[sdiy] Yves Usson's Minimoog Filter

Andrew Simper andy at cytomic.com
Wed Jun 14 14:07:55 CEST 2023


I've done some basic models of this stuff, and I found a lot of the lack of
low freq self oscillating resonance ended up being a combination of the dc
blocking caps in the resonance path shifting the phase, and the finite beta
of the bjt input lowering the gain a little. So if you want a deeper res
use bjts with large beta, possibly two in cascade, and use larger dc
blocking caps. There are lots of other things going on but these two ended
up being big factors.

edit: I just had a quick look at this schematic -
http://www.arcenson.com/projects/Modular/M104A_Lowpass_Filter_Moog_904A_clone/Specifications/M104A_VCLPF_schematic.pdf
and it already has a cascade of two bjts for at the input stage (Q22 Q23
and Q7 Q8), so all that is needed is increasing C8 a bit if anyone wants
more low frequency res with this circuit, anyone keen to give it a go and
see what happens?

Andy

On Wed, 14 Jun 2023 at 18:50, <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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