MOTM-820 / OMS-820 VC LFO / Event Generator / Lag Processor

MOTM-820 / OMS-820Quantity: 2

From the Synthtech website:

The MOTM-820 is used to add slew to an input. The most common use would be to add portamento to a control voltage feeding a VCO. Check out the features:

  • Independent voltage control of both rise (UP) and fall (DOWN) times
  • Times variable from 500us to 5 minutes
  • Ganged UP/DOWN control to emulate standard portamento effects
  • A SHAPE control that varies the glide slopes from Linear to Log

From the Oakley website:

The OMS-820 is a companion module to the MOTM-820 voltage controlled lag processor. It adds powerful new features to the superb Synthtech module.

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 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.

2. EG mode

When the mode switch is turned to EG, 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/Reset.

The envelope generator is triggered by a GATE input.

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. The RESET input has NO effect in this mode.

Switch to AD and reset mode. The MOTM output will now rise to a +5V peak when the gate input goes high. It will then fall back to zero 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 use of the RESET input. A positive signal of above 1V or so will activate reset.

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

Remember that all rise and fall times are controlled by the pots on the MOTM820 and OMS820. And can be varied from 1mS to well over two minutes.

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

The OMS-820’s two pots 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. The depth of which is controlled by the MOTM’s pots.

Richard Brewster modifications:

  • Front-Panel-Express MOTM style panel combines the MOTM-820 and Oakley OMS-820 modules into a single 2U-wide integrated unit.
  • The GATE and RESET pushbuttons were omitted because of panel space.
  • Added end-of-cycle comparator PULSE output
  • Patching PULSE out back into GATE obtains a new LFO mode that cycles 0 to +5V. This works much like the old Serge Universal Slope Generator.

MOTM-820 board view 1

MOTM-820 board view 2



3 Responses to MOTM-820 / OMS-820 VC LFO / Event Generator / Lag Processor

  1. Joachim says:

    Any hints on were the ‘Added end-of-cycle comparator PULSE output’ is connected, too?

  2. Richard says:

    The end-of-cycle comparator is hooked to U3 pin 7 of the OMS-820, which is the buffer for the input (coming from the MOTM-820 output.) I got the idea from the way the Blacet EG2070 self-cycles. The incoming voltage is compared to a fixed 150mv, so that when it reaches that just-above-ground value the PULSE out goes high. When patched to the Gate input, another cycle is triggered.

  3. Joachim says:

    Thanks a lot! 🙂

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