ARP2600 Analog Modular Synthesizer Replica

Overview The third analog synthesizer I built turned out to be a replica of the original ARP 2600 synthesizer - the idea evolved over time. I started out in the summer of 2007 with an empty set of 2x FracRac's, and the lessons from building the Pro-One modular which was 4U high and used high quality Spectrol pots. This was expensive design, and the 1/8" shaft Spectrols were of variable quality, I had to replace quite a few pots after burning in the system - not a happy badger! For my third modular I decided to use cheaper Alpha pots with splined shafts and low cost mixer style plastic knobs (15p rather than £1.00+). This reduced the cost of building a module, albeit it's a compact panel layout.



Initially I started building a collection of filter modules; various ARP designs, SSM2040 and CEM3320, and then some modern VCA's using the SSM2018 and SSM2164 chips. These became the Filter Bank. In 2008 I started tackling the ARP 4027-1 VCO, and over a number of months perfected the design. By late 2008 the idea of a FracRac ARP 2600 was becoming a reality, especially once Tony at Oakley Sound Systems announced PCB's for the 4014 and 4019. So the third synthesizer became an ARP 2600 and the filter modules became the Filter Bank which was subsequently re-engineered into Euro Rack format.

Design Details The electronic design of the ARP 2600 is well documented in the schematics and most of the components can be purchased today or near substitutes. By 2008 I had 6 years practice of designing PCB's using Eagle CAD software and the 4023 filter was one of my first clones in 2002. Confident that I could at least find, and build, the necessary electronics for an ARP 2600, I could now plan the physical format:

• The modules are all standard 3U 3" wide FracRac.
• The panels are 2 mm Aluminum made by Schaeffer
• Blacet Research FracRac rails are used - expensive!
• The +/-15V power supply is Oakley Sound with Oakley Dizzy.
• The MIDI to CV/Gate interface is from Oakley Sound.
• Pots are Alpha 16 mm, cheap but reliable.
• Slide Switches and some very high quality rotary switches.
• Lights - 3 mm LED's mounted in round plastic LED holders(Clip Lites).
• The power supply is 1.5A +/-15V and the transformer is remote.
• The cabinet is MDF covered in Black Tolex.

Design Changes & Outcome Well it took 2 years to get this project finished in the summer of 2010. Whilst there have been a few minor problems, such as making mistakes in the CAD files, these were easily corrected. There were 4 main hurdles to overcome before this synth could be finished. Firstly the ARP 4020 ADSR schematics on the web has errors, and it took me some time to track all the mistakes down using an original schematic and an actual module. That was sorted in 2009 - a year flew by.

The second problem was the MIDIBOX MIDI CV converter introduced too much noise into the ground rails, and no amount of de-coupling would sort this. The third problem was that the ADSR pulled down the 10V gate signal from the MIDIBOX and made the ADSR output too low. In the end I switched out the MIDIBOX and used an Oakley Sound System converter. This works perfectly with no ground noise problems. I have added a simple Op Amp buffer to provide 10V to the ADSR with no droop.

And finally the Digisound MN3011 Reverb circuit - I just couldn't get this to work . So I swapped in an ARP 4075 VCF - which I think is a better decision as I now have two cool ARP filters, the original Moog ladder and the later OTA filter.