AM1042 Triple VCA


2024 Update I revisited the ARP 2500 in the Spring of 2024 and designed a set of ten “lost modules” that ARP mentioned in 1970 but never manufactured. These Eurorack modules are commercially available from the AMSynths webstore here.  The AM1042 was relaunched as a Eurorack module, initially as a 16HP wide version in April 2024.

An updated version was launched in the Summer of 2024 with a revised design (soft clipping DRIVE circuit on the audio input, two CV modulation inputs, single input and output), and ARP 2500 control knobs. More details are here.

AM1042 Triple VCA

Overview The ARP 1042 module was announced back in 1970 but it never made it into production. It was intended to be a triple voltage controlled amplifier and would have provided a set of basic VCA’s to be used for CV and signal control with having to tie up a 1006 or 1005.

 

I have recreated the module using the transistor based ARP 2600 VCA design (4019). which is a slight improvement over the 2500 stock VCA and a nice warm circuit with Linear and Exponential modes. The module has had three versions:

  • 5U Panel with control knobs (2016)
  • Eurorack 16HP Module with trimmers (2024)
  • Eurorack 14HP Module with control knobs (2024)

AM1042 Module The module uses the ARP 4019 VCA circuit in all three versions, with the removal of the HF rejection circuit, and electrolytic caps on the current mirror power rails. The 5U version used matched transistor arrays from THAT and SSM, the Eurorack versions use BCM8xxBS matched transistors.

High quality op amps are used at the output and a Yellow/Amber LED is driven by a TL072 driver circuit to show the output level on the first two models. Modern era metal resistors have been used rather than a move back to ARP style 5% carbon composite.

AM1042 Triple VCA

The first two modules have the following front panel controls for each of the three VCA’s:

  • Gain
  • 2x Audio Levels
  • 1x CV Level
  • 1x black slide switch for VCA mode (linear or expo)

There are 12x 3.5mm jack sockets mounted at the base of the front panel, each VCA has:

  • Audio Inputs (x2)
  • CV Input
  • Signal Output

5U Build History I had the idea in June 2015 and laid out the PCB in February 2016 into Eagle CAD, and the prototype PCB populated on 09 April. A front panel was ordered at the same time, with 3x pot PCB’s holding the front panel components, with ribbon cables back to the Main PCB. This also makes the wiring A LOT easier! I am getting tired of hand wiring 20 cables back into the panel using x-Roland 1980’s cable. Nice though it is, I don’t have much time…

2019 Update The lettering on the 5U panel quickly rubbed off due to manufacturing defects, and the module was scrapped.

 

 

 

Copyright AMSynths 2024