[sdiy] CV input op-amp circuit

Bernard Arthur Hutchins, Jr bah13 at cornell.edu
Sat Dec 5 03:47:20 CET 2020

David G Dixon dixon at mail.ubc. wrote Sat Dec 5 02:11:46 CET 2020
"If I'm generating CVs that are meant to be precise (as, for example, VCO pitch-controlling voltages from a keyboard or sequencer), then I take the opamp output from the other side of the current-limiting resistor.  I call this resistor an "innie" and it doesn't affect the output voltage of the circuit (the opamp compensates for it).  I don't want that damned 1k resistor dividing my carefully calibrated voltage."

Which is why we have said many times to NOT use a 1k series with the MCV (Main Control Voltage:  1 volt/oct) from the keyboard (for VCOs and VCFs).  This always goes into the VCF or VCO though an (often precision) 100k so as to accommodate a standard 2k temco and v/oct trimmer.  (Or some similar standard).

The 1k’s are used here generally to prevent op-amp HF oscillations due to shielded cable capacitance, as noted.  Without the 1k for the MCV, keep MCV distribution cables short, or use unshielded wires, or use a 741 op-amp to output the MCV.

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