I decided to build a VCO using Ken Stone’s CGS48 PC board. My version is minimal; it omits the additional waveshapers that are on the board. I was interested in getting a simple, decent 1volt-per-octave VCO. The details are apparent from the panel features.
- Coarse and Fine tuning pots
- 1V/OCT CV input
- FM Input with attenuator pot, switchable between EXPONENTIAL and LINEAR resonse
- LINEAR FM can be AC or DC coupled
- Hard Sync input
- Pulse Width Modulation iInput
- Sawtooth and Pulse wave outputs, both +/-5V
History of the CGS VCO
Start with Ken Stone’s Catgirl Synth website. Scroll down and look at the VCO project.
It all goes back to 1976 when Bernie Hutchins published the schematics for the ENS-76 VCO Option #1. If you are into the design of circuits, I highly recommend getting hold of your own hard copy of Electronotes. The issues from the 1970’s are the best, including the Musical Engineer’s Handbook.
More recently, the ASM-1 project made a variant of this circuit. Read all about that >>here<<.
You can study all four different schematics: Electronotes, ASM-1, Ken Stone’s, and mine, which is built on Ken’s PC board. Here are quick links to the ASM-1 and mine.
I use some different parts values. From the ASM-1 docs: “The original value of R15 was 1.5M Ohm giving an Iref of 10 uA and a f0 of 909.09 Hz.” I use the original 1M5 value to set the initial current in the exponential converter. Combined with the 590K pulldown on the VC mixer it sets the frequency with zero volts on the control inputs to approximately 160 Hz. The Coarse pot gives a manual frequency range from under 1 Hz to 24KHz. The only trimpot on my board is for 1V/oct calibration. The 4M7 on the Fine tune pot gives a nice fine tuning range. The 56K on the exponential FM attenuated input gives a very sensitive CV input. I use a 390K on the Linear FM input for less sensitivity than Ken’s 100K but more than the 560K used in the ASM-1. I added a switch to short out the AC-coupling capacitor, so that the Linear FM can be DC-coupled if desired. Linear FM typically takes an audio frequency from another VCO, for which you want AC coupling, so you can omit this switch if you don’t need weird DC linear modulations. The VCO core, consisting of the CA3140, LM311, and the FET, is identical in all these versions and I left it alone. I modified the output buffers to get +/-5V levels on both outputs. I’ve standardized on the 3K3/1K8 ratio for my pulse outputs. It gives a nice strong level without drawing too much current and still presents a low impedance. I added the 2K2/2M2 positive feedback on the square wave comparator, same as on the CGS56 Pulse Buffer. It sharpens the edge of the waveform and keeps it snappy at subaudio frequencies. I use 18pf instead of 68pf in the feedback loop of the saw output buffer. It still suppresses parasitics, and leaves the waveform a bit sharper. Another small enhancement was addition of 100nf bypass capacitors on the power pins of the TL072 output op amp.
Here’s a finished module that uses an earlier version of the PC board.
Parts and Special Parts
The critical parts are:
- LM394 or MAT-02 matched transistor pair
- 2N4856 or J108 FET
- 1K Temperature-compensating resistor
- 2n2 polystyrene capacitor
You cannot substitute these without risk to the quality of the VCO. The CA3140 is getting harder to find; it is an old-fashioned IC. I use the LT1013 to mix the exponential CV inputs. It has good DC stability. But a TL072 would also work. Don’t substitute a different type of capacitor; you must use polystyrene. Mouser part 23PS222 works well.
I bought a kit of special parts from Elby Designs. This is called the Bi-N-Tic special parts kit because the Bi-N-Tic uses the same VCO core. It includes 1 LM394 or MAT-02, 1 J108 FET, and 1 1K Tempco resistor. It also comes with a second resistor that if used in series with the 1K can provide more accurate temperature stability. I tried it, but found that in this circuit it interferes with the 1V/octave calibration. Better off without, since you have to cut the PC board to put the resistor in series with the Tempco.
- Don’t forget the 5 jumpers. Two of them are needed to connect one end of a 590K to -15V
- The 4M7 fine tune resistor goes into a hole with a diode marking (diode not used).
- Carefully orient the LM394 by its tab.
- The J108 FET matches to the markings on the PC board. Flat side to flat side.
- The 4-pin MTA MOTM-style power connector is reversed from normal. The pins face inward.
Full parts list
|CGS48 VCO PC board||Ken Stone||1|
|Mill-Max DIP8 socket||575-193308||4|
|3-pot long bracket||Stooge||1|
|10K multiturn trimpot||72-T93YA-10K||1|
|1K Tempco||Elby Designs||1|
|Thermal compound||Radioshack 276-1372A||1|
|SPDT toggle switch||633-M201202||2|
|LM394CH or MAT-02||Elby Designs||1|
|J108 FET||Elby Designs||1|