AW: original and ingenious circuits ... (was: in How the heck does the MS-50 filter WORK?)

[Magnus Danielson]

>>>>> "HJ" == Haible Juergen <Juergen.Haible at nbgm.siemens.de> writes:

Hi!

 HJ> 	[great description of Eventide omitted]

 >> The LFO in the EH SmallStone is a nice one as well, it is based around
 >> one CA3094. Hell, I like the rest of the SmallStone as well, I just
 >> lack a few features...

 HJ> Very nice circuit. I remember that Don has mentioned the direct
 HJ> exponential voltage control of 3080's a long time ago, and while
 HJ> it was clear to me that it could work in theory, I hesitated to
 HJ> consider it practically useful ... until I saw the Small Stone schemos.

 HJ> As fas as I understood it, the Iabc inputs of the all pass otas
 HJ> are driven by a *voltage*, while the trigger thresholds of the
 HJ> hysteretic switch are sensing the according *current*. So the
 HJ> modulation depth is temperature compensated. Very nice indeed.

The allpass connection is a nice one as well, one easy falls into
considering OTAs and op-amps mixed up to form a filter, not too
unsimilar the ASM-1 VCF BTW. The CA 3094's output transitors forms the
buffer needed in a neat and compact way. The relative freedom of the
output transistor is certainly used in the LFO.

In the LFO is the 1M pot being feeded with a 5.9 k and 2 V source and
is looking into the Vbe of the diff-pair current transistor. I guess
it would not be too difficult to make this thing track a CV.

Also note the clever use of the Color switch, someone has surely done
a case study to get the most out of it.

I have been long considering cloning the SmallStone since I don't want
to wreck the one that I have with all the mods. Some of my ideas
involves:

Width of modulation, thus the range for which the LFO sweeps (I will
save depth for later). Replacing the 100 resistor from the LFO with a
pot should do the trick.

Depth of modulation, thus the dry/wet mix between the phase core and
input signal. This mix is being performed by a fixed 30k/27k network
which could be altered with a pot to source the 27k resistor.

Color of modulation, thus the sharpness. This is being perfomed by a
3k3/4k7 divider feeding the core output to the input. Replacing with a
5k pot should work. The colour mode makes this a very wet unit...

One distinct property of the SmallStone in colour mode is that when it
sweeps down in frequency to reach the very bottom it will let some
higher frequencies in, so when running white or pink noise throught it
you get a windy noise which "fries" as it reach the bottom, Jarre has
used it and I discovered it by misstake myself! Taking this to an RTA
is a great enjoymment, you have waves squashing here and there on you
RTA display at the same time as you hear them, it kind of fits while
it makes a very cool effect :) I got to get a RTA again, I used to
borrow a DN-60 and misses it a lot with all it's brokeness.

CV input for modulation offset. Still considering on how to make this
mod go into minimalistic mode, that is, without needing an op-amp. If
using a breaking jack one could split away the cap in the LFO and
insert the CV and thus reuse the buffer in the LFO if one does not
trust the signal to be low-impedance enought. This should be combined
with some measure to turn of the current output, this can be done by
forcing the pin 5 contol to ground. It is a bit ugly, but it should do
the trick....

I have been considering an offset pot, so that one can position the
point around which the effect sweeps.

Another thing one might want to try out is a core depth switch
basically selecting which of the core outputs that is going to be used
for output. This would make it become similar to the Formant 24 db
filter which has such a switch.

So, without adding more silicon this cursuits lends itself to further
experimentations, or at least so does it seems.

In some notes that I have made I have a derivation of the allpass
properties that the core has as a function of components and Iabc
current. I could dig it up.

One of the things that I really like is the sonical appearance of this
thing.

Cheers,
Magnus