6 Voice Simple Guitar Synth

This instructable outlines a guitar synthesizer with 6 voices where each string is tied to an oscillator. The vibration of the string is converted into a control voltage for each amplifier and the contact of the string to the frets controls the pitch of each oscillator.

Materials:

You can use different a hardware controller for the synth (though seeing that the whole thing is designed for a guitar you will want to have a functional guitar to modify unless you want to redesign the control voltage supply methods)

Integrated Circuits (The numbers correspond directly to the # of voices the synth will have. If

• AS3340 or CEM3340 [x6] (Voltage Controlled Oscillator)
• AS3360 or CEM3360 [x3] (Voltage Controlled Amplifier)
• TL074CN [x6] (Quad Op-Amp)
• TL071 [x6]

Common circuit components

• 1N4148 diodes [a lot]
• common resistors, particularly 100kOhm
• common capacitors

22 AWG wire

22+ pin ribbon cable

Piezo saddle pickups

Assemble the voltage controlled oscillator boards and connect them to the voltage controlled oscillator board (both of which are attached in this Instructable) following the circuit diagram. Note that the files for the PCB may need tweaking to print properly and should be inspected prior to usage. To have 6 voices 6 VCO boards and 1 VCA Board are needed. Each side of the AS3360 chip connects to one VCO board as shown in the circuit diagram. Tune the oscillators using a known 1V/oct supply by adjusting the 10k pot until the chromatic scale is self-consistent. Then use the 100k pot to tune the circuit.

The circuit above with 3 op-amps is a phase shift oscillator circuit. This circuit will drive AC signals that will be sent into each string. It may be helpful to attach an inverting amplifier to the output here to supply a high enough voltage to supply the voltage divider in the neck.

The voltage divider apparatus for the neck is illustrated above. To connect the frets into this circuit thin wire can be soldered onto the edges of the frets and run back to the guitar body along the back of the neck or in a cleverer way if you imagine one. Trim pots should be used in place of the resistors associated with each fret so that each note can be tuned later. There should be a 1/12V drop across each resistor. A tip for tuning this voltage divider is to start at the higher fret, tune it, then work your way up the neck.

Connect the output end of the voltage divider to 6 of the bandpass filter circuits shown above (containing two op-amps). For each channel (string) the oscillator circuit should perfectly match one of the bandpass filters in frequency with the frequencies of each channel spaced wide enough so that each output is fully isolated. After the bandpass filter, a rectifier circuit is needed to extract the amplitude information that has been modified through the voltage divider. The capacitor used in the rectifier will need to be chosen based on tone. This outputs into the control voltage input of the VCOs.

The control voltage for the amplifier will come from the piezo saddles output. The piezo saddles should replace whatever bridge saddles were in use before. The outputs of each piezo should be amplified using an inverting op-amp and then rectified before being connected to the control voltage of each VCO. Make sure that the VCO and VCA are each being controlled by the same string.

The outputs of the VCAs can be summed using a summing amplifier so that they can be equalized in volume and scaled up or down collectively. This can be wired directly to a 1/4" output jack or coupled into the existing electronics of an electric guitar as an extra pickup would. Powering the whole system can be done using batteries or an external power supply depending on how you personally weigh environmental care and portability.