AM1005 ModAmp

AM1005 ModAmp

Overview This is a replica of the unusual ARP 1005 module from the ARP 2500 Analog Modular Synthesizer. The module consists of a Ring Modulator which is internally connected to a standard ARP transistor OTA based Voltage Controlled Amplifier. The Ring Modulator is the 4014 circuit used in the later ARP 2600, however the 1005 has the additional capability of generating in-harmonic preset voltages which are designed to be feed into the source VCO’s. The modulation effect can be turned off and on by the front panel push buttons or by a gating input, the VCA has both linear and exponential mode.

AM1005 Module The AMSynths design uses the same schematic and similar PCB layout as the original and is built as a 5U high MOTM compatible design. The PCB is 150 x 150mm with 40mil (fat) traces. The original component choices have been retained; LM301 Op Amps, carbon composition resistors and limited use of metal film resistors, Switchcraft illuminated push buttons, high quality rotary pots and ARP 2500 style knobs. .Ceramic capacitors (as per the original) are used in the VCF ladder and a large 680nF  high quality polyester film capacitor between the VCF and VCA.

Modernisation has been limited to using matched transistors from Linear Systems, modern trimmers and a 10V precision power rail for CV control (removing some resistor selection at setup).

The module has the following front panel controls:

  • Unmod Gain
  • Amplifier Gain
  • VCA CV1 Level
  • VCA CV2 Level
  • Ratio and Tune for the Preset Voltages
  • Unmod and Mod Push Buttons
  • Slide switch for VCA response – exponential or linear

There are 10x 3.5mm jack sockets mounted at the base of the front panel, these are:

  • Mod/Unmod Triggers
  • Mod Gate
  • Audio Signal Inputs (x2)
  • VCA CV Inputs (x2)
  • Preset Voltage Outputs (x2)
  • Signal Output

Build History I bought the ARP detailed service notes back in 2004 and made a couple of attempts to design a PCB over the next few years, and even tried to fit the design into EuroRack. The MOTM format for my ARP 2500 Rack modules was finally decided upon in June 2015, and in July the large 150 x 150 mm PCB was laid out, and a prototype ordered on 18 July. I populated the PCB in August and mounted it to a nice aluminium panel. However the buttons were not controlling the ON/OFF nature of the precision output voltages or the lamps. The transistor flip flop worked but somehow the signal was failing to drive through some transistor inverters correctly. Many long hours passed!

By March 2016 I gave up trying to make the original ARP circuit work, and designed a new circuit to drive the switching. The transistor flip flop was retained, driving a new CMOS Schmitt trigger, followed by high quality CMOS switches controlling the CV and audio paths. The lamp drive circuit was also retained. The new “digital core” worked like a dream with 0 to 15V on/off voltages and the benefits of CMOS switches over the old FET’s.

I moved all the components from the old to the new PCB (took me 3 hours!), as I wanted to re-use the expensive resistors and transistor arrays. Then I fired up the new PCB in mid April 2016 and connected it up the panel. At Last…


Copyright AMSynths 2017