[sdiy] Yves Usson's Minimoog Filter

Ben Bradley ben.pi.bradley at gmail.com
Sat Jun 10 09:37:49 CEST 2023

I have two thoughts. First is the schematic says (at each transistor)
BC547C, with the C suffix indicating higher HFE. The Onsemi/Fairchild
datasheet shows the C grade to have Hfe of 420 to 800. From my study
of the Moog filter, the original "special" TIS97 transistors had a HFE
of around 800. This is higher than the average 2n3904, but not WAY
higher, so I don't know if it would make a significant difference.
I've wanted to make this kind of filter with transistor sockets so I
could easily find out, especially if there's really any magic related
to the "special" transistors as some have claimed.

The other thought is the input signal level. I recall part of the
filter's sound has to do with the input being slightly or moderately
overdriven (with the Minimoog mixer level at full volume), so try
increasing the input level until the tone changes, and see if that's
more to your liking.

On Sat, 10 Jun 2023 at 00:12, 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 appreciated.
> Dave Dixon
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