[sdiy] Oberheim Xpander VCF

[Tom Wiltshire]

Dave,

This is extremely interesting stuff.
It also strikes me as extremely counter-intuitive, which makes me  
very curious. How can a 1% variation in anything make a 40dB  
difference to something else? I know it could, but it just doesn't  
seem right! That makes me want to know more.

It does look like the resultant filter *is* musically useful, since  
no-one seems to dismiss the Xpander/Matrix synths for the rubbish  
filter, or the horrible variation between voices. According to the  
plots, this should be *severe*. So what's going on? Did Oberheim use  
0.1% resistors? Does it actually not matter much? Does the ear not  
hear large variations in filter characteristics? One or two people  
have commented already that many of the settings on the A107 sound  
the same, although on paper they vary quite a bit.

Regards,
Tom



On 16 Jan 2009, at 06:35, Dave Manley wrote:

> 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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