align the white line to the “0” tick mark, and tighten the hex screw. The silver
part of the knob has a protective clear plastic overlay that can be removed if
desired. Gently rub with your fingernail across it and it will peel off.
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CONGRATULATIONS! YOU HAVE FINISHED BUILDING THE MOTM-485!
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All that’s left to do is test it! But before we do, please read the following Theory of
Operation.
THEORY OF OPERATION – by Scott Rider, Chief MOTM-485 Designer
The MOTM-485 is a 2nd-order Sallen-Key switchable highpass/lowpass filter that is based
on the same circuit configuration as that discovered to be used in the Yamaha “NE”
submodule set that Yamaha developed in the early 1970s for their GX-1 stage synthesizer.
The filters represented by the MOTM-485 are the NE10400 lowpass and NE10500 highpass
filter “potted” submodules; that is, the NE modules were small circuit boards composed of
standard parts and placed in shallow plastic “potting” boxes with connection pins facing up,
then an epoxy resin/catalyst was poured into the boxes and allowed to harden. This is
similar to the way ARP modules for machines like the 2600 and Odyssey was made. The
Yamaha NE submodule set was used in their SY-1 and SY-2 preset solo synthesizers and
their Electone CSY-1 & CSY-2 organs in addition to the GX-1.
Since the NE10400 and NE10500 are nearly identical circuits, with the use of a DPDT
switch it is possible to route the signal path such that both filter modes can be provided in
one module. For the purposes of adapting the filter core to patchable modular use, the
original circuit has been updated to a MOTM voltage control front end for 1V/Octave
frequency as well as reversing FM input. Aside from that and the passive audio mixer
circuit to provide three signal inputs, the core filter circuit remains the same as that used
in the original submodules.
Diode rings as Voltage-Controlled Elements
Referring to the schematic, note there are two diode rings, or “bridges” that are made from
the base-emitter junctions of four transistors of Q7 as well as Q10. In each ring, the
transistors (connected as diodes) form an “equivalent impedance” circuit: a control current
enters the “top” of the ring (pins 6 and 9 of the Q7 or Q10 array) while the same amount of
control current leaves through the “bottom” (pin 3). These control currents are supplied
from the CV front end for resonance (Q7) and filter frequency (Q10). As current flows
through either “side” of the diode ring, a corresponding forward voltage is developed across
each diode. The voltage-current aka “V-I” characteristic of diodes being nonlinear, on paper
it typically appears as an exponential curve upward. The math starts to get a little complex
here is things head toward AC systems analysis and we discover we’re talking about
impedance and not simple resistance, but for simplicity just consider that for each value of
control current “I” there is a resultant voltage “V” on the curve. Taking the basic electrical
formula V=I x R (voltage = current times resistance), rewriting it as V/I=R the “apparent”
SYNTHESIS TECHNOLOGY
PAGE
12
MOTM-485 ASSEMBLY 8/11/05
WWW.SYNTHTECH.COM
Содержание MOTM-485 GX-1 Diode VCF
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