AM8430 JP-4 ADSR

Overview The AM8430 module is a replica of the ADSR envelope generators used in the Roland Jupiter 4 analog synthesizer from 1979. It has a very unusual design that was necessary to create voltage controlled envelope timing that could be stored in patch memory. Three voltage controlled CMOS oscillators, one each for Attack, Decay and Release, are in each envelope generator.

The ADSR produces the usual exponential curves but with microscopic steps that are not audible. The design is parts and labor intensive, and the 5x 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 envelope generators from 1984 onward.

ADR Tapers

How does it work? The JP-4 ADSR is a unique design which uses high frequency CMOS oscillators and analog switches to create the traditional ADSR envelope generator. They work well, but have some downsides, especially the large number of components needed and the dependency on a stable -15V power rail.

Each envelope generator has three high speed CMOS oscillators with a narrow pulse width. A 4069 chip (Hex Inverter) is used to create the oscillators with a timing capacitor of 150pF (Attack) and 47pF (Decay and Release), therefore the Attack oscillator is slower, as Roland decided to give the Attack faster settings. The frequency of the oscillators is varied by a 0 to +5V control voltage that the JP-4 reads from 50k slide potentiometers on the front panel. The timing capacitor of the attack oscillators is 3x larger at 150pF than the decay and release oscillators (47pF).

The A, D and R pots have additional 22k resistors between pin 1 and 3 which makes the taper a reverse log and helps setting values at the lower section of the pot travel. See diagram for the actual taper curve. Roland used the same trick in the JP-8 A, D and R pots as well as on the LFO Delay pot.

Each pulse train switches a SPST analog switch (4066) on and off many times per second, which discharges or charges up its own timing resistor and capacitor (C17, C18, C19), this creates the envelope stage voltage curve (A, D and R).

Each pulse creates a 220ns voltage step (the time taken to discharge the timing capacitor, which is held on a JFET op amp and a 100nF “hold” capacitor. As the pulse train keeps getting another 220ns burst of voltage it builds up the envelope curve via a series of steps that approximate to a curve. The step duration is driven by the clock frequency. At 10KHz these steps are 100us, and at 2.2MHz they are 454ns.

The faster the oscillator the shorter the duration of the envelope stage, so a 10Khz frequency creates a 1ms envelope time. Once the attack voltage reaches the voltage set by R105 and R106 (+9. 43V) the comparator Op Amp (IC21) inverts the state of the IC25 flip flop circuit. Using IC24 (triple NOR) the Attack phase is turned off and the Decay stage is switched on using the Sustain level (voltage) as the end point. A similar transitions is made into the Release stage.

The CMOS oscillators are trimmed on the Module Controller PCB to create the correct envelope timings, and to match the timings across the two envelope generators. It is not clear why Roland did not use one set of oscillators driving both envelope generators to reduce component count and ensure the same timings.

Overall Envelope Shape

These single turn cheap trimmers are very sensitive, and sweep the clock frequency across many kHz. It is really hard to align the timing across the four voices in the Jupiter 4. and a multi turn trimmer is a much better solution.

Attack timing varies from 0.6 ms to 3 s according to Roland but the fastest timing we obtained was around 1.4 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), and an inverted envelope also available.

AMSynths Design The AM8430 is an exact replica of the original circuits, adjusted for 12V power. The envelope generator uses an external Eurorack gate signal which is boosted to 10V to drive the CMOS circuit. There is also a manual gate push button with a red panel LED. The sliders are Bourns 30 mm travel, with LED’s.

Some early 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 for older serial numbers, but no mention of them is made in the service notes. The clock stability is dependent on the accuracy of the power rails, so precision rails may help the accuracy and consistency across voices. However for a single ADSR I have kept with +/-12V.

Fast Attack 1.5 ms

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. On the prototype I did try out log potentiometers (wired upside down to give reverse log) which made for better control, and eliminates the need for the added resistor. I have corrected the Sustain level for 12V Euro Rack operation.

Added Feature The The Key Follow feature of the Jupiter 6 has been added, where the pitch CV can control the timing of the A, D and R stages. Larger pitch CV’s result in faster envelope timing, the Sustain level is not changed. The amount by which the timing changes can be adjusted by a rotary potentiometer and a Range switch selects whether the pitch CV is 0 to +5V or 0 to +10V and therefore a spread of 5 or 10 octaves.

The Key Follow voltages can be set from 120% down to 0%, which enables the amount the pitch change to be actually increased as well as decreased.

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

  • External GATE input (Euro Rack standard of +5V)
  • Pitch input for the Key Follow
  • ADSR envelope output, normal or inverted.

Fast Release 3ms

Module Outcome & Availability A single prototype 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.

The complex design means it is too big to fit a dual ADSR into less than 20HP, so I have created a single 10HP ADSR module with the additional features mentioned above. Check the webstore for more details and availability.




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