[sdiy] Yves Usson's Minimoog Filter

Benjamin Tremblay btremblay at me.com
Wed Jun 14 13:25:11 CEST 2023


I could read more on this thread every day for the rest of my life! 

Benjamin Tremblay

> On Jun 14, 2023, at 6:58 AM, rburnett at richieburnett.co.uk wrote:
> 
> I suspect that this rising output impedance at the top of the ladder (and the loading of the diff-amp that recovers the output signal) is one of the main causes of the decrease in resonance ("Q") that you hear at the low-frequency end in Moog and TB-303 ladder filters.
> 
> -Richie,
> 
> 
>> On 2023-06-14 10:55, Stephen Makdisi wrote:
>> Ladder filters, both BJT and diode, have a very high output impedance at the top of the ladder, which gets worse as CV goes down. Later versions of the circuit, e.g. in the Rogue I believe, use an op-amp instrumentation amplifier as the diff amp at the top of the ladder to counteract this. Using one gives a consistent level and thus consistent resonance response across the range of the filter. Might be what you need, but it’ll require a new layout unfortunately. Otherwise increasing the input impedance of the diff amp you have may help with the resonance.
>> Or at least it should. I’m still in the simulation phase for a ladder vcf I’m working on, and ngspice seems to support this.
>> It’s not something that’s talked about in a lot of the online literature on these filters, oddly.
>> Also, the AJH MiniMod has a switch to control the resonance response. If they used an instrumentation amp, that switch may attach shunt resistors to the inputs of the amp, changing the impedance, and this resonance response. I don’t have one,  it it’s what I’m doing in my filter.
>> Cheers
>>>> On Jun 12, 2023, at 18:45, David G Dixon via Synth-diy <synth-diy at synth-diy.org> wrote:
>>> 
>>> So, I replaced a couple of the trimmers on my Minimoog filter board -- the 1k and 500R trimmers were one-turn wonders, and the 500R was actually a 470R cheapo flat trimmer that shorted out every time I put a screwdriver in it.  I replaced them with proper 18-turn trimmers, and I think the circuit works better now.
>>> So, it seemed to work pretty well.  I was getting good filtering.  It still didn't sound much like a Minimoog to me.  Also, I don't really like the way that it responds to high resonance.
>>> Then I swapped the feedback resistors on the input and output amplifiers (56k to 120k on the input, 120k to 56k on the output) to make them both unity gain and drive the filter a bit more.  This certainly did not improve things.  At the highest input attenuator setting, the filter just sounds harsh and unpleasant.  At a middle setting, it sounds much better, but simply isn't loud enough.  Hence, that was a bad idea, and I will swap them back.
>>> Then, using the same VCO and ADSR settings, I switched to my Dr. Octature Roland-style filter (but with gain cells to eliminate signal droop with resonance) -- this filter simply sounds brilliant (to me).  It's loud, snappy, has much more pleasing resonance, and "she responds like a limousine brought alive on the silver screen" to incoming CV from the ADSR.
>>> Maybe I just don't need a Minimoog filter.  Or maybe there is still something wrong with it.  I don't know.  I'm half done building the same board with 547/557 instead of 3904/3906.  I'm going to finish that board and put it in the module to see if it is any better.  If not, then I will probably just abandon the whole idea of building a Minimoog filter.
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