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AM 4004 - ARP VCO
Overview This is a replica of the ground breaking ARP 2500 oscillator that introduced a new era of pitch and temperature stability in synthesizers. Pearlman introduced a very
accurate exponential converter for his new VCO design, and thereby produced a synthesizer with a significant edge over the existing Moog design, which were not pitch stable.
Original Circuit
In 1970 ARP created a generic VCO that was used in a number of ARP 2500 modules (1023, 1004, 1045) with slight variations to the number of controls.
The VCO core is a dual slope integration osciallator that produces a triangle
waveform. It uses two exponential converters each with matched transistors and a tempco resistor. The pitch stability is 0.1% drift per hour after an initially warm up period of 1 hour. The triangle waveform is then processed into sqaure,
pulse, sine and swatooth waveforms, which can be mixed or selected depending on the VCO module type. The VCO has Coarse and Fine frequency controls, and it can be switched to LFO mode. The specifications are:
- Frequency Range: 0.03Hz to 16kHz
- Sine: -5V to +5V peak to peak
- Traingle: -5V to +5V peak to peak
- Swatooth: 0V to +10V peak to peak
- Pulse: 0V to +10V peak to peak
- Square:0V to +10V peak to peak
New AM Circuit The AM4004 module clones the ARP VCO core and waveform
processors exactly, using the same circuit design but with some improved Op Amps. The circuit uses the original exponential converters with newly manufactured 100 ohm 3500 ppm/C tempcos.
Front Panel
The front panel is an AM High Density design, 90mm wide, 3mm thick aluminium, 4U high, with black lettering. Controls pots have 3.85mm diameter shafts, knobs are 19mm and 13mm in diameter - black and chrome. Switches are sub-minature toggles and a 3mm LED if fitted to monitor the VCO frequency when in LFO mode, and to show the VCO is active when a waveform is selected.
Build History The schematic is being transferred into Eagle CAD, and the board layout needs to be completed.
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