|
Effects
Overview The analog synthesizer provides a wide
palette of sounds, which can be spiced up even further with delay and reverberation effects. In 1970 the only option was a spring reverb, but with 5ervernFrom the mid 1970's onwards a series of 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.
Therein 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 well 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 plus 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.
ARP VCO The ARP2500 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 genartors with 100 ohm Tempcos and potted sub modules. This design was then simplified for the ARP 2600 and used as the basis for most of their 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.
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, it is a very nice design similar to modern VCO's from MOTM
and Oakley Sound.
SSM2030 VCO Dave 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 1984 (E-chip in the Emulator II) and went on to design digital chips used in 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.
|
