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Module A-111-2 is the successor of the obsolete High End VCO I A-111-1.
Here are the most important features of the module:
exponential frequency controls and inputs:
access to internal bus CV (via jumper, optional, please remove the bus jumper if this feature is not used to avoid unwanted frequency modulation as then the unused CV line of the bus works as a kind of antenna)
linear frequency controls and inputs:
Range switch to select VCO/VCLFO mode, frequency relation between VCO mode and LFO mode about 500 : 1
dual color LED display (to display low frequencies below ~ 25 Hz)
Nearly "perfect" waveforms
high end triangle to sine converter with excellent sine waveshape (thank's to Tim Stinchcombe who recommended this circuit)
the micro precision VCO A-111-3 uses the same VCO core as the A-111-2, but has less inputs, outputs and controls
The control voltage applied to the socket 1V/Oct is added to the control voltage coming from the bus (interruptible by removing the jumper JP4). Connecting a cable to the socket 1V/Oct does not interrupt the bus CV connection ! I.e. the 1V/Oct socket is not normalled to the bus CV. This allows e.g. transposing sequence CV (applied to the socket 1V/Oct) by the bus CV (e.g. connected to the CV output of a Midi-to-CV interface).
The module is based on the CEM3340 circuit. The hard sync of the CEM3340 works different compared to most other VCOs. The excerpt from the data sheet below shows the details. The CEM3340 type of hard sync is also explained in the A-111-1 user manual on page 9: A111_man.pdf.