June 2020 Project updates

This is what to expect in the coming months on my YouTube channel.
I build a lot of things with the Z80, this being a specialist interest I am going to develop circuits that can be more accessible for anyone to build.


PT2399 Chorus upgrade for the Z80 Synthesizer
Z80 sequencer
Audio Power amplifier and associated accessories, VU Meters / Audio mixers etc
Programmable mono synth prototype


PT2399 Chorus
The ‘Little Angel’ design has been tested and modified to work with the higher voltage line level outputs of the synthesizer, the board will be mounted¬† above one of the voice boards and routed with a switch to enable and disable the effect.

Z80 Sequencer
All PCBs have been completed, this project will be on going as there is lots of programming to do, assembly and design.

Audio Power Amplifier
A series of videos about the amplifier build and projects you can complete yourself such as a simple audio mixer and VU meters.

I am developing a chip based on a PIC16F15356 running on a 16MHz Crystal that can output a square wave for the DCO.
This can be developed as a control chip or a DCO complete board that is more accessible to people to build.

Programmable mono synth
A project that will probably for now remain as a prototype but all designs will be public, another project that is accessible.

RH-101 Synthesizer – Complete

This is the completed version of my SH-101 style synthesizer


The synth has a single 3340 voltage controlled oscillator with square, saw and a sub octave that can be all mixed together at different levels. The sub octave uses a 4093 schmitt trigger NAND gate to generate a pulse from the saw wave (we don’t use the square output as this has PWM adjustment and we want our sub to be a 50% square) This pulse goes into a 4013 divider where it is divided down twice (-1 and -2 octave), then there is a rotary switch with 3 options and a diode steered -2 octave with 75% duty cycle.

Once the oscillators have been mixed they go into a 3320 filter, using the virtual ground summing node the filter provides. The final VCA is a 3360.


There is a single 3310 envelope generator which uses a re trigger option so the envelope can be re triggered when a new key is pressed.
The envelope output goes to the filter CV via a level pot but also distributes out to the VCA and PWM

VCA: Switch between Gate or Envelope trigger
PWM: LFO1 or Envelope with adjustable level
Filter: Envelope (adjustable), LFO2 (adjustable) and direct CV control with Resonance

The entire synth fits on a small PCB and is controlled with a PIC 16F1516 and a dual MCP4922 DAC (which does the oscillator CV and the key track offset voltage)
It’s a single mono synth with MIDI In, there is no patch storage available, this just gets too expensive and difficult sometimes.


There are 2 LFOs, 1 is for the PWM and 2 is for the filter, with and adjustable level. The LFO is square wave only and is generated from the MIDI clock with 3 options:
1: Beat
2: 8th Note
3: 16th Note

Below is a link to the whole repository including:
PCB Files in KiCAD
Software in MPLAB X  (For PIC 16F1516)


All the downloads are completely free and this project is open source, but please consider donating, there has been alot of time put into the testing and design.


Sample and Hold

Sample and Hold was the way to get more than one analog output from a single DAC.
We can analog multiplex on reading analog inputs, this lets us multiplex outputs too.

How it works

There is a single DAC which is connected to an analog multiplexer chip such as the 4051, this is an 8 channel. The output of the multiplexer has a small capacitor before a voltage follower buffer. The size of the capacitor is dependent on the refresh rate, i.e how often the CPU loops round and updates the output values. 100nF works quite well if the outputs are updated every loop.


Set DAC value
Set output address
Next address

The ‘sample’ part above uses the inhibit input of the 4051 to do just that, inhibit the output while the DAC and address are being written, then the output can be enabled allowing the voltage to be stored in the hold capactitor.

The image above is simplified, single channel, the AI is Analog In. The capacitor before the second buffer is our hold capacitor, when the C input is enabled, the voltage from AI is stored and transferred to AO. The 4051 multiplexer replaces this switch, giving us 8 outputs.

Most 80s synthesizer schematics use sample and hold for all the control voltage parameters, I don’t use this method anymore because DACs are easier and cheaper now. But however there may be occasions where this could be still used, if I design any 80s based system then it is worth using to make the circuit more authentic!.

RH-101 Synthesizer

This is based on the SH-101 synth, but simplified, MIDI only and designed to fit on one PCB.


This runs a standard design CEM/AS3340 volgate controlled oscillator just like the Roland SH-101 did, the saw wave is then taken through a schmitt trigger gate to create a pulse to drive the 4013 flip flop where it’s divided down twice. A pair of diodes then connects the 2 sub octaves together to give a -2 octave with a 75% duty cycle, just like the SH-101.

There is where the SH-101 circuits end, I use a 3320 filter, 3310 envelope generator and 3360 VCAs. This is the easiest modern way of building a synth, even though these chips were designed in the 1980s or earler, they are still made today as clones with part numbers AS332, AS3310, AS3360 and AS3340.


There are 2 LFOs which are generated from MIDI clock, so if MIDI clock is not running then the LFOs do not work, LFO1 is routed to the PWM with a switch to select between PWM and Envelope modulation, the second LFO is connected to the filter with an adjustable amount.

Power Supply

The main board is designed to be powered from +15V and -5V. There is an on board 5 volt regulator which is for the microcontroller.
These voltages might be a little odd, why not just stick to +-15 or +-12, well there’s a couple of reasons.
One. The AS chips like to work at -5Volts, any higher we need to use their internal Zeners requiring an extra resistor and this would then generate more heat inside the chip.
Two. The 3310 envelope requires 0 to -5Volts on the pots to control the Attack, Decay and Release times, this could be done by adding voltage dividers to the pots but again extra components and potential for things to go wrong.

So I opted to power the whole board from +15 and – 5V. On my rack case I have added a -5V regulator to the front, I could have put one on the board but I didn’t feel this necessery at the time.


The controller is a PIC 16F1516 with built in UART, SPI for the DAC and 595s for the LED display. This controls all the featues of the system and controls the LFOs.
There is a 2 channel MCP4922 DAC which is for the pitch CV and for the key tracking amount.

The system accepts MIDI only as a form of control, no keybed scanning.


There is only one voice on this synth, but if a second key is pressed the frequency will change and, if selected, the envelope will re trigger. This makes playing a single voice synth much easier and allows multiple keys to be used. Only one will be the playing note at any one time but it allows for a smooth glide to other keys.