RH-1 DCO DIY Synthesizer

The RH-1 is a DIY Synthesizer PCB with digitally controlled oscillator (DCO) Synth based off the Juno 106 design. It will be available as a PCB to build allowing the user flexibility of a desktop unit or rack mount unit.

It’s a mono synth with programmed ‘key rollover’ if a key is held down and another pressed it will change the pitch and re trigger the envelope.

Specification

    • Single Digitally Controlled Oscillator, Saw + Pulse (PWM Adjustable)
    • Single ADSR analog envelope (AS3310)
    • PWM and Filter envelope mod, adjustable
    • External inputs for Filter and PWM summer
    • 0 to 5 volt analog CV
    • 12 volt digital CV
  • AS3372 signal processor
  • Envelope re-trigger on second key press
  • Adjustable audio out ~650mV p-p
  • PIC16F18344 controller running with a 4MHz crystal oscillator

Power requirements are +/- 12 volts. A standard eurorack connector will be on the PCB

See below for parts list and information

Documentation
Schematic
MPLAB X Files

Recommended Modifications – Part 1

1. Supplied buffer PCB. A 4 channel buffer PCB can be used to buffer direct potentiometers. The Env and PWM mod pots need to be buffered, the cutoff is recommended and then the Resonance or PWM (experiment if you like with these).
This just gives a smoother control of these inputs. If you decide to use a DAC or other pre buffered input then you don’t need to use this buffer board.

2. External potentiometer before extra modulation inputs (PWM and Filter). These inputs are not protected and have been tested to a max of 12 volts.

3. External volume potentiometer and 10uF capacitor coupled output to the audio jack (negative of cap to the jack, positive to synth output). This will just make sure any DC voltage is blocked.

 

 

Recommended Modifications – Part 2

As with all designs, there’s always little things that can be improved. These are nothing major and not worthy of a second revision PCB (which wouldn’t be fair to those who have got the first)

The output buffer is AC coupled. I have never had any problems of output voltage drift due to the voltage followers not liking floating inputs.

The AS chip seems to always be slightly active even at low VCA, so this output remains at 0V when nothing is being played.
That being said, a simple resistor soldered under the board from U10 pin 10 to ground (pick up ground from the output adjust RV1) will make sure any voltage drift on the idle output will never happen.

A final couple capacitor and front panel pot as mentioned above is recommended too.

 

Pros/Cons & known limitations
I always like to be transparent and honest with any design.

Firstly let me say the excellent good points about this synth.
This is a 10×10 full synthesizer PCB with external modulation / LFO inputs and 0 to 5 volt controls. It’s designed to be cheap to build (as cheap as synths go) and easy to build and modify. Everything is on one PCB and it has a nice sub oscillator.

This particular synth design is designed for the lower frequencies, and they sound amazing.  This is due to the nature of the PIC 16 bit timers the higher the frequency, the less resolution you get between notes. The Intel 8253/4 as used in the Juno 106 (and my RH-106 design) gave a 50% duty output, was free running so it just needed programming and that was it.

With the PIC timer we have to create a square wave by using a flip flop (internal programmable logic) so we need to clock the timer at twice the frequency so we get a square wave at the note we want, as the flip flop with divide it. The timer also needs resetting every interrupt.

Pitch bend can have audible glithes at high frequencies due to the digital timer, and is not as responsive as the voltage controlled version. This only has basic pitch bend.

Why not Arduino?
I chose this PIC because it’s only 20 pin and has 4 CLCs (the configurable logic) so I could program this to divide the sub octaves on one chip.
Arduino I would have to use a bigger chip, and frankly one that is way over powered for this task. This I felt was a more efficient choice. I supply a pre programmed chip with the PCB

 

 

Disclaimer

While every effort has been made to test this circuit and provide comprehensive information in this documentation. This design is built and operated at the builder’s own risk. It is up to the builder to provide a suitable powersupplyof 12 volts and make sure tests are carried out prior to connection to external equipment.
The output must be tested before connecting to any external audio equipment. RH Electronics accepts no responsibility for any damage that may occur to any external equipment connected to this PCB.