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Oscillators - At The Source
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 simplified for the ARP 2600 and used as the basis for most of their analog synthesizers 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
converters and added a sync facility 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 converters 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 Creative 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 Rev 1 in a modular synthesizer. click here
Roland VCO Roland entered the analog synthesizer market in 1973 with the SH1000
and a rather unusual VCO, which they stuck with in 1974. However from the System 100 of 1975 onwards they used a much improved design, until replaced by CEM chips and then DCO's in the early 1980's. This VCO is a traditional dual
matched NPN exponential generator, often using the dual heated transistor UA726 as the exponential generator for a sawtooth core. Precision voltage rails are used to ensure better stability than many of their competitors.
The
VCO's of the System 100, 100M, 700, SH-09, SH-1, SH-2, SH-5, SH-7, Jupiter 4 and 8 are all versions of the same design, with minor differences in the waveform shapers to minimize component count and setup as the years advanced. I
have replicated the design from the 100M and improved the precision voltage references. click here
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