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AM2350 Module
- Eµ Transient Generator
Overview This module is a clone of the Dual Transient Generator in the Eµ Modular. I have decided to build it as a single TG rather than dual, to make construction easier.
The Transient Generator has four phases as well as the usual sustain level. Dave Rossum has added Delay to the usual Attack, Decay, Sustain and Release controls. The original 2350s module can be voltage controlled with the 2355,
which connects over firmware.
Original Circuit
The original Revision 1 design dates back to late 1973, when Eµ introduced the 2350 to replace the existing 2310 and 2320 TG modules. The 1350 sub-module was built from discrete components, and was available without the initial DELAY control as the 1351 sub-module (which was therefore a bit cheaper).
When Dave Rossum redesigned the Modular synthesizer with SSM chips in 1978, the Dual TG was probably reworked with the SSM2050 chip. The Revision 3 version is based around the SSM2055 TG chip, and the sub-module is called the 1352.
Cloned Circuit
SSM2055's are hard to locate, but the nearly identical 2056 is available still for US$40. I happen to have some 2050's which I want to use. The differences between these chips are mariginal, the 2055/2056 were designed for polyphonic synthesizers (such as the Korg Poly Six). I have therefore based the AM module on the Revision 3 schematic but with some modifications to use SSM2050's.
All Revisions have the additional Delay control which is implemented outside the SSM TG chip with a CA3046, LM1458 Op Amp and some CMOS digital chips. The circuit is easy to replicate and the front panel has manual control of gate
on, and gate selection between the two internal busses of the AM Modular (MIDI or Sequencer). There is no separate trigger control. A front panel 5mm red LED is controlled by the gate signal and mounted using a red Clip Lite cover.
Panel The front panel is a standard AM design, 90mm wide and 4U high, with black lettering. Controls knobs are standard Eµ Modular designs.
Specifications
The SSM2050 uses an internal voltage controlled resistor to produce exponential voltage curves, however compromises were needed when the design was integrated into silicon, and there is a deviation of +/-5% on the exponential curves.
Rather more of a problem is the wide tolerances in the envelope stage times between SSM2050's (+/-25%), although the A, D, R times for any single chip are matched to 1%. However this problem and variations in the initial timing and the
timing capacitor tolerance means that TG's based on the SSM2050 can have widely different envelope times. This is very noticeable in the SCI Model 700 Programmer which has two SSM2050's.
Whilst this is more of a problem in
polyphonic systems, it can still annoying when using lots of TG's in a Modular system. The simple solution is to include a trinmming control (which is documented in Dave Rossums AES Paper on the SSM2050), so that each SSM2050 can be set to
the same envelope times. This is how I have implemented the AM2350.
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