AM8108 JP04 Dual ADSR & VCA

Overview The AM8108 module is a replica of the dual ADSR envelope generators used in the Roland Jupiter 4 analog synthesizer from 1979, the module also includes the JP-4 VCA. It has a very unusual design that was necessary to create envelope timing that could be stored in patch memory. Three CMOS oscillators, one each for Attack, Decay and Release are used to drive a CMOS flip flop circuit with a 100nF timing capacitor.

This produces the usual exponential curves but with microscopic steps that are not audible. The design is parts and labor intensive, and the 5 CMOS chips were replaced by a single custom IR3R01 chip in the Jupiter 8 and Juno 6/60 from 1981-1982, and then by digitally generated versions from 1984 onward.

Euro Rack Module:   22HP wide
User Manual:          Download PDF

Detailed Description The AM8108 is an exact replica of the original circuits, adjusted for 12V power. The envelope section timing is created by high frequency CMOS oscillators hooked up to 50K slide potentiometers. The oscillators can be trimmed to create short and long envelope timings, and vary in frequency from 10 kHz to 800 kHz.

Overall Envelope Shape

These trimmers are very sensitive and you can see why its really hard to align the timing across the four voices in the Jupiter 4. Attack timing varies from 0.6 ms to 3 s according to Roland but the fastest timing we obtained was around 1.5 ms. The Decay and Release envelopes vary between 3 ms and 10 seconds, faster than the 14 ms minimum quoted by Roland in the user manual. The envelope generator produces 0 to +10V control signals (I measured +9.90V), with an inverted envelope also available.

The gate signal of each envelope generator defaults to the Doepfer Bus but the external gate jack socket over rides this. There is a push button which is always active for initiating both generators and the 5V gate signals are converted internally to the required 10V. The presence of a gate signal is shown by the red panel LED. The sliders are Bourns 30 mm travel, with no LED.

Some Jupiter 4’s used a matched transistor pair in the CMOS oscillators (2SC1583), even though the layout is for two unmatched transistors. The  use of matched pairs seems to be an older serial numbers, but no mention of them is made in the service notes. Roland state an approximate 1:1000 range in the timing of the envelopes.

Fast Attack 1.5 ms

Timing & Feel Both envelope generators go through an Op Amp buffer to provide the negative and positive going envelopes. There are 6 trimmers for both sets of A,D and R timing.

The Jupiter 4 has a reputation for fast envelopes, which is certainly how they feel as the attack is quick and longer times are right at the top of the potentiometers. I have used log potentiometers which makes for far better control and eliminates the need for the added resistor. I have corrected the Sustain level for 12V Euro Rack operation.

Connections There are five 3.5 mm jack sockets on the base of the front panel:

  • External GATE input (Euro Rack standard of +5V)
  • VCF ADSR envelope output, normal or inverted.
  • VCA ADSR envelope output, normal or inverted.
  • VCA signal input.
  • VCA signal output.

Fast Release 3ms

Module Outcome & Availability A single ADSR PCB was laid out in the Autumn of 2018 and ordered as a prototype in November. Testing was conducted on 15 and 22 December and a couple of changes were needed to get to a production version, along with designing a PCB to hold the pots and VCA. A new set of PCB’s was ordered in December 2018.

There are no plans to release the AM8108 as a commercial product, as its too expensive for what it delivers.



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