The Story

 

Introduction
The LFO expander is a stand-alone expansion for your analog synthesizer. It is designed to give extra functionality while performing your music. It’s main feature is an LFO generator that lets you control LFO depth and LFO speed at the same time ! It’s second feature is a bend-function that allows for pitch-bends or filter-sweeps. Finally, there’s a dedicated knob to control (attenuate) a CV signal. The LFO-expander is made of steel. Thanks to it’s small size it fits on top of many synthesizers.

History
As an owner of some analog vintage synthesizers, I felt the need to expand the functionality of some of these synths. I’m very enthusiastic about the external “vibrato”-control that can be found on the Oberheim OB-X, OB-Xa and OB-8. This input is designed to accept a foot pedal that takes over the function of the modulation-lever found at the left of the keyboard. While playing the keyboard, you can introduce vibrato with the foot pedal. This allows for some very expressive effects. However, only LFO depth can be influenced, not LFO speed (rate).

I’m also very enthusiastic about the Yamaha CS80’s LFO features. While applying aftertouch to the keys, you can influence both LFO depth and speed at the same time. The CS80 is one of the very few instruments that allows for this and once you have tried it, you would like to have this feature on every synth.
Since the CS80 has polyphonic aftertouch, you can influence LFO depth for each voice separately (not LFO speed, however). This is great for articulating specific notes within a chord but the downside of controlling these parameters by aftertouch is that it makes it impossible to apply modulation on fast riffs or create the same amount of LFO for all notes within the chord. So I came up with the idea to design an external LFO generator that would give me access to foot-controlled vibrato on my CS80. I asked my friend Mark Cox to develop the electronics while I worked on the mechanical aspect of this project. Mark Cox is specialised in repairing analog synthesizers and designs midikits and other modifications himself. He was of great help to me during this project. At my links-page, you can click to his website.

For what synthesizers ?
When working on this device, I figured that such a tool would not only be handy for my CS80 but would also be great for my minimoog and ARP2600.
On the minimoog, oscillator 3 doubles as LFO. This means you have to disable osc3 as a sound source that leaves you with a 2 oscillator sound. The LFO expander makes it possible to create 3 oscillator sounds and still apply vibrato, wah-wah or tremolo.
On the ARP2600, all controls are sliders. For making filtersweeps, a turning knob is much more comfortable. If this knob would be near the keyboard, it would be even better. So there's a dedicated knob on the LFO-expander for this purpose.
The Yamaha CS50 and CS60 do have aftertouch that can control LFO depth. However, there’s no way to influence the LFO speed with aftertouch. The LFO expander has an LFO that does allow for speed-changes so you can create CS80-like effects on the CS50 and CS60.

In general, the LFO expander can be used on every synth that has input(s) for a CV signal(s). Please click here to view a list of synthesizers that have been tested with the LFO expander. There you can also find a manual which includes connection examples.

Foot pedal ?
The LFO expander is not a foot pedal. It is a CV generator that can be controlled from it’s panel and/or by a foot pedal. This foot pedal can be an active pedal or a passive pedal.
An active pedal contains a battery and generates a voltage (usually in the range of 0 – 5V). The old moog 1120 is an example of such a pedal. It had a “TS” jack plug (Tip and Sleeve). The moog 1120 was tested on the LFO expander and it works fine.
A passive pedal is merely a resistor that attenuates an incoming signal. A Voltage coming from the synth is attenuated by the foot pedal and send back to the synth. Most expression pedals are passive. A passive pedal has a “TRS” jack plug (Tip, Ring, Sleeve).

If you don’t have a foot pedal to connect to the LFO expander yet, I can advise the Moog EP2 expression pedal. It works great and has a good build quality.

If you don’t have a foot pedal and you don’t want to buy one, you can control the LFO expander by hand. There’s a dedicated “manual” knob that takes over control from the foot pedal. By turning the manual knob, LFO speed, LFO depth and Sweep can be influenced by adjusting one knob only.

You can also control the LFO expander by feeding a CV signal to the pedal input. This makes it possible to use your modular synth’s envelope to control LFO depth and speed, or to use the keyboard voltage to influence the LFO, just to name a few.

You can also set the controls for a steady LFO signal and just play your synth's keyboard while the LFO-expander generated the LFO. You can now use your synth's internal LFO for other purposes. Since the frequency of the LFO-expanders' LFO can be set higher than on most synths, this will open doors for new sounds.

Sounds like…
Once upon a time, there was the Korg MS04 modulation pedal. It’s a modulation-pedal from the 70’s. It generates an LFO and bend CV. You can find information on internet about this pedal. When you press the pedal, the depth of the LFO increases but not the speed. Besides, it’s very difficult to access the controls on the MS04 when it’s standing on the floor, under your foot.
The LFO expander has no build-in pedal. You can connect your own expression pedal (or buy yourself an expression pedal) to the LFO expander and place the LFO expander just in front of you. You then have easy access to the knobs and switches.

How does it work ?

control panel

LFO:
Set the initial speed and initial depth with the corresponding controls. When the foot pedal is up (or when the manual-knob is fully counter clockwise), these are the values for LFO speed and depth. With 2 other knobs, set the influence that pressing the foot pedal down or turning the manual knob clockwise will have on LFO depth and speed. You can select the LFO wave-form (square or triangle) and switch from slow to fast speed. There’s a LED indicating LFO speed.

SWEEP:
The sweep-knob controls the voltage that will be generated when you press the foot pedal down or turn the manual knob clockwise. Since this knob is bi-polar, positive and negative voltages can be set. There’s a switch to change from low to high output and there’s another switch to activate the sweep-signal or to disable it.

CV:
The Key-CV generated by your synthesizer will be added at a 1:1 ratio to the LFO + Sweep signal.

KEY CV OUT from synth  + LFO + SWEEP         =          KEY CV IN to synth

You can also feed a CV signal into the CV-in and use the CV-knob to attenuate the voltage and send it back to the synth. Very handy to make filter sweeps on the ARP2600 but you can use it to attenuate all kinds of voltages.With this feature, you can also make variable keyboard tracking of the filter frequency on the ARP2600. Please have a look at the manual that can be found here