AM1033 Dual DADSR

AM1033 Module

Overview This is a replica of the ARP 1033 Dual Envelope Generator module from the ARP 2500 Analog Modular Synthesizer. The module consists of two traditional ADSR envelope generators with a Gate Delay.

The original design from March 1970 is based on 2N5172/2N6076/2n5308 transistors switching through the phases, with the voltage held on a couple of 1uF Tantalum capacitors interfaced with two 2N5460 JFETS and three LM301AH Op Amps proving the buffering. The Hold feature uses a 10uF tantalum capacitor to create the gate signal delay.

The ARP documented specification for this envelope generator shows a really snappy 1ms fast attack and decays, and the longest phase times of only 2.0 seconds. The measured response times with the same timing capacitors as the original are:

  • Delay Time: 0 – 3 seconds
  • Attack Time: 1 ms to 1.6 seconds
  • Decay Time: 5 ms to 6 seconds
  • Release Time: 10 ms to 6 seconds

Gates & Triggers There is a toggle switch to select the trigger options, a “gate on” 12V lamp and a manual gate push button. The envelope generators have a very useful Low Level Gate input which has a 1.8V threshold, which means they can be gated by Euro Rack modules.

The trigger options are either Single – where the gate signal alone is used, or Multiple – where the attack is initiated by a positive pulse at the TRIG input, provided the GATE signal is on. Just a GATE or TRIG signal is not sufficient to start the envelope. A new attack is generated each time a TRIG pulse is applied as long as the GATE is on. The delay time is applied to any of the triggers.

AM1033 PCB’s

AM1033 Module My design uses the same ARP schematic laid out on a large 150 x 150 mm PCB. 1M log 1/8″ shaft potentiometers are nearly impossible to locate, but I eventually found some in the Mouser catalog and bought a set, enough for three modules. They are CTS model 270X at about 4 GBP each. The circuit is laid out in a similar pattern to the original but I have not done an exact copy as I can improve on the layout as my PCB is double sided.

The transistors and LM301 TO5-8 Op Amps are all still available after nearly 50 years, which is quite remarkable! So I used the same parts and built an initial set of PCB’s with metal film resistors and then a final pair with original carbon composite resistors, as I love the look of the colour branding on the brown casings. The AM1033 produces 0 – 10V control signals (10,32V measured), positive and negative.

Controls I was going to use 45 mm sliders with ARP style coloured caps, with an Initial Delay control on a rotary pot, when I couldn’t find the right 1MA pots. Maybe I will do this in a Euro Rack module. The final front panel design is very similar to the original, using rotary pots, and has the following front panel controls for each Envelope Generator:

  • Delay, Attack
  • Initial Decay
  • Sustain Level
  • Final Decay
  • Toggle switch for Multiple or Single Trigger
  • RED Incandescent Lamp for Gate On
  • Manual Gate Push Button


Connections There 10x 3.5mm jack sockets mounted on the rear the panel, for each Envelope Generator we have:

  • Gate
  • Trigger
  • Low Level Gate
  • Inverted Output
  • Normal Output

Module Build & Outcomes The PCB was initially laid out in September 2006, and then updated to use a large 150 x 150 mm PCB in September 2015 (9 years ….). A total of 4x PCB’s were ordered, with the first one as a trial using metal film resistors, as I suspect the circuit was going to be tricky to get working.

I left the build until February 2016 as I had a backlog of 2500 PCB’s to sort out and get working. A new version of the PCB (REV04) was created and manufactured in February 2017. Two AM1033 modules were built and completed in 2017, with a third in 2019 using the original carbon composite resistors. The circuit worked but with two issues; I had made a mistake with 2N5460 pin out which was easily corrected, but the decay and sustain controls interacted.

This problem seemed impossible to fix until I built the 1045 ADSR in 2019 and noticed that I had added a 10K resistor to +15V in the wrong place, this was simply removed. I also noticed that a 10nF capacitor was removed from the ARP 1045 schematic but was in the 1033 version. I made both changes on the 1033’s and they worked perfectly, at last!

AM1033 in 2019

2019 Update After building another set of 1033 PCB’s I found some worked and some did not, so I spent a few hours investigating the reasons. The 1046 Quad Env Generator schematics show the correct pinout of the JFET’s on the schematics. It turns out I had Source and Drain reversed in my schematic which counteracted the 2N5460 incorrect pin out. So reversing the JFET’s was not necessary after all!

The 1045 ADSR’s worked ok, but the Delay circuit in the 1033 was giving some problems on some PCB’s. This turned out to be my mistakes; diode reversed, wrong resistor value. All four DADSR circuits now work perfectly. During extensive testing I learnt that the 10nF capacitors in the delay circuit and in the Gate input can both be fitted, they work fine. It does not matter whether you use carbon composite or metal film resistors, or use polyester or ceramic capacitors for 1nF and 10nF. Similarly tantalum or electrolytic capacitors can be used for the timing.

As part of the final commissioning I fitted smaller washers and nuts onto the 270X potentiometers which means the knobs are now flush to the panel. This improves the look of the module and the parts are: Nut is the Keystone 1449, Washer is the Apem U215.

2024 Update I researched the “lost modules” of the ARP 2500 in the spring of 2024 and created a video here. The module plan includes the 1046 Quad Env Generator, which did make it into production.  The reason to do this is to provide full ARP 1003/1033 ADSR capability; two time settings (fast and slow), break out panel with all four sets of Gates, Triggers and the Sustain Pedal input.


Copyright AMSynths 2024