Hello Tim, thank you very much for your efforts. Highly appreciated.
Thanks to your suggestion and my solder sucker I've swapped the B50K
pot for a C50K in a fly. The result is much more to my liking.
However it effects the signal/self osc ratio which is no prob. since
one can turn down the input level to adjust the ratio. Now, setting
the res. pot fully cw the filter resembles the MS10 one. At least IIRC
since I sold my one a long time ago.
Still I think the overall smooth character of the vactrol filter and
the rather rude self osc. seems to me a bit contradictory even though
others will actually like this.
I still like to give your 2nd mod a try but I'm not sure wether I get
it. Even worse my A101-2 is a prototype thus there are no part numbers
on the board. Pls verify if I've understood how to proceed:
I'm going to link the res. pots wiper (= 2nd pin i.e. the one in the
middle) with one resistor on the board. Since I've no part numbers:
You're talking about one of the 2 resistors located btn the brackets
of the res. pot: the 47k one One of it's leads is already connected
to the wiper! The remaining lead should go to the wiper as well
Sorry, just want to make sure.
Many thanks for you efforts! You did me a big favor! Regards, Ingo



--- In Doepfer_a100@yahoogroups.com , "Tim Stinchcombe" <timothy@...>
wrote:

>
> Hi Ingo, Dieter, list,
>
> After looking at the schematic Dieter kindly sent I believe the
> reason for the 'suddenness' of the resonance/self-oscillation is the
> type of filter itself: it's a standard Sallen-Key low pass topology,
> and I've read in several books that for normal use these are not
> regarded as very practical filters, due to the sensitivity of the
> resonance to the component values used, i.e. it is hard to guarantee
> any given value of resonance because small changes in component
> values give rise to large changes in the resonance. I came across
> this before in the work I've done to date on the Korg35-based
> MS10/MS20 filters (there's a little algebra on it in my report), as
> these are also Sallen-Key types, and as then, I'm finding it hard to
> come up with a good way of sensibly representing this problem, and
> how it might be affected and/or alleviated by different circuit
> topologies (and for example different pot laws). However I have
> breadboarded a very simple fixed-frequency Sallen-Key filter, and
> comparing experimental results with some simulations, I now have a
> greater trust in what the simulations are saying (which can be hard
> to interpret), and the upshot is I believe it should be possible to
> make the resonance come in in a more controlled fashion. This should
> be achievable by replacing the resonance pot P5 with a 'reverse log'
> type, law 'C' (keeping it at 50k); and another thing which would seem
> to be a further step in the right direction would be to connect the
> pot wiper to the end of the pot connected to R17 (=47k).
>
> Unfortunately since I don't have one of these modules I cannot try
> it, and so cannot be sure for certain. However Dieter has said he
> will look back at the notes made during the module development to see
> if anything similar was tried then. The simulations show that one
> possible problem with doing this is that the peak in the resonance
> might occur for a lower knob setting: trying it out for real is going
> to be the only way to find this out, but even if it does, the
> solution should be quite simple - tweaking the value of R17 (although
> since we know the topology is very sensitive to small changes in
> component values, the required value might mean the use of a preset).
> Also, since I have only simulated the *filter* configuration of the A-
> 101-2, it is possible that the change has an undesirable effect on
> the VCA side of things (! ).
>
> I've placed a small file in the 'Files' section showing some plots of
> the effect of changing the pot on my simple experiment. The schematic
> shows the Sallen-Key set-up: the key thing it shares with the A-101-2
> is the topology around the final op amp providing the gain; other
> component values were chosen for convenience (and note since I have
> equal value capacitors, the max gain for self-oscillation needs to be
> three, as opposed to 2 in the A-101-2, hence the different values
> there). The blue trace shows how the filter gain shoots up in about
> the last 5% of the pot travel for a linear pot, which effectively is
> the problem. The pink trace shows how the gain rises much more
> steadily throughout the pot rotation, using a law 'C' pot with the
> wiper connected to the one end as stated above. I should point out
> that the traces were, of necessity, obtained via slightly different
> means for the different pot types: using switches I rigged it so that
> as well as measuring the filter gain, I could simply switch the
> filter caps and resistors out and measure the gain of the non-
> inverting op amp set-up. For the linear pot, having measured both
> gains, I inverted the theoretical relationship between the pot
> position and the op amp gain (simple, since the pot is linear), so
> this enabled me to plot the filter gain versus pot position - this
> gave a satisfyingly very close trace to that output from the
> simulation (in which it is easy to relate pot position to everything
> else), which is what convinced me that I could trust what the
> simulations are telling me. For the 'C' pot (robbed from my A-102 as
> I don't have a spare one lying around!), I have no way of telling
> exactly what the relationship between pot position and its resistance
> is (the log pots I use in my simulations are 'guesstimates' from
> datasheets), so I have nothing that I can invert, so the figures are
> literally taken from the physical settings I used (and hence why the
> graph is nowhere near as smooth).
>
> I'm hoping that Dieter or someone can try this suggestion easily
> enough and let us know whether it is as useful as I'm hoping it is!
>
> Tim
>
> --- In Doepfer_a100@yahoogroups.com , <hardware@> wrote:
> >
> > > Hi Ingo,
> > >
> > > > Even though the
> > > > filter itself is rather smooth sounding the self oscilation
> comes all
> > > > of a sudden
> > >
> > > I'd be quite happy to cast an eye over the circuit to see if I
> can come
> > > up with a suggestion as to how to change this, but unfortunately
> I
> > > don't actually have this module and so do not have access to the
> > > schematics, so unless someone can send me a copy ... Dieter
> Anyone
> > >
> > > Tim
> >
> > Tim,
> >
> > I'll send you the schematics.
> >
> > Dieter
> >
>