[sdiy] Yves Usson's Minimoog Filter

Tom Wiltshire tom at electricdruid.net
Sat Jun 10 14:44:11 CEST 2023


The CA3080 is another example of that soft differential pair distortion that was mentioned earlier. If you add a bit of soft drive into the filter and a bit more into the VCA, you start getting to that Moog sound. Or at least, that's how the Taurus does it!


> On 10 Jun 2023, at 13:17, Benjamin Tremblay via Synth-diy <synth-diy at synth-diy.org> wrote:
> 
> I have built the 904 design from the original patent diagram. (And always eventually end up using op amps for the input and output buffers).  
> What I love most about the ladder filter is how much gain you have even when the cutoff is dialed way down. Big boomy bass, with potentially lots of harmonic distortion but filtered smooth as Moog butter. 
> It sounds like a Moog, but yes input level matters. A weird thing I observed, is if I pump in white noise at a very high level and then crank down the cutoff to sub-octave level I hear this weird “bubbling” effect where the over-driven input is causing the frequency of the filter to jump around in random stair steps. It’s super-weird and almost s/h-like. 
> 
> The Taurus engine used the CA3046 and it sounded great and was much more stable. (Also used a mere CA3080 for a VCA and it sounded great.)
> 
> Benjamin Tremblay
> 
>> On Jun 10, 2023, at 6:40 AM, René Schmitz <synth at schmitzbits.de> wrote:
>> 
>> 
>> On 10.06.2023 06:08, David G Dixon via Synth-diy 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.
>>> 
>> That minimoog sound comes partly from overdriving the filter.
>> 
>> To replicate the sound you need to create the same level situation as in the minimoog. 
>> 
>> Yves has a resistor in the design that is meant to be tuned for that.
>> 
>> 
>> 
>> I built mine from BC547s too. 
>> 
>> Overall I find that it hardly ever makes a difference which transistors are used, as long as you use ones with similar beta. 
>> 
>> (Transistor parameters stray anyway.... And the TIS97 was pretty much a jellybean go-to part. )
>> 
>> The CA3046 might have much lower beta, compared to the original TIS97. 
>> 
>> 
>> 
>> The caps are are different. 68n vs 47n. 
>> 
>> 
>> 
>> You might want to have a look at my version as well, much more bare bones, a bit more faithful to the original.
>> 
>> (But mine has different caps, I used the 22n from the Prodigy filter.)
>> 
>> 
>> 
>> One problem I had when I built mine, was that due to the different collector voltages in the diffamp one side heated 
>> 
>> up more than the other, imbalancing the diffamp. (And removing much of the resonance gain.) 
>> 
>> That could also happen in Yves version. (There run some 5-6mA through there, with 15V thats 90mW... Original voltages were lower.)
>> 
>> I placed a resistor in the other leg (the one with 22 / R25 in Yves schematic), to balance it. 
>> 
>> 
>> 
>> 
>> 
>> Best,
>> 
>>  René
>> 
>> 
>> 
>>> Dave Dixon
>>> 
>>> 
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>> 
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