Behringer 911 Envelope Generator

911 ADSR

Introduction In August 2020 I bought a set of Behringer System 55 modules, as my first journey into Moog modular. My plan is to recreate the Klaus Schulze Moog IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

Rather than rack up a set of Behringer modules and ride away making music, I thought it would be useful to explain how the modules work, how to calibrate them, and how I have set them up and augmented them. I will also check their specifications, to see if they replicate the originals accurately and so I can understand them better.

911 Rear of PCB

The 911 Envelope Generator This module is a simple transistor based design with the traditional 4 stages but with Sustain and Release pots reversed in panel location. The module uses the Moog S-trigger standard rather than the familiar Euro Rack gate signal (or V-trigger), with a S-trigger being a momentary connection to ground. Once ground is released the envelope generator moves from the Sustain to the final Release stage.

You will need either a V-trigger to S-trigger cable, a S-trigger keyboard (like the Arturia Key Step) or use the 961 module to do the conversion for you. I am building a bigger modular system, so I have used the 961.

Envelope Times

The output level of the control signal from the 911 should be 0 to +5.5V with a 10% tolerance, and we should see Attack, Decay and Release timings with a range of 2 ms to 10 seconds. Although earlier Moog documentation states 10 ms as the fastest time. You can see the output timing plot at maximum times and 50% sustain level in the picture, the grid is at 10 seconds.

Envelope Timing Testing my 991 gives these measurements;

  • Attack time: 1 ms to 7.5 seconds
  • Decay Time: 4 ms to 20 seconds
  • Release Time: 10 ms to 20 seconds
  • Sustain Voltage: 0V to +6.0V

These envelope timings are slightly different to the Moog original, and I have checked that the timing potentiometers are all 1M log, so these timing are more to do with how the circuit has been implemented in SMD. Whilst the timing labels on the front panel do not match the actual times, that is not really a concern. There is an End of Travel (EOT) limitation on the timing pots where the full CCW position will have a step into the first position on the taper itself. For example the Attack pot steps from 1ms to 15ms with no  intermediate values. I have replaced the Behringer pots with more expensive Alpha 9mm pots.

911 Trimming

Setup The Behringer user manual is not always in the box, so here is a link to the 911 Quick Start Manual. The 911 has been aligned accurately at the factory and there is no mentioned of the trimmers in the manual. There are in fact three single turn trimmers accessible from the rear;

  1. ESUS ZERO adjusts the sustatin level to zero
  2. ESUS MAX adjusts the sustain max level
  3. OUT ZERO adjusts the output offset to zero

The original Moog setup (picture on the right) explains these briefly and has a nice diagram of the glitch between Attack and Sustain maximum values which we do not want. The only minor adjustment I made was to adjust the OUT ZERO trimmer to get the output offset down from +10mV to +1mV, at maximum Sustain. This was not easy to achieve as its a very sensitive single turn trimmer. In reality 10 ms is perfectly acceptable, and some people like to set a slight – value to ensure the VCA is fully shut off.

The Attack peak was the same as the Sustain level when monitored on an oscilloscope, so I left the ESUS trimmers alone. If you don’t have an oscilloscope then listen to the Attack to Sustain stage change with the ADSR patched into a filter. The 911 produced a nice +6V at peak, which matches the Moog standard.

911 with red LED

Summary The 911 Envelope Generator performs as expected (nearly) and is a good basic ADSR envelope generator. It has been setup at the factory correctly and should be just “plug and play” into your modular system. There is no LED indication or a local push switch to trigger the envelope, but that is just how Moog designed them! There is no inverted envelope output either, and that requires the CP3A-M Mixer to achieve control voltage inversion.

I have added a red LED to my 911’s using a custom AMSynths daughter PCB which attaches to the main Behringer PCB using spacers. The red LED is driven by a 10k resistor and NPN transistor from the output jack socket. A hole has to be carefully drilled in the front panel for the cylindrical THD LED to fit in. I have also replaced the 1MA pots used for Attack, Decay and Release timing to improve the EOT response.


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