Wednesday 20 April 2022

912 Envelope Follower Investigation

 While I'm waiting for delivery of  the PCBs for my next two "Moog" module projects (991 passive filters, and 911+ envelope generator) I thought I'd take a look at the 912 Envelope follower module.

It seems simple enough, but will need probably need two stacked PCBs as it looks too much for a single PCB. Controls and trigger extractor on the front PCB, envelope follower circuit on the rear PCB.

The obsolete diodes will need some investigation. There is one resistor marked "25" on the Moog schematic. Given it's position I think it is "R25" not "25 ohms". I've overlayed pictures of the top and bottom of the original Moog PCB to try and identify it:

Moog 912 PCB
(Source 912 PCB pictures taken from

Tuesday 12 April 2022

904C Filter Coupler

 Now available, my latest "Moog" eurorack module: the 904C filter coupler.   

This module needs to be used with a standard Behringer 904A low-pass filter and a 904B high-pass filter, and gives band-reject and band-pass modes.

8HP, single PCB design with easily obtainable through-hole parts.

The Moog module used a 3-pole 9-way switch, which is expensive and not suited to the smaller Eurorack format. My design subsitutes an electronic switch for some of the switching, other parts of the circuit are as the Moog original.

Some soldering is required to the Behringer filter modules to provide the input, output and CV connections to the 904C:

  • Inter-connections to the 904A and 904B can be made easier using Fitzgreyve “Universal Daughter” PCBs.

  • No soldering is required to, or near to, any surface mounted (SMT) components on the Behringer modules.

  • Full instructions are provided in the build document. 


    See the "Moog eurorack modules" page for details.  
    Demo video coming soon.