912 Envelope Follower development

 

Debugging of the 912 envelope follower is proceeding. I found a couple of minor errors in my PCB design (missing tracks!) that are easily corrected.

Once adjusted using the published Moog procedure, it seems to work OK - mostly.

The one issue I need to resolve is that althought the control output is fine down to 1mV RMS input signal, it becomes unstable when there is no input signal present.

 

Below are a couple of oscilloscope traces of the "control output" (upper) when the input is a repeating 250Hz sine wave envelope (lower).

 

I believe there is an error in the published Moog Schematic - the CL-1 diode in the trigger generator is the wrong way round ? With the schematic, the diode simply provides a path from +12V to 0V via R9A (330 ohm) - R9A just gets hot!

I believe the diode needs to be reversed, this is supported by pictures of actual Moog 912 units, where the diode cathode is clearly connected to R9A.

Another 904C Filter coupler video

Couple of videos of the 904C Filter Coupler in action, this time one built by a customer:

 

 

Eurorack "Moog" Modular

Having started with "Moog" modules in eurorack just over a year ago, my planned two cases are now complete:

I orginally planned this as a Moog IIIC lookalike. I made the decision that the lower ("console") row would be normal eurorack height, not the reduced module height used in the original (5U high) Moog modular.

Key to modules: (B) Behringer, (F) Fitzgreyve, (A) AMSynths:

Top cabinet:
(B) 914, (F) 992, (B) 904A, (F) 904C, (B) 904B, (B) 904A with AMSynths daughter board, (B) 994, 2 x (F) 911+, 3 x (B) 902, (A) 993, (B) 911, (B) 911A, 2 x (B) 911.

Bottom cabinet, upper row:
(F) 901ABBB, (F) 901ABBC, (F) 901, (B) 921, (B) 995, (B) 960, (B) 962, 2HP Tune

Bottom cabinet, lower row:
2 x (F) CP3, (F) CP3A, (B) 923, (B) 961, (A) 924 mixer PCB with (F) panel.

The cases are recycled vintage mahogany. I've already got more modules than case space, so another case is being build. Not fitted, lying on top: (F) 991, spare (F) 901B, another (B) 921.

911+ Envelope Generator

 

The Behringer 911 Envelope generator is excellent value for money, but suffers from difficulty getting values at the extremes of control travel due to the use of budget potentiometers. This is particularly a problem at the low end of the T1,T2, T3 controls which leads to difficulty in setting short envelope times. The panel control values also do not generally match the actual envelope times.

This module is designed to address the above issues, add some additional functionality (looping modes, linear envelopes, "Punch", Time and Level modulation), while maintaining the Behringer System 55 “look and feel”.

The Fitzgreyve 911+ uses nothing resembling the original Moog circuit, being based on a PIC microcontroller. Single PCB, all through-hole components.

 

 

 

ADR envelope in LFO mode.

 

991 Filters

Our latest "Moog" module for eurorack:

The  991 Filter and Attenuator is a passive module (i.e. no power required) that contains a switched low pass filter, a switched high pass filter, and an attenuator.

Single PCB with easily obtainable compoenents.

May be built using vintage Mullard C280 aka "tropical fish" capacitors (as in the picture below), or with smaller modern capacitors. 

901 Voltage Controlled Oscillator

 Originally it was not my intention to build this module, but having got the 901A and 901B working this was more of an academic excercise to prove that following the Moog approach of combining a 901A and 901B in a single module actually works.

There is no Moog documentation for the 901, so some educated guesses have been made.

The 901 Voltage Controlled Oscillator module combines the two "rear" PCBs from my standard 901A and 901B modules with a new single 16HP "front" controls PCB to give a single module oscillator. 

It includes the same fixed and variable level waveform outputs as the Moog original, but I have added a "mix" output (using an in-built CP3 mixer with selectable positive or negative output), plus a pulse width modulation input. 

 

 I have now created a video of the 904C filter coupler: