Jurgen Haible Trapezoid VCA

Quantity: 2


The Trapezoid VCA is a DIY project from Jurgen Haible, renowned modular synthesizer engineer.  It emulates the EMS VCS3 and Synthi A Trapezoid Generator.  I am not familiar with the EMS module, and I wasn’t quite sure what to expect of this one.  Now I’ve built and used it, it is indeed everything I had hoped for in a combined Envelope Generator/VCA.

Download the FPD Panel

The Envelope Generator (EG) part is unique among envelope generators in that it produces a trapezoid shaped waveform with an Attack-On-Decay pattern.  When triggered, the output rises linearly to maximum at a rate determined by the Attack pot, holds for an amount of time determined by the On pot, and then decays at a rate determined by the Decay pot in combination with the Decay CV input.  At settings less that fully clockwise (max time) the Off pot determines how long the output stays at the minimum value (i.e. off) before automatically re-triggering.   I would in fact call it a Trapezoid LFO with variable rise rate, on  time, fall rate, and off time.

Be sure to check out the Dragonfly Alley page for the Trapezoid.

VC Decay

The VC Decay input has its own reversible attenuator (D CV) pot, which controls the amount of CV applied positively when right of center, and negatively when left of center.  A more positive CV increases the decay time, like turning the Decay pot clockwise.  An immediate application is to patch the minus Out envelope back to D CV, which results in an exponential decay curve more like you get with a regular EG.  This is so useful that I made it a jack-normal connection.

Trigger Modes

When Off is at max then you have three ways to trigger a cycle of Attack-On-Decay.

  1. Press the handy Manual button
  2. Patch a gate into GATE IN
  3. Enable Auto Trigger mode

In Auto Trigger mode the EG is triggered by the audio input to the VCA reaching a threshold, which is set by a pot or a trimpot.  I chose to use a trimpot.

Envelope Outputs

The envelope is available as four different outputs from the PC board (more on this later).  I brought out the ‘S’ outputs and made a modification so that the positive output goes from zero to +5V and the negative from +5V to zero.  The ENV LED indicates the OUT + envelope level.


Although the PC board provides many different input and output choices for the builder, I chose to use one buffered input and the buffered output.  There is no separate CV input for the VCA available; it is driven by the internal envelope.  It has trimming for zero and initial gain internally.  The GAIN pot establishes the Initial Gain level when you wish to allow some of the signal through when the EG is at zero.  Unlike the initial gain on other VCA modules I’ve used, turning up the initial gain does not lead to a higher output level when the envelope is at maximum.  That is, it does not sum with the envelope.  In fact if you turn GAIN all the way up, the envelope will have no effect!  The envelope merely increases gain up to the maximum.


I used the MOTM power connector with the on-board 12V regulators.  I made my own mounting bracket using aluminum stock from Lowes.  It worked quite well.  Also for this project for the first time I used 0.1″ MTA connectors with headers on the board.  This turned out to be really useful, since I had to remove the board to add some trimpots.  The PC board uses a clever design for pot connections, so that the builder can choose to install either a trimpot or a 3-pin connector for an off-board pot.


I added three trimpots, which are seen in the prototyping area of the board in the photo. A 100K trimput in series with R70 adjusts the output gain of the VCA, but it turned out that it wasn’t needed.  The full-on gain using the buffered input and output is about 1.1 with the unmodified circuit and did not need to be increased.  The other two trimpots adjust the levels for the plus and minus envelope outs.  The 100K gain trimpots were inserted in series with R38 and R42 and allow attenuating the envelope levels, which from the S outputs is roughly 10V out of the box.  Using those in combination with the offset trimpots I was able to set the plus out to be zero to +5V and the minus output to be +5V to zero.


2 Responses to Jurgen Haible Trapezoid VCA

  1. consumed says:

    fantastic looking build, as always. thanks for the info…id been eyeballing this project.

  2. mono-poly says:

    Great as always Richard!

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