SYNTH 09


Overview The SYNTH 09 is a replica of the Roland SH-09 mono synth that was released in the Autumn of 1978. It proved very popular with over 8000 sold over the next 3 years. The Roland SH-101 replaced it with a on-board sequencer and a CEM3340 VCO. The SH-09 is a stripped back to basics mono synth with only one VCO. But the fantastic 24dB filter, the sub oscillator and fast envelope, make this a great sounding bass (and lead) synth.

Buying an original is now expensive at £700 or more, and the metal panel is prone to rusting. So how about a Eurorack friendly version in 60HP, with all new components and a few modifications? Well here we go, the SYNTH 09.

Why do this? You could of course assemble a single oscillator analog synth from individual Eurorack modules, but the SYNTH 09 has some unique features;

  • 30mm travel sliders for accurate control
  • An ADSR with re-trigger for fast solos
  • Accurate ADSR curves on 12V power
  • An ultra stable analog VCO
  • An envelope follower is included
  • 1% Polystyrene  filter capacitors
  • Super matched OTA’s in the filter

Technology Used By 1978 Roland had produced a number of mono synths and were very experienced at perfecting the design to be low cost. They had gradually improved the oscillators to ensure temperature and pitch stability, using JFET Op Amps as well as BJT Op Amps like the 4558. They had started to design custom chips, like the BA662 OTA. In the SH-09 they used low cost components such as carbon resistors and mylar film capacitors. However in key areas they put in metal film resistors, and they were careful to feed the VCO precision 10V. The 09 is a simpler version of the SH-2 but with some improvements such as polystyrene filter capacitors.

AMSynths Design The SYNTH 09 has two PCB’s that are interconnected using 5 and 10-way SIL sockets and headers, with 11mm spacers;

  • The MAIN PCB contains most of the synthesizer circuits
  • The PANEL PCB has the front panel controls and the Envelope Follower

The MAIN PCB is laid out to the same component and module pattern as the original, and both PCB’s are 297mm wide and 105mm high. The power supply is based on LM2941/LM2991 LDO’s with the output voltages set close to +/-11.5V to give as much headroom as possible. Large 470uF smoothing caps have been used to ensure the LFO does not bleed into the VCO via the power rails.

The VCO is the standard Roland JFET Op Amp design that was developed for the SH-2 and appeared in the 100M and Jupiter 8. The SH-09 version is slightly different with a trimmer for 50% square wave and no sine waveform. I have replaced the ua726 heated transistor pair with a SSM2212 matched transistor and 1K tempco. The Roland design has a linearity trimmer for getting accurate tracking at higher frequencies. This trimmer and the frequency and V/octave have been upgraded to multi turn versions.

The VCO has a range control for five octaves, a tune control trimmer for +/- 6.5 semitones, and a waveform selector switch for the four waveforms; Sawtooth, Square, Pulse and Noise. I have retained the two rotary switches and added a large tuning control.

The VCF is the standard low pass 4-pole filter built from matched BA662 chips to ensure self oscillation at low frequencies and unusually for Roland polystyrene filter capacitors (also used in the 100M). I have replicated this design with matched AS662 chips and 1% polystyrene box capacitors. I have built a AS662 matching rig, that enables the transconductance of each chip to be accurately measured.

The VCA is a simple OTA circuit that uses a low offset BA662 OTA chip. There is space on the R0land PCB for a trimmer put this was never fitted. I have used a low offset AS662 chip and retained the rather low value AC blocking capacitor of 100nF in the VCA input. I may increase this in testing.

The ADSR is an all transistor switching design with an Op Amp for buffering. It is an excellent design culled from the SH-2 with fast attack, decay and release timings. The original Roland 1981 brochures state a maximum decay and release time of 25 seconds, but this is inaccurate, it is the 10 seconds stated in the user manual.

The ADSR has a trigger capability, as well as the usual gate on/off. When a new note is played whilst the previous one is held the ADSR re-triggers into a new attack. This is very useful for fast solos and crisp bass lines, as the player does not need to have let go of the previous key before playing the next one. The trigger is derived from the keyboard pitch and requires no external input from a keyboard (for example in the Roland 100M system).

The ADR curve shapes have been accurately retained, with no changes in resistor values in the ADSR. To compensate for the lower output level, the resistors carrying the signal into pulse width modulation, filter cutoff and the VCA have been adjusted accordingly.

The LFO is a simple Op Amp based design that varies from 0.5 Hz to 30Hz and has three waveforms; square, sawtooth and a nice S&H.

The Front Panel is 60HP wide and the standard Eurorack height of 128.5mm. It is made from 2mm anodised aluminum and I have deliberately not copied the originals black and white colour scheme. The Bourns slide potentiometers have a 30mm travel and have been manufactured for AMSynths. They have coloured LED’s that are coded as follows:

  • Blue – LFO and VCO
  • Amber – Mixer
  • Green – Filter
  • Red – ADSR

The panel also has the rotary controls for: VCO Tuning, Waveform, Range and Volume. There are two smaller trimmers for setting the external modulation amount into the VCO and VCF.

Connectivity The SYNTH 09 has input jack sockets for Gate and Keyboard Pitch and Modulation Input on the left. A patch bay is on the right of the synth, with 16 3.5mm jack sockets which effectively modularises the circuit blocks as well as providing the final audio output.

Availability & Outcomes I ordered the PCB’s in August 2022 and built up a prototype in the autumn. The individual circuits worked well except for:

  • Some minor trace errors needed correcting
  • Attack Clock interfering with the LFO speed, needs a PCB trace rerouting
  • LFO bleed into VCO via power supply (resolved)

A second (production) PCB set was ordered in November along with a first run of front panels.

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