Bill and Will's Synth
Jürgen Haible Trapezoid VCA


December 2010 - We began this page way before we'll get around to building this thing... but we wanted to begin documenting our thoughts about the module.

In July of 2010, Jürgen Haible announced his intention to develop a Trapezoid Generator circuit to emulate the venerable classic one in the EMS VCS3 and Synthi-A synthesizers.  We signed up.

The Trapezoid Generator is intrinsically different from an ADSR-type Envelope Generator and the EMS one is quirky at that.  Jürgen says that his has this quirkiness too.  But in addition to the Trapezoid Generator, Jürgen has included a "unique, FET-based VCA" on the PCB.

There are several ways we figured we could have implemented this design. We could have built just the Trapezoid Generator sans VCA. We could have built the Generator and the VCA but with no input attenuators. In any of these cases, we could have implemented the inputs and outputs such that they were the more nearly identical to the EMS circuitry.

We chose to build the module with Trapezoid Generator and VCA and with input attenuators.  We accommodated the cool Audio Trigger feature and we provide controls and outputs for the DC Shift feature with its separate trapezoid outputs - calling it "Trapezoid 2" or "TZD2" on the panel.  We have inverted outputs - we might as well - we have room on the panel.

Table of Contents

Here's a table of contents that we hope will make this page easier to traverse:

Background - presents an explanation and Jurgen's initial description of the module

Construction/Feature Options - presents  different possible implementations

Parts - presents a Bill of Materials and notes about it

Panel - presents our ideas for a MOTM style panel

Construction Phase 1 - Resistors, Capacitors, IC Sockets, Power Plugs, MTA headers

Construction Phase 2 - Trimmers - OK, this is where we've left off for now

Set up / Testing

Use notes



Jurgen lists the Module's features like this:

"Trapezoid VCA"

  • Emulating the EMS VCS3 and Synthi A Trapezoid Generator without copying the actual circuit
  • Unique FET-based VCA included
  • Parameters not independent of each other - quirky and inspiring like the original
  • Positive and negative Polarity with variable DC shift
  • Self-Cycle Mode smoothly activated towards the end of Off-Time knob range
  • Standard Voltage Trigger and Switch Trigger inputs
  • 3 Power options:
    • +/-15V with MOTM and connectors
    • +/-12V with "Euro standard" 10-pin connector
    • 15V AC Wallwart - ideal for making a standalone box
  • Voltage controlled Decay with positive or negative amount
  • A variety of extra functions that you can implement, or omit.

What's a Trapezoid Generator?

The Trapezoid Generator is similar to an Envelope Generator but has three "on" stages.  An Envelope Generator (EG) like the MOTM-800, has four "on" stages; Attack, Decay, Sustain, and Release.  The "contour" parameters will be self-evident for anyone who has a modern synth, but to us, with my background of prior ownership of a Synthi-AKS, it wasn't until we read Paul's user guide that we realized what the parameters meant.

These EGs always wait to be triggered by an external Gate or Trigger.

Wikipedia's great article on synthesizers outlines it like this:

"The contour of an ADSR envelope is specified using four parameters:

  • Attack time is the time taken for initial run-up of level from nil to peak, beginning when the key is first pressed.
  • Decay time is the time taken for the subsequent run down from the attack level to the designated sustain level.
  • Sustain level is the level during the main sequence of the sound's duration, until the key is released.
  • Release time is the time taken for the level to decay from the sustain level to zero after the key is released."

ADSR Envelope Contour

So - the EMS "Trapezoid Generator" (TG) has three "on" stages: Attack, On, and Decay. But there's a fourth parameter for "Off" time.  Just like on the Synthi, this is controlled by an "OFF" knob, and like on the Synthi, turning this knob fully clockwise makes it wait for an external, "Manual," trigger like typical EGs.  Turning the OFF knob to the left of extreme clockwise, controls an "auto repeat" function - the further it's counterclockwise, the less time it stays off - or the faster it starts its Attack / On / Decay contour again.

Trapezoid Contour

On the Electro-music forum, member "BananaPlug" attempted a full description thus:

  • "A/On/D always completes.
  • Long gates will sustain.
  • The On time extends the post-attack sustain regardless of gate. It begins after attack is complete and gate has dropped.
  • Decay follows the On period.
  • If cycling, Attack begins again after Off time. Turning Off all the way up stops it from cycling
  • Minimum off period is around 10ms so it becomes an AM effect at that end of the range.
  • Range of the envelope signal is +/-5v (well, bipolar anyway) with further adjustment available at output."

Jurgen replied:

"This is probably how a theoretical Trapezoid generator would work. <Smile>

"The EMS (and mine also) is more quirky. For instance, the ON-Time begins before the Attack Time is fully completet, so the two always depend on each other. I never made an attempt to describe all these little specialities in words. You "feel" them when you turn the knobs and change the parameters, and the whole thing reacts in a pleasant, yet not entirely expected way."

So What's an FET-based VCA?

Well - an FET is a field-effect transistor... an OTA is an operational transconductance amplifier... but how does a VCA based on FETs differ from those based on OTAs - and what is the sonic difference?

Jurgen wrote:

"I'd say *if* the signal level is high enough for that FET VCA to colour the sound, it will colour it very differently than an OTA, or other classic VCAs.

"The closest device I can think of, to that EMS VCA, is the UREI 1178 compressor. - Yes, I think that nails it. A poor man's 1178 modified as VCA."

Schematic Diagram:

for Jurgen's .pdf file click here.

Construction / Feature Options

Option Details


Will and I have developed a parts-list / bill-of-materials in the form of an XL spreadsheet. Jürgen has been very patient and helpful answering our pesky questions. As of today, 9 December, 2010, it's almost complete.

Corrections to BOM:

None yet -


None yet -

Click here to download the spreadsheet (apx. 350K). 


We came up with these designs and desided on the one with the VCA Input Attenuators (in the middle).


here's our FPE design

click here for our fpd file, 

Construction Phase 1

All the stuff in Phase 1 gets soldered using "Organic" Solder.  At every break in the action, we wash the board off to get rid of the flux.

Construction Phase 2

All the stuff in Phase 2 gets soldered using "No-Clean" Solder and the PCB doesn't get washed off from here on.

Set up / Testing

Use Notes


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