[sdiy] Yves Usson's Minimoog Filter

David G Dixon dixon at mail.ubc.ca
Sun Jun 11 21:27:05 CEST 2023

Hi Jay,
First of all, thanks to everyone for your comments.  I haven't had time to
formulate a suitable response yet, but I'm taking them all under advisement.
Concerning pinouts, I'm well aware that 3904/3906 and 547/557 are opposite
pinouts with respect to the flat side of the TO-92 package (another thing
that just flabbergasts me about the abject stupidity of electronic
packaging, along with the fact that most common quad opamp ICs are
upsidedown with respect to the power pins -- clearly, it would have taken 10
seconds for the designers to check on the correct pinouts, instead of
creating a situation where every user for 50 years onward has to remember
that quad opamp chips go in upsidedown).
The filter works -- it just doesn't work well.  It doesn't have very
pleasing resonance -- it goes from almost nothing to screaming like a
banshee (and, yes, I've used the correct pots).  It also doesn't respond to
CV very well.  I can't get it to respond to an envelope in the normal way.
Also, it just doesn't sound like anything -- my Roland-style Dr. Octature
filter sounds infinitely more Moogy.
Of all the suggestions, I think that upping the input drive is the best one,
and I'm gonna try that as soon as I have time to get back out into the
workshop.  I will replace that 56k feedback resistor with 120k to at least
make the input inverter unity gain.  I'm also wondering about that 10k
resistor feeding into the 10uF cap on the input to the ladder -- that
resistor will divide down the input a lot, given that it then feeds into
470R + 220R to ground.  I'm thinking about shorting that 10k resistor to see
what happens.
This is the problem I always have when I build circuits that I don't
understand (and I really don't understand how this ladder filter works, and
my Multisim simulations have been no help at all).  I generally avoid doing
so, but I really wanted a Minimoog filter in my system.  I'm tempted to do a
complete mathematical analysis of the filter, so I can get a handle on how
it works, but I just haven't had time.


From: Jay Schwichtenberg [mailto:jschwich53 at comcast.net] 
Sent: Sunday, June 11, 2023 10:33 AM
To: David G Dixon; David G Dixon via Synth-diy
Subject: Re: [sdiy] Yves Usson's Minimoog Filter

[CAUTION: Non-UBC Email]	
Going with something simple. They have different pin outs? 
Jay S. 

On 06/09/2023 9:08 PM PDT David G Dixon via Synth-diy
<synth-diy at synth-diy.org> wrote: 
Hello SDIY Team, 
I've built a ladder filter on my own PCB layout from Yves Usson's Minimoog
ladder filter schematic, and I must confess that the results are less than
stellar.  The filter works, but not terribly well, and it sounds nothing
like a Minimoog.  I used 2N3904 and 2N3906 instead of BC547 and BC557, but I
matched all of the transistor pairs in the ladder, and matched the four
transistors at the top and bottom of the ladder as well as the other four
transistors, using Ian Fritz's transistor matching circuit (which I also
laid out and built on a PCB).  I did not try to match the one PNP to the
current driving NPN below the bottom of the ladder. 
Does anyone here have any experience with this schematic?  Will it change
anything if I replace the 3904s with 547s, etc?  Everything else was exactly
as specified in the schematic.  This was my second attempt to build from an
Yves Usson ladder filter schematic.  My "Raging Bull" Taurus filter didn't
work at all. 
I just want to know if there is some fundamental flaw in this schematic
before I spend too much more time with it.  Any insights would be most
Dave Dixon 
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