This is an excerpt from a March 1970 Tonus Catalog featuring the 2500 series synthesizer. 
Not many of these modules atcually made it to production which is a shame. Some of the  
modules they never got around to making would have great.

Rob Williams
rw@goldengate.net
02-18-98

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COMING SOON FOR THE ARP 2500.

 1024 Minisequencer: A compact 8 position inexpensive sequencer in a single 3-inch
 wide module.

 1028 Slave Module: A sequencer extender module which provides three extra voltage
 outputs per step.  Board with miniature potentiometers may be unplugged and saved.
 Extra, inexpensive boards of pots plug into panel.

 1019 Oscilloscope: A small oscilloscope especially designed for observing audio
 waveforms generated by the synthesizer.

 1029 Frequency Shifter: Radical new circuit permits laboratory quality frequency
 shifting at dramatically reduced cost.

 1013 Octave Fixed Filter Bank: Set of octave wide filters covering the entire audio
 spectrum in a compact and efficient format.

 1015 Half-Octave Fixed Filter Bank: 19 fixed frequency band-pass filters designed
 for minimum phase shift.

 1031 Dual Multipurpose Filter: A combination high-pass, low-pass, band-pass, notch,
 and peaking filter with voltage controlled ft and Q.

 1025 Dual Reverberation Module: High quality reverberation systems designed
 specifically for the ARP Synthesizers.

 1045 Voltage Controlled Voice: Combination module containing a voltage controlled
 oscillator, a dual envelope generator, a voltage controlled low pass filter, and a
 voltage controlled amplifier.  The module is designed to function as a compact pre-
 set voice, although the functions can be used separately since functional circuits
 are not hard-wired internally.

 1032 Mixer/Distributor: Eight voltage controlled amplifiers in a voltage controlled
 mixer/distributor; 8 in, 1 out  --- 1 in, 8 out.

 1035 Triple Modulator: Three balanced modulators and control circuitry in a
 single module package.

 1037 Dual Voltage Controlled Envelope Generator: Dual envelope generator has all
 voltage controlled parameters and generates both linear and exponential envelopes.

 1036 Dual Random Voltage Source/Sample & Hold: Two uncorrelated noise sources
 followed by sample and hold circuitry produces random voltage output.  Internal
 clocks determine sample rate.  Sample and hold function may be isolated.

 1038 Dual Preset Envelope Generator: Dual envelope generator has eight preset
 envelopes.

 1042 Triple Voltage Controlled Amplifier: Three voltage controlled amplifiers in
 a compact module.

 3002 Four Voice Keyboard Controller: Keyboard controller capable of four-part
 polyphonic playing.

 tonus, inc 
 45 Kenneth Street/ Newton Highlands/ Massachusetts 02161/Tel*phono(617)969-0810