Oakley OMS-820, MOTM-820 User Manual

Oakley Sound Systems

Oakley Modular Series

OMS-820 issue 2

The MOTM-820 Companion Module

User’s Guide

V2.01

Tony Allgood B.Eng
Oakley Sound Systems
CARLISLE
CA4 9QR
United Kingdom

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Introduction

The OMS-820 is a companion module to the MOTM-820 voltage controlled lag processor. It
adds powerful new features to the superb Synthesis Technology module. No component
changes are needed to the MOTM module, and the two modules are connected together by
just one removable connector hidden behind the front panels.

The module is designed to fit into a 1U MOTM style panel. It requires the standard MOTM or
Oakley power supply.

With this module you can make the MOTM-820 into a voltage controlled LFO with variable
waveshape. Or you use it to make a powerful voltage controlled envelope generator, with
manual set and preset modes. You will be amazed by the uses you can find for this module.

1. LFO

When the mode switch is turned to LFO, the output(s) of the MOTM module will oscillate
between +5V and -5V. The rise time of the waveform will be set by the UP control and the fall
time will set by the DOWN control. The frequency will change according to the settings of
both UP and DOWN. You can also change the rise and fall time together with the UP/DOWN
control on the MOTM. You can easily make saw, reverse saw, and triangle type waveforms.
The LIN/LOG pot on the MOTM will alter the shape of the waveform by introducing the
usual non-linearites of the exponential rise and decay.

Top frequency is 1KHz or so, whilst the lowest frequency is so low I got bored waiting for
one cycle.

2. EG mode

When the mode switch is turned to EG (envelope generator), you get a voltage controlled
AD/AR generator and more besides. This section has four sub-modes. These are selected by
two switches: AD/AR and Gated/Resetable.

The envelope generator can be triggered by two means, either a gate input as normal ADSRs,
or a push button, GATE, on the front of the OMS-820. When in AD (attack-decay) and Gated
mode, the output of the MOTM-820 will then rise until +5V is reached, whereupon it will fall
back to zero. Rise and fall times are governed by the MOTM-820 of course, and fully voltage
controlled. Removal of the gate signal will cause the decay phase to start prematurely.
Standard A-D EG behaviour.

Switch the unit to AR (attack-release) and gated mode, and the output will rise to +7V or so,
and stay there until the gate is released. Standard A-R EG behaviour.

For either of the above modes, pressing the RESET switch will start the decay or release
phase regardless of the gate's condition. The RESET input has no effect in this mode.

Switch to AD and Resetable mode. The MOTM output will now rise to a +5V peak when the
gate input goes high or the GATE switch is pushed. It will then fall back to zero

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automatically. Removing the gate has no effect on the output. This is 'one shot' mode.
However, the output may be forced to decay prematurely by pressing the RESET push button,
or by use of the RESET input. A positive signal of above 3V or so will activate reset.

Switch to AR and Resetable mode. The MOTM output will rise until +7V is reached whereby
it will stay there indefinitely, or until the RESET button or RESET input is activated.

Remember that all rise and fall times are controlled by the pots on the MOTM-820 and
OMS-820. And can be varied from less than 1mS to well over two minutes.

Four LEDs are included on the front panel. Two of these indicate the status of the gate and
reset inputs. Another will show the EG's status, this will light when the MOTM’s output is
rising or stationary. A fourth LED, a bicolour type, will give visual indication of the final
output signal.

The OMS-820's two pots together with the MOTM's own pots, control the rise and fall time.
This will enable you to set a time of say, 1 sec with the OMS-820's pots and then introduce a
modulation signal at the MOTM’s CV inputs. The depth of which is controlled by the
MOTM's pots.

The OMS-820 and the MOTM-820 are connected together by one 4 way 0.1" connector. It
can be simply removed if the modules need to come apart at anytime. The wires from the
connectors are simply soldered to special key locations on the MOTM-820. No component
changes are required to the MOTM-820 at all. Inserting a jack plug into the IN socket of the
MOTM will override any control signal sent by the OMS-820. The MOTM-820 will then
behave as a conventional VC Lag module.

Of Pots and Power

There are just two main control pots on the PCB. If you use the specified pots and brackets,
the PCB can be held firmly to the panel without any additional mounting procedures. The pot
spacing is on a 1.625” grid and is the same as the vertical spacing on the MOTM modular
synthesiser. The PCB has four mounting holes, one in each corner should you require
additional support which you probably won’t.

The design requires plus and minus 15V supplies. These should be adequately regulated. The
current consumption is about 20mA, although this varies slightly as the OUT LED turns on
and off. Power is routed onto the PCB by a four way 0.156” Molex type connector. Provision
is made for the two ground system as used on all Oakley modular projects, and is compatible
with the MOTM systems. See later for details.

Circuit Description

Let us first refresh ourselves with the operation of the MOTM-820. Basically it is a capacitor
that can discharged and charged at a controlled rate. The level to which the capacitor charges
to, or discharges to, is determined by the input voltage applied to the MOTM-820. The

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voltage across the capacitor will directly control the output of the MOTM module. When the
OMS-820 is connected up it will supply the input voltage to the MOTM and thus the
maximum level applied to that capacitor. It also needs to keep an eye on the output voltage
too.

As with many electronic circuits the OMS-820 can be split up in several little chunks. Grab
hold of the schematic now and let’s run through each bit in turn.

