T911-A Dual Trigger Delay and T993 Trigger Envelope Voltage Router


These two modules are combined in many System 55 instruments together with three 911 EGs and three 902 VCAs to do an internal pre-wired patching of two STrigger events to three envelope voltages to corresponding VCAs. This is done by the T993 Trigger Envelope Voltage Router which handles two independent STrigger events, e. g. from a keyboard, a MIDI interface or a sequencer. These STrigger events form a logical OR and can be connected internally up to three EG modules. The ouput envelope voltages of these EGs are connected back to the T993 where they can be connected to three corresponding VCA modules. This can be done by switches on the front panel. So the T993 is not really a router as the module name implies because the targets of the trigger events and the envelope voltages are predefined and therefore cannot be determined. The benefit of the T993 is to provide an internal standard patching which saves external patch cords and reduces patch complexity. If the internal wiring is switched off the EGs and VCAs can be included in other patches or used for other purposes than the standard functions.

The other part of this dual module consists of a so called "Dual Trigger Delay", which receives an incoming STrigger event and routes this event to an output after a selectable delay time. Please note: when the input trigger event terminates, the output trigger terminates as well, so the termination is NOT delayed, just the beginning of the trigger event.
The T911-A consists of two cascadable delay units.
The two modules form a functional unit, because the second and third EG are initiated by the delayed output trigger of the T911-A.

Although both modules were sold separately by Moog I've never found any pictures ore documentation of original modular systems which contained only one of both. They were always sold as functional unit, so I combined the T993 and the T911-A to one single module. With this combination module the functional unit is complete.

T911-A Dual Trigger Delay and T993 Trigger Envelope Voltage Router combination module

Complete functional unit of the Moog System 55 clone:

Internal pre-wiring of the functional unit:

Differences to the original

Like for my T902 or T911 clones I decided to combine both modules (T911-A and T993) to a module combination behind one front panel for the reasons mentioned above.

Like for my TCP3A clone I replaced the illuminated switches of the original 993 module, which I couldn't obtain, by standard switches in combination with LEDs indicating the "On" - status.

Other add-ons are the 6mm LEDs of the T911-A which indicate incoming and (delayed) outgoing STrigger events. I don't understand why Moog did not include this in the original 911-A modules as this helps to see what's happening and to check module behaviour optically.

Complete Module

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Frontend / Human Interface

T993 Trigger Envelope Voltage Router

The two switches on top of the front panel determine which incoming STrigger event is processed. The module can handle two STrigger events, so there are two switches. As the label says the STrigger events - if switched on - are routed to the input of the top unit of the T911-A Dual Trigger Delay on the right.
The left switch of the second row routes the incoming STrigger events mentioned above to the left envelope generator (see functional unit above) at the same moment. The left switch of the third row routes the (delayed) output trigger event of the top delay unit to the center EG. The left switch of the fourth row routes the output of the lower delay unit of the T911-A to the right EG module.
The right switch of the second row routes the resulting envelope voltage of the left EG to the left VCA (see functional unit above). The right switch of the third row routes the resulting envelope voltage of the center EG to the center VCA. And the right switch of the fourth row routes the resulting envelope voltage of the right EG to the right VCA.
All sources and targets are labeled on the front panel.

T911-A Dual Trigger Delay

On the top of the front panel one can find the first STrigger delay unit. The potentiometer determines the delay time until the incoming trigger event is forwarded to the output. LEDs indicate the incoming and outgoing STrigger event. The T911-A can be used without the T993 router, for this purpose the Cinch Jones connectors can be used as input (female) and output (male) terminals for the STrigger events. The second delay unit works identically.
In the center of the front panel the coupling mode of the two delay units can be determined. "OFF" means that no coupling is done, "PARALLEL" means that both delay units are fed with the same STrigger input event, and "SERIES" means that the second delay unit is fed by the output of the first one.


A nice effect can be achieved if the two delay units are coupled in "SERIES" mode and the keyboard STrigger event is routed to the first EG/VCA combination which controles the sound amplitude, the delayed output of the first delay unit is routed to the second EG which controls a T904-A VCF, and the delayed output of the second delay unit is routed to the third EG which controles the pitch of the sound. If you press a key the sound starts unmodulated, after a short time the VCF starts to modulate the sound and after another short time period the pitch of the sound rises. A (delayed) sequence of modulation events like this is very important in electronic music, much more than starting complex modulations at the same time.


Schema of the trigger delay unit (one stage only): Attention: Clicking the schema means acceptance of disclaimer on page bottom! Please refer to the original schematics for more details / component values.

STrig and the Cinch Jones connector

Cinch Jones jack and plug

The gate event of a key beeing pressed on the keyboard or of a step of a sequencer is beeing reached follows a different concept in many older Moog synthesizers, inclusive the modular systems, compared to other synthesizer systems. Other synthesizer manufacturers and modular systems use a so called "gate" event which is physically spoken a binary control voltage of 0V (gate off) and somewhat between 5 to 10V (gate on). This control voltage is not handled separately from other control voltages and can be used for other purposes than note playing also.
The Moog concept differs. The "gate" event is not represented as a voltage but as a current sink (STrigger) and therefore should not be mixed with other control voltages. To prevent an accidental connection between signals, control voltages and the STrigger event no 1/4" phone plugs are used to submit the gate event to corresponding modules but so called 2-pole Cinch Jones jacks and plugs. The Cinch Jones jack serves as input for STrigger events, the plug as output.
The 2-pole Cinch Jones connector contains two blades of different width on the plug side and the corresponding females on the jack side (see pictures). As the STrigger concept is a current sink, only one pole would be necessary to transmit the event. This is done via the smaller blade. The wider blade works only as ground reference for the connected modules, not as shield like in phone plugs.

Circuit description of the T911-A Dual Trigger Delay

An incoming STrig event causes a voltage drop at the STrig input node of the circuit. This causes a voltage drop at the base of Q1 and Q2. Q1 opens (PNP), Q2 closes (NPN) and the timing capacitor C1 is charged via R5 and the front panel pot P1 (DELAY TIME). The voltage of C1 is buffered by the FET J1 which works as source follower. The internal adjustment trimmer TR1, R9 and Q3 form a comparator with a threshold determined by TR1, so this trimmer defines the max. load time until the comparator opens Q3. Q4 closes, and Q5, which is the current sink for the STrigger output / next delay stage, opens.
The incoming STrigger event / input is also buffered by R13 and Q6 and therefore indicated by the LED D2. The same indication is implemented for the outgoing STrigger event via R15 and Q7.
The second delay stage works the same way.
For the coupling of the two delay stages a rotary switch forwards the input of stage 1 directly to the input of stage 2 in "PARALLEL" mode and forwards the output of stage 1 to the input of stage 2 in "SERIES" mode. "OFF" decouples the two stages / does not do any connection at all.

Part replacements

I tried to build my module as close to the original as possible, but I had to do some changes due to actual components I use instead of the originals:

Component changes

Beside the functional addition of the optical STrigger indicators and the basic changes mentioned above I changed the following component values:

Board of the T911-A clone

  • Board dimensions: 160 x 100 mm
  • Left side: Power supply, T993 routing connectors to the T902 VCAs
    In contrast to the original concept of the Moog modulars I supplied the board with a +/-15V power supply. Voltage regulators on the board convert it to +12V and -6V. Benefit of this is a higher stability of the power supply.
  • Lower side: Routing connectors to the T911 EGs and the T993 LED connectors
  • Upper side: The two delay units

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YouTube video

Please send questions or remarks to:
Carsten Toensmann


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