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  • CS-style VCO core

    For a  long time im interested in the sound of the old CS Synths. So I started in spring 2013 making a clone of the VCO.
    After studying the Service Manuals, Interrnet Research (google Patent search) I breadboarded several circuits with more or less sucess.
    Some don’t oscillate reliable, some wheren’t linear etc.

    In a German Synthesizer Forum, Sequencer.de  I got help from Uwe Schüler and he designed a simple CS-Style VCO. Thank you Uwe.
    I read that one of the secrets where the 30µs rectangle step.
    You can see the waveform in the following pictures

    CS Style Oszi1_web
    CS-Style Saw

     

    CS Style Oszi2_web
    CS-Style Saw enlarged.

    The VCO is a linear sawtooth oszillator with a V/Hz characteristic.
    You can here the VCO in combination with my homebuild MIDI to Voltage Converter.

    As you can hear, it’s detuned in the low octave. The Reason for this is the 12Bit resolution of the DAC.
    In the following table you can see the small differences in the control Voltages, I think a 14Bit DAC will be fine.

     

    CS Style VCO thru a 24dB Ladder Filter:

     

    Note    CV
    C           0V
    C#        0,5297V
    D          0,5612V
    D#        0,5946V
    E          0,63V
    F          0,6674V
    F#        0,7071V
    G          0,7492V
    G#       0,7937V
    A          0,8409V
    A#       0,8909V
    B          0,9439V
    C’         1,0000V

    Here a picture  from the bench.

    Aufbau 1_web

     

    UPDATE 08.01.2014

    After Uwe, the developer of the VCO, wrote me an e-mail where he asked me about the used OpAmp, i wrote him that i used an OP07 (bipolar), he told me to use jfet type.

    So I changed it  to a LF356.

    Here the results:

    CV                   F
    0.125V              49.8Hz
    0.250V             99.01Hz
    0.5V                 197.4Hz
    1V                    393.8Hz
    2V                    784.9Hz
    4V                   1520Hz
    8V                   3140Hz

    I think this is very linear.

    I did a simple temperature check (hair dryer)

    15C°    1000Hz
    20C°      999Hz
    30C°      993Hz
    40C°      984Hz
    46C°      977Hz

    The circuit was build with standard metalfilm resitors with +- 50 ppm

    Thank you Uwe.

    Schematics:
    CS Style Schematic.pdf

    Patent No. for google search:
    Patent No: 3959743
    Patent No: 4191932
    Patent No: 4061987

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