SynthBend Analogue Synthesizer Controller


When The Peasant started to plan the building of the SynthBend Controller, a comprehensive and innovative design was envisaged. The intent was to create an extremely versatile analogue synthesizer controller for driving the SynthCase System.

A classic control-voltage type keyboard and an Appendage ribbon controller would be the heart of the project, with many added features. A large quantity and variety of manual CV modulation controls, such as joysticks, potentiometers, and pressure sensors would also be included. The design would be rounded out with some manual triggers/gates and switches. Hopefully the comprehensive design of the SynthBend Controller will provide an exceptional playing experience.

The first step in the design of this project was to decide on the basic module layout and chassis configuration. A number of salvaged plastic cases with hinged lids, as shown below, were chosen to house the controls and electronics.

The layout was chosen to be ergonomic and comfortable to play, and consists of the keyboard module placed in the front, with the Appendage ribbon directly behind and a little above it, to allow comfortable hand positions when playing both. Just behind the ribbon and angled steeply forward is the control panel for the ribbon controller. Above and behind the ribbon control panel, but still within easy reach, are the joysticks and other modulation controls in their panel. Finally, behind and below the modulation panel and facing to the rear, is a module with all of the patch connectors for the controller, built together into one panel.

Two pieces of 1 1/2" X 1/8" aluminum stock were purchased and formed into brackets shaped to support the plastic module cases. The brackets were trimmed, ends drilled and bolted together, and reinforced in the center with some long stand-offs.

The two brackets were attached to two aluminum rails to create the finished module support frame. Plastic feet were then installed on to the ends of each of the rails.

Two more small aluminum pieces were chosen to attach a metal case for the controller power supply.

The five plastic cases were then mounted on to the frame in their respective positions. Here you can see the completed assembly resting on a keyboard stand.

Choosing a keyboard mechanism to use with the project was required. As The Peasant is not a trained or experienced keyboard player, a full sized keyboard was neither necessary nor desired. In addition, this project was to be more focused on the large variety of control options available and not on any particular one. So a compact keyboard that fit well with The Peasant's small fingers as well as the limited space on the controller itself would work best here.

Luckily a solution was at hand! The Celtic Peasant had previously started a project converting a small Casio keyboard to a volt-per-octave type controller:

She had already dissassembled the unit and stripped all of the parts from the pcb, but had set the project aside when she began work on her full-sized keyboard controller.

Hoping that she wouldn't notice, the Casio was plucked from her project pile while she wasn't looking, and the work began!

Here you can see all of the parts from the original keyboard that would be used:

The pcb traces for the key contacts were cut and reconnected into a single-bus circuit design. A string of tiny 100 ohm 1/8 watt tightly matched metal film resistors was soldered in place on the pcb.

A keyboard "velocity" type control feature was desired for this project, and it was decided to implement this using force sensitive resistors (FSRs). To facilitate this, the rear keyboard mounting hardware would utilise flexible "shock mount" brackets to act as pivots, allowing the front keyboard supports to apply pressure to a pair of FSRs in proportion to how hard the keyboard was being operated.

Below you can see the assembled keyboard module with the shock mounts installed in the rear and the front supports with their respective FSRs.

A circuit for converting the FSR resistive output to a DC control voltage was designed. It includes linearity and sensitivity controls as well as two independantly adjustable gate/trigger outputs and an inverted voltage output. This will allow the operator's key pressure to control not only volume, but any other voltage controlled parameter as well as triggering and switching other musical events.

Here is a picture of the plastic case with the opening cut out for the keyboard and another picture of the keyboard assembly installed into the case.

The keyboard interface was designed around the MFOS single-bus keyboard controller pcb. The raw CV signal from this pcb is provided as an output, and octave switching that affects all CV outputs has been added. The voltage output of the MFOS pcb then feeds two separate CV channels each with it's own glide, pitch bend, octave switching, and vibrato circuits. There are also built-in "autobend" generators based on the Yusynth design.

The pitch bend control can be set to affect either or both CV channels and can be switched to shift the channel B pitch CV inversely to the channel A output. As well, channel A has two extra CV outputs that can be switched to the correct intervals to form major and minor chords when driving three or more oscillators. In addition to the octave switch, Channel B has an interval switch that shifts the CV output by a third or fifth, a feature found to be very useful on The Celtic Peasant's keyboard controller.

The vibrato circuitry is very comprehensive, consisting of a sine wave VCLFO whose frequency can be controlled by a built in AD/ASR envelope generator or an external input, as well as with a manual control. The VCLFO is adapted from a MFOS design and the envelope generators are from Ian Fritz. The envelope generator itself can be controlled by either the keyboard trigger, gate, or delayed gate signals, and it's output can be inverted for added flexibility.

