Re-Red Red Synth

Ladbroke Grove
VCO Array

Introduction

 

Documentation

 

Implementation

 

Build Notes

         

The Ladbroke Grove VCO Array faithfully reproduces the vision and circuitry of the "Living VCO" developed by Jürgen Haible in early 2009. Like the original, it is comprised of four entirely separate components—three VCOs and a "driver"—on one PCB. But further, the PCB was developed taking into account the experiences and comments of the people who have built its predecessors—and so it reflects an evolution of the module.

The whole idea of this module is to re-create the sound of three early EMS oscillators playing in near-unison—a distinctive, softly phasing sonic effect. In his 2009 introduction to the module, Jürgen states, "I've never heard better drones than from three EMS VCOs running at almost the same frequency, and beating against each other in an ever changing pattern." As any owner of a Synthi-AKS can attest, this particular sound came at the price of the notorious instability of those EMS oscillators. Hense, Jürgen goes on to say, "My goal was to build a set of VCOs that have the untamed bass range power of early EMS and Moog VCOs, but which are tracking a keyboard voltage over 5 or more octaves nevertheless."

VCOs

Almost all VCOs are comprised of a few basic building blocks; a control voltage (CV) summer, an exponential converter, the core, and optional waveshapers. This module represents a melding of EMS VCS3 oscillator features and the Yamaha CS-80 pulse width modulation circuitry. It uses the "noisy" four-transistor exponential converters of the VCS3. The oscillator cores are "integrator-with-reset sawtooth" type designed for stability over five octaves' range. The PMW circuitry emulates the limited pulse-width range of the CS-80—and, therefore, the ability to "overdrive" it. Each Voltage Controlled Oscillator has a linear "detune" feature which allows the user, having first tuned the oscillators to unison, to then "detune" them a desired amount.

This diagram illustrates the components of each of the three VCOs:

Driver

The Driver section of the module provides common tuning including an octave switch, common Volt/Octave and Control Voltage inputs, a vibrato Low Frequency Oscillator, and a portamento feature. There are three amplifiers to increase the raw, Moog-standard 1.2V outputs to MOTM standard 5V.

Features

The module's features were originally described by Jürgen thus:

VCO Features:

Discrete (no opamp), noisy servo in expo converters (VCS3 feature)

Emulation the low frequency behaviour of linear (Hz/V) VCOs and of leakage (EMS VCOs)

Capable of very pleasant beat rates between the VCOs over the entire keyboard range

Allows rich and powerful animated bass sound, and smooth "phasing" slow-beating sound in higher octaves

Discrete (no opamp) pulse width modulators (capable of smooth audio rate PWM)

Limited PW range allows PWM overmodualtion (CS-80 feature)

Low level (Moog) or high level (MOTM) outputs

Driver Features:

VCO driver with two CV inputs, LFO oscillator with limited speed range and voltage controlled amount

Accepts potentiometer (passive modulation wheel etc.) or CV for vibrato control

Exponential (RC-Shaped) Portamento

Either one shared, or 3 individual potentiometers for Initial Pulse Width

What's in a Name?

Why "Ladbroke Grove?" Well, we wanted to distinguish this PCB from Jürgen's original and we wanted to acknowledge the distinctly EMS-English sound that inspired him. When Jürgen wrote about how he named the module "Living," he explained that there was no deep significance to it; that he just liked the name. So we decided to take some latitude in how we honored the module's inspirations. Ladbroke Grove was West London's sort-of Haight-Ashbury from 1967 to the late seventies. Among the artists who frequently played there were, of course, Pink Floyd who featured the EMS sound most notably, perhaps, as Jürgen himself pointed to, on their Obscured by Clouds album and their Dark Side of the Moon. But in recognition and direct honor of our friend, Jürgen Haible, we include his name, or initials, and an Ichthys symbol on the pcb.

"... The EMS VCS3 makes incredibly good drones (listen to Pink Floyd's "Obscured by Clouds" album!); it shows a very similar behavior in the low range as the synths mentioned before—but it has exponential, V/Oct VCOs! But on closer inspection of the linear part (after the exponential current source), you find an offset like on Hz/V VCOs, but not in form of an offset voltage, but an offset current instead! The reason is the input bias current of the integrator, built from discrete bipolar transitors. (No FET input opamp here, and no current compensation technique as in low bias current bipolar opamps, either.)"—Jürgen Haible

 

Purchase

PCBs are now available for purchase at $25 US per board plus shipping. Please email us to inquire.

 

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