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.

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: +6.0V

These envelope timings are a bit different to the Moog original, although the snappy Attack is very useful, as well as the long Decay and Release times. I have checked that the timing potentiometers are all 1M log (which they are), so these timing are more to do with how the circuit has been implemented in SMD. The timing labels on the front panel do not match the actual times, but this could be do with Moog using a different Log taper on the potentiometers.

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 dont have an oscilliscope 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.

Summary The 911 Envelope Generator performs as expected (nearly) and is a good basic ADSR envelope generator, with a snappy Attack and Decay. 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 would like to add a red LED to my 911’s and there is some space on the PCB to the left of the Sustain control that I can use. A red LED is driven by a 10k resistor and NPN transistor from the output jack socket. These parts are mounted on a custom PCB with uses a SMD LED and a plastic LED and lens mount.

 

Copyright AMSynths 2019