[sdiy] Oberheim Xpander VCF

Fri Jan 16 22:45:58 CET 2009

For what it's worth, the complete Xpander service manual, with schematics,
is here:
http://experimentalistsanonymous.com/diy/Schematics/Full%20Synths%20Drum%20S
ynths%20and%20Misc%20Synth/Oberheim%20XPander.pdf
The schematic specifies 1% resistors. There are no trimmers. I had an
Xpander and I'd bet a box of donuts that it didn't have a 40db
voice-to-voice variation in the low side of the BPF response. One of its six
voices was audibly different than the others, but it wasn't a 40dB
difference and my memory is too vague to really say what the difference was.
I'd be surprised if it was a variation of even 3dB of any part of the
spectrum.

It's possible that Oberheim hand-matched the resistors, as I described I had
done, but even that seems unlikely in a production environment.

Now I'm even more anxious to get my modified MOTM-440 running. Can anyone
suggest any free (or relatively cheap) Windows software for making
frequency- and phase-response measurements with an audio interface? I've
looked for such a program several times and never found anything very
satisfactory.

--Adam

> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl 
> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of 
> Scott Nordlund
> Sent: Friday, January 16, 2009 11:37 AM
> Cc: synth-diy at dropmix.xs4all.nl
> Subject: RE: [sdiy] Oberheim Xpander VCF
> 
> 
> I believe the precision is necessary because certain 
> frequency bands must be canceled out to get the different 
> responses.  To achieve maximum attenuation, then, the levels 
> have to be exact, though it doesn't necessarily mean that 
> incomplete cancellation renders it useless.  For something 
> like a notch filter, I doubt it would be very audible.  
> 
> ----------------------------------------
> > From: tom at electricdruid.net
> > Subject: Re: [sdiy] Oberheim Xpander VCF
> > Date: Fri, 16 Jan 2009 11:06:46 +0000
> > To: dlmanley at sonic.net
> > CC: synth-diy at dropmix.xs4all.nl
> >
> > 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
> >>>>
> >>>>
> >>>
> >>> _______________________________________________
> >>> Synth-diy mailing list
> >>> Synth-diy at dropmix.xs4all.nl
> >>> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >>>
> >>>
> >>>
> >>>
> >>
> >> _______________________________________________
> >> Synth-diy mailing list
> >> Synth-diy at dropmix.xs4all.nl
> >> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> >>
> >
> >
> > _______________________________________________
> > Synth-diy mailing list
> > Synth-diy at dropmix.xs4all.nl
> > http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 
> _________________________________________________________________
> Windows LiveT: Keep your life in sync.
> http://windowslive.com/explore?ocid=TXT_TAGLM_WL_t1_allup_expl
> ore_012009
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
> 