AM8027 Athena Dual VCO

ARP 2600 Dual VCO Clone

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 a stable and wide ranging oscillator 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 the VCO is driven my 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 2 VCO’s to be sync’d together using the jack sockets.

The AM8027 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 1970. The 4027-1 has a fixed High Frequency trim circuit to ensure the VCO tracks accurately over a wide range of frequencies - with no adjustmen, 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 controlled by preset octave Range
The Hard Sync facility added from the ARP Odyssey
Low Frequency option removed as it destablises the core
Improved intgerator 2N4392 FET
25 turn cermet trimmers & Metal film 1% resistors
1% Polystyrene integrator capacitor instead of Mylar
The AM8027 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 has its own output and the Sync facility can be independently patched to which ever VCO you wish and controlled by a simple slide switch (ON/OFF).

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