AM 8020 Module - ARP 2600 ADSR and AR

Overview This is a replica of the ARP 2600 Envelope Generators, both the ADSR and AR. The module is built into a single FracRac module.

Original Circuit The original design first appeared in the ARP 2600 in 1970, but the circuit is a derivation of the ADSR in the earlier ARP 2500. It is a transistor based design with a Op Amp used as an output buffer. The schematic printed in the ARP 2600 Service Manual has at least one mistake, possibly three:

• R22 is duplicated, the left hand R22 is actually 68K not 180K
• R8 is shown as 10K but should be 33K
• R4 is shown connected to -15V, it should be GND

The ADSR does not function unless the first error is corrected. A took me a few months to work out why my replica didn't work, as I didn't have access to the original ARP blueprints.

AM8020 Module My FracRac module replicates the original design exactly except for the following changes:

• The Op Amp has been upgraded to a TL072
• This provides both positive and negative outputs
• The hard to locate TZ581 and 2N5367 are replaced with 2N3702
• The 2N4392 can be replaced by a 2N5459
• There is a front panel LED and GATE push button

Replicating the module provided difficult and the initial attempts failed to give an ADSR set of CV's. After a few months of trying to diagnose the faults I gave in and bought a ready made 4020 module from Anglian Art and discovered the following differences/errors in the ARP schematics:

• 2N3702 transistors used for all PNP's
• Trigger circuit modified with input and ground basing resistors
• Resistor errors in ARP schematics, including Op Amp gain

Once these change were made the ADSR worked properly, although larger DECAY settings cause the SUSTAIN voltage to fall. This was only 200 mV on the Anglian Art version but nearly 1V in mine. The exact cause remains a mystery until I build further production versions using all 2N3702's for PNP's. I have also adjusted one of the resistor value in the Op Amp gain circuit to give a near perfect 10.0V output.

The module has the following front panel controls:

• Attack
• Decay
• Sustain
• Release
• Attack and Release for the AR
• GATE ON push button

There are five 3.5 mm jack sockets mounted on the left hand side of the panel, these are:

• GATE input
• TRIGGER input
• AR output
• ADSR Inverted Output
• ADSR Output

Front Panel The front panel is a 3" wide FracRac with Alpha potentiometers and push fit 15mm mixer style knobs.

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Original ARP 2600