Mods & Modules

902 + 911 Daughter Boards

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 IIIP synthesizer and Dual Sequencer setup that Klaus bought on 22nd December 1975, and used successfully for many years.

Whilst the Behringer modules cover most of the Moog modules I need, there are a few additional modules and changes I have recreated:

The projects can be made using standard components with the new PCB’s. I have used Lumberg jack sockets and non-Moog 14mm control knobs, but Behringer style Moog knobs can be used if you sacrify a , or I may use slightly larger knobs that are commercially available.

AM905 Reverb

Module Dimensions Here are the important dimensions of the Behringer modules:

  • Panel Thickness = 1.9 mm
  • Panel Height = 128.4 mm
  • Panel Width = 40.2 mm (8 HP)
  • Width of Silver Edge = 1.0 mm
  • Jack Hole Diameter = 8.7 mm
  • Pot Hole Diameter = 7.8 mm
  • Module Name Lettering Height = 3.0 mm
  • Module Name Lettering Inset from Top = 8.0 mm
  • Module Lettering Height = 1.8 mm
  • LED Hole Diameter = 3.0 mm

911 ADSR LED This is a simple addition of a red LED to the output of the ADSR, so you can see when its active. I designed a small PCB that fits to the rear of the 911 re-using the three existing lower mounting points with new M3 standoffs to the new PCB. A cylindrical red LED is used, with a new 3mm hole drilled in the front panel between the jack sockets and through the Behringer PCB. This is a safe area of PCB with no traces but the LED wires must be insulated as they pass through the PCB to avoid shorts.

The LED is driven by a NPN transistor fed via a 10k resistor from either the ADSR output, and a pad on the new PCB is right over the output pin of the jack socket. A PCB pin is soldered to the jack socket and into the pin pad. No further soldering is required as the power to the 911 comes from the new PCB, which has a new power inlet socket. The new PCB is all THD and easy to assemble and fit. The PCB also has 2-pin Molex connector for using with the 993.

AMSynths 993 Panel

993 Trigger & Envelope Voltages This is a replica of the original Moog design but without the colour illuminated slide switches. Even Moog could not find these when they re-issued the System 55 and had to have them 3D printed. I have used black DPDT slide switches which switch on the circuits as well as an indicator LED. I have used red LED’s for the Trigger Inputs, warm white LED’s for the trigger routing, and green LED’s for the 911 to 902 routing. They are cylindrical LED’s mounted on standoffs, which match the Behringer SMD LED’s.

A new PCB has been designed which has a 12V power inlet socket and pin headers for the connections to the 911’s, 911A and 902’s. Single cable Du Pont leads are connected to the pins and soldered to the various Behringer jack sockets. These are not normalised, so no PCB traces need to be cut on the Behringer modules.

904C Filter Coupler This is a simple module that does some routing and mixing of the 904A and 904B, so that you can switch them into Band Pass or Band Reject mode without re-patching. The original Moog uses transistor based audio and CV mixers but I have used modern Op Amps for both the signal and CV routing. The massive 9 pole 3 way switch (!) is replaced with high quality analog switches and a 3 way 2 pole switch.

I have designed a front panel and new PCB set, I use THD components for ease of build but it needs more PCB area than the Behringer SMD. I could not find knobs or control pointers as small as the ones Behringer have used, so a 995 or CP3A-M module has to be used as a donor for the knobs and jack sockets.

The module is hand wired into the 904A and 904B, into both the CV and audio signals. The Behringer module inputs and outputs are NOT normalised (to GND), so we can connect signals to the jack sockets without having to cut any PCB traces. It is an important module as Klaus used the filters in band reject mode.

928 Sample & Hold This is a replica of the original Moog design and more details are here. It is an 8HP wide module with a traditional sample and hold circuit and glide.

1631 Ring Modulator This is a replica of the module that Bob Moog built for Keith Emerson, and it was based on circuits in the Bode Frequency Shifter. The original circuit has two multipliers, one based on the UA796 and the other based on the MC1494. I have used a UA796 based circuit from the Roland 700 Modular, because I have the chips in stock and the circuit is sufficiently simple to fit on a small 8HP wide PCB.

PPG 303 LPF Klaus added a full set of PPG 300 analog modules and two PPG analog sequencers in 1976, and placed them on top of the Big Moog, where the ARP 2600 and EMS 8 Octave Bank usually lived. The PPG setup replaced the Synthanorma SQ312 analog sequencer, and the ARP 2600. He kept the PPG Modular setup until he moved onto Digital synths in the early 1980’s, and seems to have sold them quickly, whilst the Big Moog survived in his studio until 1996.

The Moog IIIP only has a few filters, so I am sure Klaus enjoyed the additional PPG sounds. There are two types of PPG filter; the 303 VC Modifier with a 24dB transistor ladder filter (and a VCA) and the 317 VC Filter, which looks like a combined 904A/B/C.






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