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 keyboard that can generate S-Triggers (e.g. the Arturia Key Step) or use the Behringer 961 module to do the conversion for you. An excellent alternative is to use the AM993 and AMCP-1 or the AM-CP4 Routers, which have 3 or 2 V-trigger inputs, with selection switches. Another option is to use the 4HP wide AM997 Dual V-Trigger to S-Trigger module.
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 911 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.
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;
- ESUS ZERO adjusts the sustain level to zero
- ESUS MAX adjusts the sustain max level
- OUT ZERO adjusts the output offset to zero (within 1mV)
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 mV 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.
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.
AMSynths Upgrades 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, and the PCB. I have also replaced the 1MA pots for Attack, Decay and Release timing to improve the End of Travel (EOT) response.
I have also added a second timing capacitor, which is selected using a new front panel toggle switch. The original 10uF electrolytic capacitor is removed, and a small L-shaped PCB placed on top of the Behringer PCB. It is attached via 2 wires soldered into the pads left by removing the timing capacitor. A 5mm diameter hole is drilled in the front panel (using a template) and the toggle switch mounted into the hole.
The PCB contains two SMD tantalum capacitors; 2.2uF for a fast envelope (attack time of 0.25ms to 1.2 seconds) and 10uF for a standard envelope (attack time of 1ms to 6.5 seconds). This modification enables nice and snappy envelopes to be easily dialed in.
