VCO design research results

gstopp at gstopp at
Wed Sep 13 21:47:02 CEST 1995

     Hi all,
     As a result of all of the VCO experiments I've been doing lately I'm 
     going to use my current favorite as a "production" design. I've placed 
     an order for 20 MAT-03FH Matched PNP Pairs at a cost of $8.25 each. I 
     will re-sell these (minus the few that I keep) to AH and Synth-DIY 
     members etc. for what I paid plus postage. Lead time is mid-November. 
     The tempco resistors are being investigated, and like I said before are 
     not ultra-critical.
     The VCO has gone through a few changes and ended up to be exactly what 
     is shown in the schematic for VCO Option 2 from the Electronotes 
     "Preferred Circuits Collection", which is a reprint from EN issue 75, 
     page 11 (the only difference is the sync stuff that I added). Here are 
     the specs:
     VCO Type:                     Integrator/Schmidt Trigger Tri-Square
     Control Voltage Response:     1 volt per octave
     CV Input Impedance:           100k ohms for 1 volt per octave
     Frequency Range:              0.1 Hz to 57.7 KHz guaranteed, CV input
                                   sweep = -15v to +15v
     Power Consumption:            +15v @ 31.1 ma, -15v @ 27.8 ma
     Waveform Outputs:             +/- 5v Sine
                                   +/- 5v Triangle
                                   +/- 5v Sawtooth (rising ramp)
                                   +/- 5v Square
                                   +/- 5v Pulse
     Output Impedance for all:     1k ohms
     Linear FM Input Impedance:    560k ohms
     Sync Input Type:              Soft sync to Hard sync depending on sync
                                   signal input level
     PWM Control:                  Initial Pulse Width/Saw Shape pot, PWM in
     Additional Features:          Symmetrized Ramp Modulation (SRM) on sawtooth
                                   waveform output, controlled by pulse width 
                                   control voltage
     Trimpots:                     Volts per octave trim
                                   Saw Shape trim
                                   Sine Symmetry trim
                                   Initial Frequency trim (not on schematic)
     Semiconductors used:          (3) TL082
                                   (3) CA3080E
                                   (2) LM748
                                   (1) CA3140E
                                   (1) Analog Devices MAT-03
                                   (1) JFET (PN4391, 2N3819, etc.)
     Overall this VCO is as close to my original desgin ideal as I think I'm 
     going to get. All waveforms are absolutely perfect at all frequencies, 
     except for the sawtooth which gets a little funnuy-looking over 20 KHz. 
     This is probably due to the 100pf de-spiking cap in the sawtooth converter 
     as well as the finite rise time of the square wave which is used as the 
     control signal for the FET in the converter. I don't consider this a 
     problem since the sawtooth is fine below 20 KHz.
     My VCO is built on a piece of Vector prototyping board p/n 45P80-1, which 
     is a 0.10" grid of 2-sided pads on a board that's 4.5" X 8". I can fit 2 
     VCOs on this. The construction technique is my usual lead-clipping and wire 
     soldered rat's nest. Layout is not too critical but I have to mention that 
     you must de-flux the board once it's built and let it dry completely. If 
     you leave the flux on you will have potential tracking problems, asymmetric 
     waveforms at low frequencies, and the VCO will not go down to the lowest 
     frequency. To de-flux I used the industrial PCB cleaner spray we use here 
     at work. Isopropyl will probably work but don't use rubbing alcohol because 
     it has lanolin in it.
     If somebody wants to CAD up a circuit board that would be GREAT - I will 
     probably try eventually but don't hold your breath.
     My thoughts on the other VCO designs -
     Most sawtooth-based VCOs use NPN transistors in their exponential 
     converters and therefore tend to gravitate towards the CA3046 transistor 
     array for the matched pair on pins 1-5. The other transistors in the array 
     are frequently used in a chip-heater circuit to eliminate the need for a 
     tempco resistor in the volts/octave prescaler divider. I have found that 
     the 3046-based VCO's don't go to the extremely low frequencies, which is 
     okay for audio VCOs but not for LFO-type modulation.
     Most of the 3046-based sawtooth VCOs have quite a few components involved 
     compared to this tri-square VCO. These designs include the ones used in 
     the Micromoog, Multimoog, Rouge, Prodigy, and Moog 921. Now large 
     quantities of passive components is okay for production machines but 
     really a pain in the butt for non-printed-circuit-board assembly so aside 
     from the superior performance the tri-square VCO is easier to build to 
     boot. Notice that the Moog 921 is a 3046-based sawtooth design but is has 
     about five times the number of passive components as the typical design.
     The Chroma 3046-based VCO has a low parts count, but I have not had good 
     luck with this (probably cockpit error). I'll probably look into this one 
     more in the future but for now I have settled on what I think is the best 
     all-around design for my home-built systems.
     I will make a generic fax that I can send out, of the schematic and the 
     related text, so Email me with your fax number if you want one sent to 
     That's it for now,
     - Gene

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