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Oscillators
Overview The analog synthesizer relies on
the voltage controlled oscillator (VCO) to create pitched notes as the basis for most of the amazing sound potential of an analog design. The VCO dates back in to the 1950's, but the first commercial VCO's arrived in the Moog
Synthessizer of 1964, the 901 VCO. With a reputation for high quality waveforms but instability of frequency and tracking, the 901 was replaced by a more stable design the 921.
There-in lies the tale of the VCO, the design MUST
be stable and accurate from a frequency perspective AND it must deliver sonically useful waveforms, which may not need to be highly accurate in shape. For the last 40 years VCO design has evolved and been perfected. I have built a
number of different VCO;s, both new and replicas, these web pages describe the designs in more detail and I have made accurate measurements so you can see how the VCO's perform.
Moog VCO Whilst it would be fun to
clone the 901 its instability due to a lack of Tempco and matched transistors in the expo generator and its complex high and low frequency trimming doesn't make it an attractive project. The 921 is a step forward with a CA3046
transistor array and a Tempco providing much needed frequency stability. However the design I have replicated is the later MiniMoog VCO which uses a dual heated transistor UA726 as the exponential generator for a sawtooth core. I have
upgraded the design to improve accuracy and to do without the rare UA726. Details to follow...
ARP VCO The ARP 2500 set a new benchmark of VCO stability in 1970 but its VCO's are too complex to replicate quickly. They have two NPN/PNP
pairs as a "dual core" exponential generators, along with 100 ohm Tempcos and two potted sub modules. This NPN/PNP transistor pair design was considerably simplified for the ARP 2600 and used as the basis for most of their analog
synthesziers for the next 10 years. It is this ARP 2600 VCO that I have replicated and improved, with its sawtooth core, NPN/PNP transistor exponential generator, 1k87 Tempco and fixed high frequency trim. I have accurately replicated
the waveform convertsers and added a sync facilitty based on the Odyssey Mk3 VCO's. click here
E-mu Systems VCO This designs dates back to 1972 and the rare E-mu Systems Modular synthesizer. Dave Rossum implemented a very nice and
stable sawtooth core design, using dual matched NPN transistors, 1k Tempco, high frequency trim and a potted sub-module. I have cloned this circuit and made various improvements, including precision reference voltage rails for the core
and the frequency controls. it is a very nice design similar to modern VCO's from MOTM and Oakley Sound, and it has its own design of waveform converets and a final mix stage as well. click here
SSM2030 VCO Dave Rossum went on (in 1976) to design a range of chips with analog circuits within them, including VCO, VCA and VCF. He
switched to digital chip design in 1983 (the E-chip in the Emulator II) and went on to design digital chips for Cereative and the Audigy Sound Card. The SSM2030 VCO was used in the Prophet 5 Revision 1, and whilst its not the most
stable VCO in the world, its nice to have the sound of the Rev1 in a modular synthesizer. click here
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