AM8027 Dual ARP 2600 VCO

AM8027 Dual VCO

Overview  This module is an improved version of the legendary ARP voltage controlled oscillator, first seen in the 2600 synthesizer of 1971 and then subsequently the VCO in all ARP products, including the Odyssey and Quadra. This module provides two stable and wide ranging oscillators with an accurate replication of the original wave shapes (Sawtooth, Triangle and Pulse). A Range switch varies across 5 octaves and the Pitch pot across +/- 2 semitones.

The core of each VCO is driven by ultra precision voltage regulators and is potted using a thermal compound, just like the original. Pulse Width can be varied from the front panel sliders, along with Frequency Modulation and Pulse Width Modulation. A hard Sync facility has been added, enabling VCO1 to be synchorinsed to VCO2.

AM8027 PCB

Detailed Description  The AM8027 Dual VCO module is based on the ARP 4027-1 voltage controlled oscillator which in itself is an improved design over the original 4017/4027 VCO’s developed by ARP in 1971. The 4027-1 has a fixed High Frequency trim circuit to ensure the VCO tracks accurately over a wide range of frequencies – with no adjustment, and a short reset time on the sawtooth core, which means there is less need for HF compensation.

The ARP 4027-1 VCO is a standard sawtooth core using a capacitor and integrator. However it uses a NPN and PNP transistor pair exponential generator that were apparently hand matched. This design flies in the face of the standard dual PNP matched transistor pairs (Moog, Oakley, MOTM).

The AMSynths design uses a potted sub module for the VCO core, as after extensive research (including using an original ARP2600), this was found to provide the most stable frequency. The AM8027 replicates the original ARP electronic design precisely, with these improvements and variations:

  • On board high precision +10V and -10V Reference Voltages
  • Frequency control is buffered for additional stability
  • Frequency control preset in a 5 octave Range
  • A Hard Sync facility
  • Low Frequency option removed as it destablises the core
  • Improved integrator with 2N4392 FET
  • 25-turn Cermet trimmers & Metal film 1% resistors
  • 1% Polystyrene integrator capacitor instead of Mylar

Each VCO has a set of slide potentiometers to adjust 2x external Frequency Modulation CV’s, the Pulse Width of the Pulse output and PW Modulation for an external source. The VCO is connected to the Keyboard Note CV in the Doepfer bus, using an additional Op Amp buffer to ensure stability. Each waveform (sawtooth, triangle and pulse) has its own output jack socket.

Outcomes & Availability The Dual 8027 design was based around the VCO’s I made for my 2600 FracRac in 2007, and after a variety of prototypes a completed REV06 PCB set went into small scale production, with one customer sale in 2017. The module was accurate and sounded great, but was time consuming to build. The availability of the TTSH 2600 clone (and many others), and a busy day job, resulted in the 8027 being a low priority project.

2024 Update With the arrival of a Behringer 2600 VCO clone in 2021, I have no plans to manufacture a 4027-1 module. However the schematics of the ARP 4017 have emerged and there is now an AMSynth 4017 module which includes the sync in/out and HF trimmer of the original.


Copyright AMSynths 2022