This is a licensed adaptation of the original Serge VCF

Type: State variable, 2 pole, 12dB/oct LP & HP, 6dB slopes on the BP.

A little on how it works.

The circuit consists of a voltage processor, allowing the filter frequency to respond either proportionally or inverse-proportionally to the incoming CV. The output of the voltage processor is then fed to a pair of exponential converters that control the gain of the CA3080 transconuctance amplifiers in the filter core, thus varying the frequency.

Click here for the Schematic

Construction

The component overlay and basic wiring for the VCF. The pots are viewed from the rear. No components need be installed in the buffer section (below the thick line) if it is not needed.

Before you start assembly, check the board for etching faults. Look for any shorts between tracks, or open circuits due to over etching. Take this opportunity to sand the edges of the board if needed, removing any splinters or rough edges. (With the boards supplied by me, the edges are already milled, and etching faults are very rare.)

When you are happy with the printed circuit board, construction can proceed as normal, starting with the resistors first, followed by the IC socket if used, then moving onto the taller components.

Take particular care with the orientation of the polarized components such as electrolytics, diodes, transistors and ICs.

When inserting ICs into sockets, take care not to accidentally bend any of the pins under the chip. Also, make sure the notch on the chip is aligned with the notch marked on the PCB overlay.

The holes without pads near the input and output pads are to allow a wire to be fed up from the bottom side, looped over and then soldered to the pad. The slack is then pulled back through the hole, creating an anchor that holds the wire by its insulation, reducing the chance of wire breakage. It is also helpful for a system such as Serge, where wires are traditionally connected from the solder side of the PCB.

See the photo below re mounting the J201 FETs on the VER1.0 PCBs.

Notes:
• If you cannot find an 11M resistor, substitute a 10M.
• The 22M can probably be omitted. I have not tried this though.
• A TL071 can be substituted for the ancient LM748. If you do this, leave out the 5pF capacitor
• The resistors marked RG1-4 are for setting the gains of the optional buffers. It is unlikely you will need to install them, and then only if you are using the buffers as gain stages for something other than this filter.
• All parts surrounding the TL074, as indicated by the heavy line marking of the area of the PCB, can be omitted if the buffers are not required.
• Possible use for the buffers include buffering the input and CV input to compensate for their lower-than-usual input impedance, or buffering the outputs after they have been fed though a level pot, as per the panel in the photo.
• A DPDT switch can be added to short points F and G to 0V. Doing this will switch the filter into low-frequency mode, as per the VCFX (for CV filtering etc.). While the original Serge boards had this function, it is doubtful it was ever brought out to the panel.
• The LD connection can be connected to one of the outputs to give a visible indication of an output signal. The LED, the associated transistor and resistors can be omitted.
• Even though the panel says "Cynthia" on it, it does NOT mean that this is a module she is offering for sale!
• Offsite link to an mp3 example of how this filter sounds.

Setting up

Adjustments on the vcf board are basic. Set trimmer so that the VCF's center frequency as at an appropriate pitch.

Notes:

• PCB info: 2" x 6" with four 3mm mounting holes 0.15" in from the edges.
• Will run on +/- 12 volts or +/-15 volts.
• Please email me if you find any errors.

Part Quantity Capacitors 5pF (omit for TL071) 1 47pF 5 100pF 2 220pF (Styrene) 2 10n 4 22n 1 100n (monoblock) 3 100n MKT or better 2 10uF 25V 2 Resistors 330R 7 470R 4 1k 5 2k2 2 3k3 2 4k7 1 15k 1 18k 1 22k 5 33k 3 47k 1 68k 8 82k 5 100k 9 330k 2 470k 2 820k 4 1M 2 3M3 1 11M 1 22M 2 RG1-4 see text 4 22k (25k) trimmer 1 20k lin pot 1 50k log pot 1 100k lin pot 2 Semi's LED 1 1N4148 1 2N3904 1 2N3906 4 J201 2 LM748 or TL071 1 CA3080 or LM3080 2 LM3900 1 TL074 1 Misc. Ferrite Bead 2 0.156 4 pin connector 1 cgs77 PCB 1

Parts list

This is a guide only. Parts needed will vary with individual constructor's needs.

Classic Serge style knobs can be purchased from
Small Bear Electronics LLC
phenolic clone of Davies 1900H
ABS clone of Davies 1900H
Colored ABS clones of Davies 1900H

If anyone is interested in buying these boards, please check the PCBs for Sale page to see if I have any in stock.

Can't find the parts? See the parts FAQ to see if I've already answered the question. Also see the CGS Synth discussion group.
 

Article, art & design copyright 2006 by Ken Stone

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