The LFO section is that bit centred around dual op-amp U1. This bizarre looking circuit is
actually called a precision Schmitt Trigger. The input comes in to the BUFFERED node. The
voltage at this point is a copy of the output of the MOTM module. U1 (pins 1,2,3) acts as a
comparator. When the voltage on pin 3 is more positive than the voltage on pin 2, the output
at pin 1 swings highly positive (high). When the voltage at pin 3 is more negative than the
voltage on pin 2, the output goes highly negative (low). Now because pin 2 is connected to
zero volts, or local ground, any positive voltage at pin 3 will make pin go high.

The output of the op-amp drives a transistor output stage. This set of four transistors simply
act as a switch that will give either +15V or -15V depending on the polarity of the output of
the op-amp. Op-amp outputs can’t, in general, actually go right to their supply rails. So the
high state is more likely to be +13V or so, while the low state is probably -13V or so. To
make a good symmetrical LFO waveform we really should have a accurate swing from plus to
minus. ‘Probably’ +13V is not good enough. I want exactly 15.00V... well, not exactly, but
the closer the better. So simply put, those four transistors provide a near perfect symmetrical
output swing from an imperfect op-amp output.

The U1 (pins 5,6,7) op-amp performs two important jobs. Firstly it steps down the +/-15V
swings to a more reasonable +/-7.5 volts. This gives us the peak charging voltage for the
MOTM-820’s input. This is not very important for when the MOTM-820 operates in the
linear mode, but it sets the asymptote of the log curves in the LOG mode. Leaving this at +/­15V would give an unsatisfactory log response, making the LIN/LOG pot do very little. The
second function of the op-amp is to invert the output. Inversion is all important to get the
thing to oscillate, more about this later.

Let’s go back to the first op-amp again, and have a look at pin 3. Pin 3 gets its instructions
from two sources. Firstly the voltage on the BUFFERED node, ie. the output voltage of the
MOTM-820. Secondly, the highly symmetrical output of the transistor switcher. The ratio of
R12 to R13 sets the weighting each input has on the final output. It is this weighting that is
crucial to the operation of the LFO mode. Say the output of the switcher is at -15V, then with
zero volts at the BUFFERED node, the switcher’s output will stay stuck at -15V. Its staying
in the low state by its own output forcing pin 3 low. However, consider what happens as the
voltage on the BUFFERED node starts to rise, so does the voltage on pin 3. At around +5V,
the voltage at pin 3 is approaching zero. As the voltage rises and crosses zero, the op-amp
flips state to its high level. The switcher responds and flies to +15V. This is fed back into pin 3
via R13 and forces it higher still. The op-amp is now stuck in a high state.

But remember that the BUFFERED node is being driven by the MOTM module, which in turn
is being driven by the output of the inverting amplifier, U1 (pins 5,6,7). This output is now
negative, so the MOTM-820 now forced to discharge, and hence its own output will fall. The
falling voltage now causes pin 3 to slowly fall from a positive value back to zero. And once

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again, as it crosses zero volts, the op-amp swings and the switcher responds... and the process
repeats again and again and again.

Of course, the speed at which the process repeats is set by the UP and DOWN pots. You can
get very fast at around 1000 complete cycles a second, and you can get very slow. Very slow
indeed.

On my MOTM-820 I noticed slight instability in the waveform when driven very fast in LOG
mode. This is feature of the MOTM’s circuit core, and not a fault of the OMS unit.
The envelope generator (EG) is more complex, but the principle is very similar to the LFO
really. However, an EG will not cycle endlessly, but needs trigger signals to initiate certain
phases. The OMS-820 in conjunction with the MOTM-820 form a very powerful envelope
generator, that behaves quite different from your usual ADSR type. It has four modes, of
which the attack-decay (AD) mode is the most complex.

The GATE input is applied to initiate the attack phase. This is a switch type signal that is
either at around 0 volts when off, or any positive voltage greater than 3V when on. The OMS­820 can easily handle greater voltages without damage. D8 protects Q9 from any negative
inputs.

A push switch, G-S, named GATE on the front panel, can also be pressed to initiate the attack
phase. C4 and R19 smooth off any contact bouncing in the switch. D6 prevents C4 from
smoothing any incoming gate signal, which would not be a good thing leading to unwanted
delays.

When a positive gate arrives, Q9 turns on and pulls its collector down to ground or 0V. This
inverse version of the applied gate signal is sent to two destinations. One is another transistor,
Q12. This is configured as another inverter. Thus the output of Q12 produces a copy of the
gate signal that swings from 0 when off to +15V when on. R35 passes some current back to
the first transistor. This creates a type of Schmitt trigger action which makes the transistors
change state faster. It also allows slowly varying signals to trigger the OMS-820. For example
you can use a slow sine wave or aftertouch CV to fire the EG.

The output of Q12 is passed on to a CR network that acts as a differentiator. This circuit
produces a positive voltage spike when the gate goes high. The duration of the spike is
determined principally by the values of C6 and R23. D5 prevents a negative spike being
produced when the gate goes low. The positive spike triggers an RS flip-flop circuit based
around two NOR gates, U4.

A flip-flop is a sort of a one bit memory, or latch. Once triggered by a positive going pulse at
pin 1, it stays latched. You can only reset it by removing the power or a reset pulse at its other
input, pin 9. When the flip-flop is latched, pin 10 goes high and pin 4 goes low. The output at
pin 10 is passed via R16 to the MOTM-820’s input, thus causing the MOTM to start to
charge upwards. R16 is chosen to interact with the MOTM’s input impedance of 100K to give
an input signal of 7.5V in the high state. When the flip-flop is active the SET LED is lit.
Several sources are able to reset the flip-flop, which one depends on the mode you have
selected.

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