 
doubt 1% gain control is easily achieved.  5% gain variation yields 
anything from a high pass to an all pass! 

Again, I'm not saying the resultant filter wouldn't be musically useful, 
but I don't think the plots previously presented would be seen in a real 
world circuit.  The LP will look like a low pass, but will you get a 
quality bandpass or high pass?

I stopped considering this circuit because of these results.  If someone 
has built it I'd like to see/hear the real world transfer functions of 
the different modes.

-Dave


Adam Schabtach wrote:
> I have one of Marc's PC boards for that add-on. It's one of my back-burner
> projects; I haven't finished it because I've been waffling over how I want
> to do the panel and the mode display. I really should at least get it to the
> point at which I can test it. I used to have an Xpander and the filter was
> probably my favorite part of it. I have two MOTM-440 filters and like them
> very much, so giving one of them a number of different topologies is very
> attractive to me.
>
> I attempted to reduce the gain errors by hand-matching 1% resistors. I also
> realized that many of the resistance values form simple integer ratios, so I
> used resistors in parallel, chosen to create the required ratios as
> accurately as possible. I don't know how much it will actually help but it
> seemed like a good idea at the time.
>
> --Adam
>
>   
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl 
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of David Moylan
>> Sent: Thursday, January 15, 2009 8:26 AM
>> To: George Hearn
>> Cc: synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] Oberheim Xpander VCF
>>
>> This type of versatility is available to cascaded integrator 
>> filters in general, by mixing outputs of the separate stages. 
>>  Marc Bareille created an add on for the MOTM-440
>>
>> http://m.bareille.free.fr/modular1/vcf_multimode/extrapole.htm
>>
>> Although it's nice to be able to switch in discrete steps you 
>> can probably have some fun with a 4 way mixer as well.
>>
>> Dave
>>
>>     
>
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