DIODE-01 : A System 100 Clone

Introduction The idea of making a replica of the infamous Roland System 100 as a set of 60HP modules was born in Boston USA in 2018. The inspiration is from the first Human League records which I listened to after returning to the UK in 1980. Once I had secured a job as a Network Engineer in Luton I was able to buy a few records; Travelogue, Emotional Rescue, Double Fantasy, Scary Monsters. Very different styles of music, with the most radical being the Roland System 100 drum sounds by Martyn Ware.

The DIODE-01 is a replica of the 101 and 102 synthesizers that were introduced at the start of 1976, selling over 4000 units over the next 3 years. The 101 was £795, and the 102 a little bit cheaper, which is equivalent to over £5,000 today, but half the price of a Minimoog! I have blended both synths together into one 60 HP module, with almost every feature retained. The keyboard has been left out, along with portamento and the S&H lag facility. No headphone amplifier or A440 reference tone either. BUT everything else is on board and replicated exactly, to deliver the same raw analog sound, the DIODE -01.

A Semi Modular Synth In December 1975 Roland updated their SH-3 monosynth with the mighty SH-5, and introduced the System 100 with its semi-modular synthesizer the 101 and its expander the 102. The SH-5 and System 100 share a lot of circuitry; the same VCO’s, same diode ladder filter and CA3080 based VCA. The VCO design was cascaded into the 1978 SH-1, along with the 24dB filter from the System 700. The VCO designed was upgraded with JFET Op AMps and continued into the 100M (1979) and Jupiter 8 (1981).

The VCO was a new design in 1975 that uses a BiFET Op Amps and a heated transistor pair to deliver accurate and stable waveforms. The BiFET Op Amps require a special power supply as they can only cope with 16V across the power rails.

The VCF is an upgraded version of the diode filter in the SH-3 which is a copy of the EMS VCS3 filter. The matched transistor buffer amp at the top of the filter (rather than a JFET Op Amp) means the filter will not resonate at low frequencies. The addition of Q compensation means the output level does not drop off as resonance is increased, unlike the SH-3. The compensation circuit has been reconfigured as a VCA design to enable a single 30mm LED slider pot to be used rather than the originals dual slider.

A single pole high pass filter was added before the 4-pole low pass filter, using an Op Amp which went onto many future products such as the Jupiter 8.

The ADSR is a discrete transistor design with a uni-junction transistor at its core, very similar to the one in the SH-5 but with no minimum setting resistors, so this one is very snappy! The ADSR is placed on the top left of the control panel, a most unusual location that is unique in any synthesizer! The logic is that it places all the control signal modulators (LFO, ADSR, S&H) in the same place.

The VCA is a simple circuit based on the classic CA3080 OTA, followed by a final summing op amp so that signals from another 101/102 can be mixed into the final output. The DIODE 01 has been designed so that it can be used on its own or in pairs or more!

The 101 synthesizer has a rather nice noise generator with both white and pink outputs. The Noise Generator is on the MAIN PCB with an analog switch to remotely control the noise colour, and reduce the amount of “noisy” wiring to the front panel.

The 102 extra features (S&H, and ring modulation) have been included but without the S&H Lag feature, as the panel space is limited. The S&H outout can be run into an external Euorack Lag Processor before returning to the DIODE 01.

AMSynths Design The iconic 1975 electronics have been carefully replicated but with a switch to modern components and replacement of  obsolete components. The UA726 heated transistor pair gets the usual AMSynths upgrade to a SSM2212 and 1K tempco. Linear voltage regulators have been used to power the BiFET Op Amps to reduce component costs. The 1970’s Japanese transistors have been replaced with modern versions to make procurement easier and to keep costs down. The tantalum capacitors have been retained in the ADSR and S&H, and the signal path electrolytics upgraded. The use of Mylar capacitors in the low pass filter is very important to retain the original’s sound.

The module circuitry fits onto one PCB with is mounted onto the back of the PANEL PCB with SIL connectors. The PANEL PCB holds the pots, switches and jack sockets and some circuits such as the GATE generator.

The power supply uses LDO regulators set at ±11.6V, with an incoing Euorack 12V supply. There is signifiant power rail decoupling and filtering across the PCB’s to ensure the LFO and S&H clocks do not bleed into the oether circuits. The VCO has it own dedicated precision ±10V power supply for the front end and ±8V power supply for the CA3130 chips. There is also a precision +5V regulator for the RANGE switch, which replaces the originals potentiometer, making it easir to set the VCO frequency accurately.

Development Challenges The original circuits typically have a few challenges when it comes to replacing with modern components.

ADSR The first circuit to be built it proved to be the most challenging. The circuit works with 2N3904/06 transistors but the Attack curve stops too early. The 101 design is very similar to the System 700 ADSR (705) which uses an Op AMp rather than a JFET to buffer the voltages. This design translated to modern 2N3904/06’s transistors very easily.

To get the ADSR working properly I modified the 101 circuit between the JFET and UJT to that of the SH-5, it involves moving one diode. This works perfectly, and is maybe why Roland went with the SH-5 design, which was just a few months after the System 101. The ADSR generates a CV signal of +6V maximum which is the level used within the 101.

LFO This circuit worked well first time and the extra power decoupling capacitors ensures it does not bleed into other circuits.

VCO Suprisingly this worked first time, and I used CA3140E chips which I know work in the similar AM8008 VCO (SH-5 VCO). The wave shapers needed a variety of resistor changes to get the vaeforms accurate as per the Roland specification. The pulse width resistor also needed changing from 1M to over 5M. One of the LM301 Op Amps needed a 10pF compensation capacitor to stop it oscillating.  The VCO has a five octave range control that uses a +5V precision regulator. This makes it easier to set the pitch of the VCO. The fine tuning PITCH pot controls a range of +/- 12 semitones and uses the ame precision voltage rails.

HPF & VCF The high pass filter has a cutoff range from 16Hz to 8,800Hz, so at the lowest setting it does let all the VCO signal thru.



Ring Modulator

DIODE-01 Panel

Front Panel Design The original panel is a complex design, with ARP like descriptive module naming boxes, with signal path lines in white, all overprinted on green/grey block areas and a base black painted panel. Reproducing this accurately is tricky, so the prototype uses a colour printed plain aluminum panel with three colours (black, blue and orange). The control and audio signals are in blue and orange respectively, with the module layouts highlighted in black outlines.

The production panels will be powder coated; a dark grey base coat with silk screened white, orange and blue for the text and graphics. The dark grey has been colour matched to the Jupiter 8 front panel colour. The originals grey/green colour  could have been used but the contrast with the white silk screen really is not strong enough. The DIODE-01 is half the width of the original, so fitting in the grey/green panels was also tricky and messy looking.

Project Status The prototype PCB’s were ordered in August 2023 and built up over the late summer. They are use THD components to enable easy trouble shooting and correction. The production PCB’s will be available as THD for synth DIY customers. A set of SMD PCB’s will be manufactured for the production run of x50. This lowers the cost and makes assembly faster. The prototype DIODE-01 will be demonstrated at Synthfest 2023 in early October and production units will be available in 2024.



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