E-mu Systems Analog Modular Synthesizer Replica

Overview The first analog synthesizer I built was a replica of the original E-mu Systems Modular synthesizer that was manufactured in California by E-mu Systems from 1972 to 1980. Over 125 originals were built, and the last model was given to Scott Wedge when he left E-mu Systems in 1993 - he still has it.

The E-mu Systems modules were updated with new circuit ideas during their life time, and Dave Rossum exploited SSM chips in many of the later module revisions from 1976 onwards. Some of the circuits are unique designs, not available from any other manufacturer. For example the Resonant Filter and the Universal Active Filter (an improvement over the ARP 1047 filter in the ARP2500 Modular).

Why Build a Clone? The idea of attempting to build a E-mu Systems Modular clone really took hold when I was in a motel in Scotts Valley in February 2002. I had just visited Scott Wedge and fired up his mint Modular. I was very impressed with the build quality and magic sound, and whilst buying one was not an option in terms of availability and cost, building one was a vague possibility.

A few days later I met up with Riley and Ed at E-mu Systems and realised that they might have some schematics, as they were both into Eu Modulars. I didn't manage to get any details whilst in the USA, but a few months later when back in the UK both Riley and Ed shared some schematics with me over the Internet. In October 2003 I bought a set of additional Modular schematics from Martin Newcombe (remember the UK Sythesizer Musuem?).

The Challenge  Cloning a set of 25 year old analog circuits is NOT easy, even if they were designed by a genius like Dave Rossum. The first stage was to get the schematics onto paper and touch them up so they could be read. The next step was to see what kinds of transistors and IC's were used, and try and locate them. Whilst there are some easy 2N5828's and 2N4250's, there are also rare SSM chips and ultra rare dual matched transistors. Most of the Op Amps are still around, like the trusty 741 used in control paths, whilst the obsolete 556 can be substituted for. It was soon clear that I would need to replicate the circuits exactly and only once a module was working could I move onto substitution with modern parts. That meant a six month trawl of the Internet looking for rare components...and piece by piece I have managed to acquire every rare part in sufficient numbers to build a large modular!

Designing the AM2000 Confident that I could at least find the necessary electronics, I could now plan the physical format. The original physical design was based on high quality electronics and mechanics, and I set the same objectives albeit with some compromises and changes:

• The original modules were 6" high and 3", 6 or 12" wide. I am using a height of 4U  and a width of 90 and 135mm. This provides more space for the panel layout and enables the modules to be rack mounted.
• The aluminium front panel designs stick closely to the original, but with some tidying up of layout and the omission of the blue painted surround. Black lettering is etched into the 3mm thick panels (the originals were 1/8").
• Two cabinets built in MDF and painted blue. Each cabinet can hold up to 10 modules within a 19" racking system. I'll replace them with teak versions eventually!
• The separate sub-module and module concept of the original has been ditched in favour of one circuit board - I don't plan improvements to each module!
• An Oakley MIDI interface has been added, so the beast can be controlled via a keyboard.
• Pots - they need to be 6.35mm shafts to fit the control knobs. So I have used a mixture of Bourns 91A and Spectrol 248.
• Minature "flat" toggle switches - as per the original.
• Lights - 5mm LED's mounted in round plastic LED holders (Clip Lites).
• The power supply is 1.5A +/-15V and the transformer is remote.
• +5V is used on many E-mu modules but is generated on each PCB that needs it.
• The power bus structure is based around two Oakley Dizzy PCB's in each cabinet. These distribute +/-15V, ground and screen ground as well as two separate CV/gate buses.
• The CV/gate busses are selectable from some of the module front panels (just like the original) and the busses are configured as MIDI and SEQR.
• The MIDI bus is driven from the MIDI interface and has an external MIDI IN.
• The SEQR bus is driven from an external CV/gate