[sdiy] What would be THE best, most versatile VCF available on DIY community ?

Harry Bissell harrybissell at wowway.com
Tue May 29 17:26:55 CEST 2012


you could do the filters with OTA, but the best you can do is get a dual OTA whereas the CEM and SSM
would have four matched OTA on a single chip. You might need to hand select OTAs to get decent performance...

H^) harry


----- Original Message -----
From: dan snazelle <subjectivity at hotmail.com>
To: Aaron Lanterman <lanterma at ece.gatech.edu>
Cc: synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Tue, 29 May 2012 11:00:05 -0400 (EDT)
Subject: Re: [sdiy] What would be THE best,	most versatile VCF available on DIY community ?

> One thing to be careful of when looking at the Synthex schematics and thinking of adopting it is that the CEM3320 variable gain elements are not typical OTA cells. They're these weird things that I haven't seen elsewhere, and have some biasing requirements that confused the daylights out of me when I first looked at the schematic.
> 
> - Aaron



this is something that has long bothered me...when looking at schematics that use the CEM chips or SSM chips, is it possible to simply transfer the entire design to OTA's? (13700s)
or is there some sort of MAGIC (something in the patent?) in those chips which makes them hard to copy?

for instance.....the Anderton Multiple Identity Filter...could you make the same thing with OTAs? in his introduction he mentions making it with SSM chips instead of CEM chips...I took that as a good sign.


thanks



On May 28, 2012, at 11:48 PM, Lanterman, Aaron wrote:

> 
> On May 28, 2012, at 1:19 PM, Andrew Simper wrote:
> 
>> How about the Elka Synthex Multimode Filter (LP/BP/HP) as outlined
>> here: http://www.electricdruid.net/index.php?page=info.cem3320
>> 
>> It can be made from any ota core and provides "stronger" high pass and
>> band pass responses than the Xpander style summing of output taps.
>> I've got some IR3109s and some BA662s I'm going to give this a go
>> with.
> 
> The Synthex filter is unique.
> 
> The way it forms a two-pole filter with variable "Q" is to cascade two usual first-order LP filters (for instance) and then feed the output back to the input. I can't seem to find my notes on it now, but I remember working out the transfer function. Changing the feedback, IIRC, doesn't give you a Q (as typically defined for a 2nd-order filter) that varies independently of the "fc" (usually called the "cutoff"), as in the case of a State Variable Filter or a Sallen-Key. The fc also changes. So there's a subtle difference in how it responds.
> 
> 
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Harry Bissell & Nora Abdullah 4eva



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