The output of the VCLFO is routed to each CV channel via independant VCA circuits with identical envelope generator and manual control features as the previous circuit. There is also an inverter available for the channel B vibrato signal to allow the vibrato effect on both channels A and B to be out of phase with respect to each other if desired. The VCLFO output as well as each channel's VCA controlled LFO outputs are made available externally for controlling other modules, as are all three envelope generator outputs.

This module also has the aforementioned gate delay circuit as well as an adjustable delayed trigger output. Below is a block diagram of the complete keyboard module, including the FSR circuitry:

Here is the schematic for the main keyboard circuitry:

And here is the modified schematic for the MFOS single-bus keyboard pcb. All modifications are in blue. R4 was changed to 27K for +/-15 volt operation, and a 10 ohm resistor was added to the bottom of the keyboard resistor string to keep the CV output from going slightly negative. The 10M resistor R7 was increased to 100M to improve CV accuracy, with two more diodes added to the D1 and D2 string keep the idle bus voltage low enough. The output op amp stages were reconfigured to add octave switch functionality, and the gate/trigger wiring was slightly modified as shown.

Finally, below are the trigger and gate delay circuits, buffers, power supply regulators, and the rear CV mixer circuits.

After the keyboard circuitry design was complete, the front panel layout was designed and a 300dpi panel graphic file was produced. Here is a reduced-size picture of the final panel design. Note that the two blank spaces on either side at the top are included to provide clearance for the hinges holding the panel cover on to the plastic case.

Moving on to the next module, the heart of the ribbon controller is Scott Stites' excellent Appendage circuit coupled with a 500mm Softpot ribbon. All of the features of the Appendage system were included, as well as extra output level controls on some of the signals. For even greater control flexibility, a 500mm FSR ribbon was included in the module, to be conveniently placed just behind the Softpot ribbon on the control surface. The FSR circuit is identical to the previous one used for the keyboard pressure control, with the addition of the pressure CV signal being routed to the Appendage main mixer as well as to a second quantizer mixer.

Speaking of quantizers, having one in this module was considered essential, so a MFOS quantizer pcb was included with the design. The circuit can be fed with either the INIT or SLIDE outputs from the Appendage, and a glide circuit is included on the output. This output is available externally as well as being fed to a dedicated quantizer mixer along with the BEND and SLIDE Appendage signals and the aforementioned FSR pressure signal. This mixer output is also available externally.

This module also uses the same gate/trigger delay, vibrato/envelope generator, and dual CV channel system as the keyboard controller. However, the two CV channels do not have a pitch bend or glide circuit, and they have selector switches on each input allowing their use with a variety of CV signals. The six input choices available are: Appendage main mixer output, INIT, SLIDE, quantizer output, quantizer mixer output, and external input.

Below is the complete ribbon controller module block diagram:

And here is the schematic for the CV processing circuitry, which is very similar to the above keyboard circuitry. The trigger and gate delay circuits, buffers, and power supply regulators are identical to the keyboard circuits also posted above.

And here are pictures of the ribbon control panel layout and the control surface background design that the ribbons will be attached to:

The third control module begins with two standard dual-axis joysticks with level and offset controls and inverted outputs, positioned on the left hand side of the case. Next to them are two manual trigger/gate buttons. After that, there are two sets of FSR based finger pressure CV bend controls. A circuit similar to the previous FSR control circuit was developed for this application and is posted below.

Next to the FSR bend controls are a series of seven manual CV controls with adjustable outputs. Finally there are 3 manual switches included, one of which is a SPST type and the other two are configurable as either DPDT or SP3T switches. Below are the schematics for the rest of this module's circuitry:

Here is a picture of the panel layout for this module:

And now we come to the rear connector module, which consists of a panel with 138 banana jacks installed. Included in this module are five CV mixer sections, used to combine internal and external CV signals as desired.

The panel layout graphics files were then sent to a printer where they were printed and laminated with low-gloss matte laminate. They turned out looking every bit as nice as was hoped:

The laminates were carefully taped to each individual panel and all of the control hole locations were center-punched. Here is the connector panel ready to be drilled.

All of the various holes were then drilled and filed to the correct sizes and shapes. The ribbon controller softpot and the FSRs had slots made for them with a Dremel tool to allow their connector strips to be routed through the panels so that they could be connected to the circuitry inside the case. In addition to the holes in the front panel, the joysticks needed a clearance hole cut through the bottom of the case as they were thicker than the inside depth of the plastic module.

