[sdiy] Yves Usson's Minimoog Filter

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Wed Jun 14 12:50:55 CEST 2023

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.


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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