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AM 4003 - ARP 2500 ADSR
Overview This is a traditional Op Amp and transistor envelope generator from the ARP 2500 modular synthesizer. The ARP 1003 design dates back to August 1970 and it provides 4 stages
of voltage control.
Original Circuit
The original cicruit is built round a couple of 1uF electrolytic capacitors which are charged and discharged via FET's. There are 3 LM301 Op Amps to buffer and invert the control signal and a complex transistor circuit to create the initial delay. The circuit has some nice features including a sustain switch, multiple triggering, two levels of gate trigger and a lamp to indicate when the gate signal is on.
The circuit is well worth using in a modern context, but notice the envelope timing is fast and short. The module needs little setting up, all the parts are easy to find, but the parts count is high (nearly 20 transistors....).
The specifications are as follows:
- Attack 1ms to 2.0s
- Decay 1ms to 2.0s
- Sustain 0 - 10V
- Release 1ms to 2.0s
- Output voltage 0 to +10V and inverted 0 to -10V
The timing of the stages can be changed by using different capacitors in the core.
AM Circuit The AM circuit is an exact replica, and the same LM301 Op Amps can be used, as the compensating capacitor has been accomodated for on the PCB. We have upgraded to OPA134 Op Amps which are more accurate. The 2N5172,
2N5308 and 2N6076 bipolar transistors and the 2N5460 FET's are easy to locate.
Front Panel
The front panel is an AM High Density design, 90mm wide, 3mm thick aluminium, 4U high, with black lettering. Controls pots have 3.85mm diameter shafts, knobs are all 13mm in diameter - black and chrome. Switches are sub-minature toggles and 3mm LED's are used. A CV signal LED monitor is ditted with a 10 LED bar graph.
Build History The schematic has been transferred into Eagle CAD, and the board layout needs to be completed.
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