Using 3M Super77 spray adhesive, the panel graphics were carefully attached to the face of each module. The four FSRs for the modulation control panel were installed into position prior to the panel graphics for that module being glued into place.

Each hole was then cut out of the laminate to the correct size and shape with an exacto knife and all of the panel components were firmly attached. The mini-keyboard assembly was installed into it's module case and the FSR and softpot were attached to the front of the ribbon controller module.

Here is the complete controller assembly, looking good with all of the modules installed.

And here is a rear view of the controller, showing the connector patch panel.

Seeing the panels all populated with controls is exciting! It's hard to wait for it to be completed so that it can be used with the SynthCase system!

In this picture all of the keyboard module pcbs have been mounted into the case, as well as the keyboard itself. On the left hand side is a trimmer potentiometer board, with holes in the case to allow all of the CV calibrations to be adjusted from the outside. The next pcb has trigger and gate buffers as well as the channel A and B AD/AR generators. The third pcb from the left has most of the channel A and B CV processing circuitry as well as the vibrato oscillator and AD/AR generator circuits. In front of that is the voltage regulator pcb and heatsink assembly. The next pcb to the right contains the FSR circuits as well as the gate and trigger delay circuitry. Finally on the far right is the MFOS single-bus keyboard pcb.

Here is a view of the keyboard module completely wired up and tested. Getting all of those parts and wiring to fit into the small case was a real challenge!

This is the rear connector module with the CV mixer circuit pcb installed.

And here is the same module with the keyboard module wiring completed:

Below is a video of the keyboard module connected to the SynthCase system showing some of it's features:

Next comes the Appendage ribbon controller and associated circuitry. Below is the module with all of the pcbs in place and wired to the voltage regulator pcb. On the front right is the MFOS quantizer pcb and behind it is the FSR, delay, and buffer pcb. To the left of that is the main Appendage pcb with the voltage regulator pcb in front of it, then the LFO circuitry pcb, then the rest of the CV processing circuitry on it's circuit board. Finally, on the far left is the trimmer potentiometer pcb, similar to the keyboard module trimmer pcb. There was not much room left over inside this module once all of the pcbs were installed!

Here are some close-up pictures of the voltage regulator and trimmer potentiometer pcbs.

Here you can see the appendage board all wired up and connected to the softpot mounted on it's case. Everything so far is calibrated and working.

Below is the ribbon module with all of the wiring completed. The module is now finished except for mounting on the frame and connecting to the rear connector module.

The third module with the various CV controls has the least amount of actual circuitry, however, all of that circuitry had to be built up on perfboard. Fortunately, much of it was designed using the same basic circuits repeated over and over again, simplifying construction. Here is the module with all of the pcbs installed and connected to the regulator board:

Speaking of the regulator board, below is a picture showing the heatsink design used for all of the regulator pcbs in this project. The devices themselves are attached to a piece of aluminum inside the case, which is then bolted to a second aluminum heatsink outside the bottom of the case. Thick aluminum standoffs and heat sink compound are used to conduct the heat from the inside to the outside heat sink.

As shown below, the hinged covers for each module are held closed using standoffs at various points. The standoffs are attached to the front panels using countersunk screws hidden under the panel graphics. After the cover is closed bolts are inserted through holes in the bottom of the module to secure it.

Here is a picture of the completed wiring for the module.

Below is a closeup picture showing the wires attached to the FSR devices. Connector sockets are normally recommended for this, but if a person is quick and careful, wires can be directly soldered to the leads of the FSRs without damaging them.

When the rear connector module was designed, it was somehow forgotten that the joysticks have TWO axis outputs, not just one! Extra conectors had to be installed to correct this error, as shown below:

And here is a picture of all of the connectors wired up on the finished panel:

Finally we are coming to the last stage of this project, building the primary power supply. This small chassis is composed of a linear regulated power supply module, power switch, fused IEC power socket, DC output connector, and LED power monitors. The linear regulated power supply is a standard +/-15 volt type adjusted to +/-19 volts. All of the components are mounted on a salvaged aluminum metal box with a couple of ventilation holes.

Below you can see the aluminum base that the power supply box fits over and attaches to when in place inside the SynthBend controller. The DC power connector can be seen just above it, ready to be plugged into the power supply. The wiring running between the different SynthBend modules is held in place with spiroband and p-clips, as can be seen here.

And lastly below are a couple of pictures of the power supply mounted in place and powering the system. The SynthBend controller project is now complete!

Here is the SynthBend controller all moved into it's new home connected to the SynthCase system.


Don't forget to check out The Peasant's SynthCase projects: SynthCase 1, SynthCase 2 and SynthCase 3!

The SynthCase is also controlled using The Peasant's Banjo Processor.