[sdiy] Oberheim Xpander VCF (newbie alert)

[Paul Maddox]

David,

   if a dual gang pot works, use it, no such thing as un-elegant in my  
book
In Defender I use two VCAs :-)

Paul

On 15 Jan 2009, at 21:26, David G. Dixon wrote:

> This talk of VCFs has led me to another newbie question:
>
> What is the preferred method for implementing a cross-fade circuit?   
> I'm
> assuming one channel's gain is increased from 0 to 1 while another's  
> is
> decreased from 1 to 0, and they both add up to one at any given  
> setting?
> How exactly is this done (it doesn't involve dual-gang pots, does  
> it?  That
> wouldn't be elegant at all!)?
>
>> Original message:
> Antti,
>
>  you can indeed, and it doesn't have to be a "real" filter, it works
> with DSP filters too.
> With the Monowave II and Phoenix apps I did for the Chameleon I did
> this.
> take output after 1st pole, invert it... take output after fourth pole
> and cross fade between the two. In the middle you get a psuedo
> bandpass filter, but by doing the crossfade you can get a four pole
> filter with a bit more fizz and a bit less bottom.
>
>   MonowaveII examples here -
> http://www.vacoloco.net/synths/monowave2/sounds.shtml
>
>  Just look for "pole sweep" or similar.
>
>  I did the same in hardware for the Defender voice card, you can here
> the filter being controlled by the two LFOs on this page ;-
> http://www.vacoloco.net/synths/defender/sounds.shtml
>
>   It's a great way to obtain a very very flexible filter for very
> little extra work!
>
> Paul
>
> On 15 Jan 2009, at 15:10, Antti Huovilainen wrote:
>
>> On Thu, 15 Jan 2009, George Hearn wrote:
>>
>>> I've been pointed in the direction of the Oberheim XPander VCF.  I
>>> think
>>> it's quite incredible how 16 filter modes can come out of one
>>> filter.  Does
>>> this not make it about the most versatile analogue filter design
>>> ever?
>>
>> You can get the same modes (and over a dozen more) from any 4 pole
>> cascaded RC style filter (Moog ladder, CEM, OTA etc).
>>
>> Antti
>>
>> "No boom today. Boom tomorrow. There's always a boom tomorrow"
>> -- Lt. Cmdr. Ivanova
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