DIY Analog VC-LPF

Build a voltage-controlled resonant filter on perfboard for just a couple of bucks.  I designed this as an experiment.  The purpose was to see if a classic synth circuit designed for +/-15V split supply would work  on 9V single supply.  To adapt the circuit, I used a TLE2426CLP ‘rail splitter’ virtual ground IC.  It creates a low-impedance point at 1/2 the supply voltage.  Inputs and outputs are referenced to B- and coupled with capacitors.  In this way the circuit behaves normally to the outside world but its virtual ground, hidden inside, is actually +4.5 volts.  The circuit design is adapted from the original LM13700 datasheet.

Exponential frequency control is accomplished by a 0..+9V potentiometer voltage divider.  You could easily cut the wire to the pot and add an external CV input.  It will be fine for positive control voltage 0..+10V.  Do not apply negative voltage or damage may occur.  Response is exponential but not precisely 1V/octave.

So, does it perform the same as it would on +/-15 volts?  Yes and no.  It distorts easily because there is approximately 1/3 the headroom with 9V single supply.  This circuit handles up to 5V peak-peak analog input.  That should be sufficient headroom to process audio from other devices running on 9V power, such as any of the Andromeda Space Rockers instruments.

Project Plans

You could use any pad-per-hole 0.1″ grid perfboard.  I used ‘PC3′ type, available from MPJA, All Electronics, and others.  The board costs less than $1.

I used parts from Mouser.  In the Project Plans, some items have Mouser part numbers listed in brackets.  You must source your own parts. Do not write me about where to find them, catalog numbers, etc.

• black wire = GND
• white wire = Virtual GND
• red wire = +9V
• yellow wire = SIGNAL1, SIGNAL2

The board is flipped vertically in this view.  I used teflon-insulated 30-gauge wire for SIGNAL1, SIGNAL2.  26-gauge teflon wire was used for GND, Virtual GND, and 9V.  This wire is also called Kynar or Wire Wrap, and can be found on eBay, or from Circuit Specialists Inc.

The offboard wires are soldered to terminals for B+, B-, GND, IN, and OUT.  I make the terminals from 22-gauge bare copper wire.  Bend it into a U-shape with legs 0.1″ apart, and solder it into the perfboard.  I bend the legs of the “U” at right angles on the underside to add strength.  To attach an offboard wire, tack it on with solder from the top side.  This is a convenient and semi-permanent way to connect wires.

Also notice that the resistors are elevated about 0.25″ off of the board.  This makes it easy to experiment with different values.  Simply cut the original resistor’s leads at the body, leaving the leads pointing straight up.  Pick a new resistor, bend it into shape and clip its leads to about 0.25″.  It can then be soldered to the old leads that are poking up from the board.  This is much easier than replacing the entire resistor from the bottom side.

ASSEMBLING THE BOARD: first acquire all the materials, perfboard, and wire. You need a wire stripper suitable for this thin wire. Grip the wire with needle nose pliers when stripping, using the pliers as a fulcrum to avoid stressing the connection. Print out the PDF Project Plans and use the PLACEMENT diagram as a reference, and solder all components in place - but DO NOT trim the leads yet. Now look at the SIGNAL1 page in the Project Plans. Many of the short connections can be made simply by bending leads and soldering them into place. Use a highlighter to mark over all the connections as you make them. Then you may ‘mow’ down any leads that are still sticking up. Proceed to the GND page and begin using teflon wire to make all the indicated connections. As before, mark over the connections with a highlighter to keep track of the remaining work. Do Virtual GND, 9V, SIGNAL1, and SIGNAL2 the same way. When no connections are left unhighlighted, the wiring is complete.