[sdiy] help finding roland filter schematics?

[David G. Dixon]

Dan,

I've been playing with filters as well, although more on simulation software
than actual breadboards.  I haven't thought too much about state variable
filters yet, as I'm more interested in 4-poles at the moment.

My basic approach, following the advice of Don Lancaster in the "Cookbook",
has been to design a decent 6dB filter block using an OTA as a variable
resistor, and string these together, feeding the amp bias inputs of each
from parallel pnp's off a common expo converter.  For this, I've found that
the little VCF circuit in the LM13700 datasheet (which drops the input
voltage by a factor of 500) is quite adequate.  There are other approaches.
For example, some like to run with higher input voltages and use the
linearizing diodes to keep things relatively undistorted.  However, I think
this works better with CA3280s, which behave differently to LM13700s and (I
think) respond much more effectively to their linearizing diodes.  When I've
tried this with LM13700, I have found that the filter bandwidth is severely
limited (e.g., 9 kHz or less compared with 25 kHz without the diodes).

The other thing I'm doing is putting OTA-based gain blocks between each
pole, because I want to recover equal-strength sine waves from each pole.
Again, a very simple gain block based on 450:1 (100k:220R) attenuation of
the input signal does the trick nicely.  These I'm feeding with a linear
current source (basically, at this point, a buffered voltage source through
four resistors in parallel).  For my current design, which is basically a
"Mankato wannabe" with VC resonance, I've found that it is best to feed the
LPF blocks directly into each other, and also tap off the the outputs and
put them through the gain blocks.  This way, when the resonance is turned
all the way down, I still have unity gain between each block, but can get
the gain easily up to self-oscillation by cranking up the resonance current.

The key to handling the resonance is to understand the phase relationships
of each LPF block.  At cutoff, each block will lag the previous one by 45
degrees.  That means that the 4th block will lag the input by 180 degrees.
Hence, this one needs to be inverted before being recycled into the 1st
block (or, alternatively, it can be put into the negative feedback loop of
the 1st block).  Also, the resonant gains for the 2nd, 3rd and 4th blocks
should be equal, but the gain fot the 1st must be significantly higher,
because it has to amplify the 4th block's output to mimic the full strength
signal (remember: at resonance, each filter block attenuates the signal by
3dB).  I've found that about a ratio of 10:3:3:3 is just about ideal for the
four gain blocks.  I know that there is a mathematical underpinning to this,
but so far I've been too lazy to derive it.

I'm nearly done with this filter design, and then I'm going to look at a
couple of other filter topologies and see what I can do with them.

The moral of this story, I think, is this:  Don't be afraid simply to jump
in and come up with your own design, based on a calm assessment of what
filters do, and what you are trying to achieve with your filter.  I think
you may be happier in the long run with this approach than with trying to
reverse-engineer every filter you've ever encountered.

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada
 
Tel 1-604-822-3679
Fax 1-604-822-3619
 
"PERFECTA FINGAMUS SERVIAT NATURA"
 
The information in this email and in any attachments is confidential and
intended solely for the attention and use of the named addressee(s).  It
must not be disclosed to any person without the writer's authority.  If you
are not the intended recipient, or a person responsible for delivering it to
the intended recipient, you are not authorized to and must not disclose,
copy, distribute, or retain this message or any part of it.
> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Dan Snazelle
> Sent: Wednesday, April 29, 2009 10:25 AM
> To: okvern at ix.netcom.com; sdiy
> Subject: RE: [sdiy] help finding roland filter schematics?
> 
> 
> great advice and the answer is yes.
> 
> in the last 2 weeks i have built over 15 filters on the breadboard.
> 
> it has been very educational and at times aggravating as some of them go
> off without any problems at all and some i just can not get working right.
> 
> 
> AND THIS IS WHY i am trying to intuitively understand more about how the
> circuits are working. i have read the ENS76 filter article, reading the
> filter cookbook by lancaster, going through chamberlins book, and also
> barry kleins....
> 
> and today i am watching aarron lantermans lectures on ota FILTERS
> 
> so i am trying to on the one hand learn by DOING and also, learn by
> reading and watching...two different modes for me and right now until my
> math gets better,there is a ton about filters i just dont GET.
> 
> i keep coming back to roland because they have a lot of schematics that if
> all you need to do is replace their ota chips with the ones we have
> now...well that is a lot of circuits to learn from, mess about with,etc
> 
> 
> and ray wilsons soundlab filter and sub commander are also 2 great
> starting circuits.
> 
> i like them better than the datasheets because there are notes on what is
> happening in the circuit!!
> 
> i do keep finding though that no matter what circuit i build, i keep
> wanting to change the original values.
> 
> my wife keeps telling me to stop...she thinks i am suddenly going a bit
> nuts over filters!!
> 
> thanks
> 
> --------------------------------------------
> check out various dan music at:
> 
> http://www.myspace.com/lossnyc
> 
> (updated monthly)
> 
> http://www.soundclick.com/lossnyc.htm
> 
> 
> 
> http://www.indie911.com/dan-snazelle
> 
> (or for techno) http://www.myspace.com/snazelle
> 
> ALSO check out Dan synth/Fx projects:
> 
> AUDIO ARK:
> 
> www.youtube.com/watch?v=TJRpvaOcUic
> 
> www.youtube.com/watch?v=BqIa_lXQNTA&feature=channel_page
> 
> www.youtube.com/watch?v=V4nJPjGgOcU&feature=channel_page
> 
> and soundtrack/design work:
> 
> NEW: check out Dan's sound design from the 1998 award winning film SAFARI
> by catherine chalmers
> 
> http://www.catherinechalmers.com/videos.cfm
> 
> 
> 
> 
> 
> 
> ----------------------------------------
> > From: okvern at ix.netcom.com
> > To: synth-diy at dropmix.xs4all.nl
> > Subject: Re: [sdiy] help finding roland filter schematics?
> > Date: Wed, 29 Apr 2009 09:46:31 -0700
> >
> > Dan Snazelle wrote: "in my quest to better understand OTA filters...."
> >
> > I don't know about Roland filters (maybe I should change the subject
> line?),
> > but I've been following your questions about OTA filters, and it sounds
> very
> > similar to my experience. I've been wondering--have you built the
> examples
> > shown in the datasheet/application notes for the LM13700? I found it
> > incredibly helpful to get those examples working before I moved on to
> more
> > complex circuits. Building the circuits without voltage controlled
> > resonance, for example, would probably help isolate the problems you
> were
> > having with that feature. Then I'd look at the excellent--but relatively
> > simple--SEM-style circuits from MFOS and Takeda-san.
> >
> > Just a thought--this approach helped me a lot when I was learning OTA
> > filters.
> >
> > Thanks,
> >
> > Ole
> >
> > _______________________________________________
> > 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