On Sun, 16 Sep 2001
mate_stubb@... wrote:
> A CS-80 filter will be ultra cool. What are the details - authentic
> restricted resonance and cutoff range, both HP and LP sections built
> in....?
>
> Any special MOTM enhancement goodies planned?
As the resident CS loonie on the list, I guess I can make a few
comments, given that this module is one of my prototypes at the moment.
The MOTM-480 started out not as as MOTM at all, but rather part of my
effort to create replacement circuits for the series of custom chips
Yamaha made for their CS mono and poly keyboards. While tinkering with
the first prototype (which currently is hacked into one of the 'M' boards
of my CS-50), it occured to me that with a few relatively easy changes the
filter could become MOTM. I have ongoing correspondence with Paul and a
few others as to the specifics of the MOTM design, but all of the grunt
work currently boils down to my 2nd prototype, this time built for MOTM.
This MOTM prototype is actually six circuit boards: a 'base' board similar
to the regular 4.25"x5.50" board as used in most MOTM modules, and five
2"x2" plug-in boards which create my 'modular module'. These plug-in
boards are organized in circuit sections: 2 for CV to expo current
conversion, 2 for the 12dB state-variable filter elements, and one for
dual 'SVF adders' and dual resonance feedback control.
Of course, the production version will be a single board, but this
modular module allows me to try different components without having to
make an entire new board from scratch. I can test CA3280 OTAs, then swap
two boards and test NE5517 OTAs in 30 seconds. I can remove the expo
converters and try linear ones (this is more for CS machine study).
Anyway, you can see why, for a prototype, plug-in cards make sense.
On to the specifics. The MOTM-480 is a 2U module with two second-order
state-variable filters with intentional restrictions placed on minimum
corner frequency and maximum resonance. The filters are high-pass and
low-pass, hardwired together. There are three panel knobs for inputs IN1,
IN2 and IN3. There are panel knobs for initial HP freqency and initial LP
frequency. There are resonance panel knobs for HP and LP. The 8th panel
knob is the FM input attenuator.
A single 1V/Oct jack controls both filter cutoff frequencies, as is done
on the CS. A single resonance CV jack controls both resonant peaks as is
done on the CS. There is the (expo) FM CV jack for modulation of both
filters. Three signal input jacks, and two signal output jacks: LP output
from the 2nd filter as is done on the CS, and a jack to provide a bandpass
output from the first filter--something not done on the CS, and not
available (yet) from any other MOTM module.
There are a lot of parts on this module. 9 dual op-amps, 3 dual OTAs, 2
integrated matched transistor pairs, 4 trimmers and a zillion resistors.
There actually aren't very many caps, but a critical few are mylar, which
are essential to the sound. Resonance is restricted: 0V CV=0.5, 10V CV=5.
The lower limit of cutoff is restricted: 0V CV is about 40Hz (as best as
I can tell using a spectrum analyzer).
A few final notes: this module is not eating into Paul's R&D time, so
things like '310s and '130s can proceed as best they may. The MOTM-480 is
a thing of subtlety, which earmarks the CS line. Do not expect wild
responses from this filter--it is designed to emulate acoustic harmonics
(so said Yamaha), and as such it had limits placed on it to allow the
musician to explore the subtlety between those limits rather than explore
the 'edge'. I will make a few audio sample wav files once the 2nd
prototype is debugged to my satisfaction, comparing it to the IG00156
filters in my CS-80.
Oh, if someone wants to mock-up the panel:
x x
INIT FREQ HP IN1
x x
INIT RES HP IN2
x x
INIT FREQ LP IN3
x x
INIT REQ LP FM
x x x x
1V FM RES BP OUT
x x x x
IN1 IN2 IN3 LP OUT
MOTM-480
CS RESONANT FILTER
Crow
/∗∗/