CGS utility LFO
CGS58 the CGS utility LFO is a dual utility LFO offering manually variable wave shapes. Standard waveshapes are falling ramp variable through triangle to rising ramp, and variable pulse width. A fixed square is also available, as is a combination waveshape that is variable between the saw/tri output and the square. This is a MIX not a morph. Optional switches allow the variable outputs of the two LFOs to be intermixed.
This module will work on 12 volts, though the output swings will be reduced by 20%.
A little on how it works
The Utility LFO is based on the standard integrator/schmitt trigger oscillator core. Two pots in the integrator charge path allow for the adjustment of speed, and along with two diodes, for the adjustment of charge versus discharge time. Changing this ratio will allow the triangle/saw output to be adjusted through falling ramp, to triangle to rising ramp. At the same time, the output of the schmitt trigger will reflect this in the length of it output pulse.
A 0 volt referenced comparator connected to the output of the integrator generates a square wave as there is always an equal portion the ramp/triangle wave above the 0 volt reference, though adjustment of the shape of the ramp/triangle wave does change the phase of this square wave.
Following the comparator is a buffered voltage divider to bring the square wave done to a more suitable level and also to drive one side of the variable output mixer pot. The other side of the variable output mixer pot is connected directly to the integrator output. The variable output is buffered is used post mixer by another op-amp.
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.
There is provision on the copper side of the PCB for tacking some extra decoupling capacitors directly across the power rails of the chips, should they be needed. Capacitors in the range of 10n to 100n would be suitable.
For greater range, or a slower LFO:
Method 1: Use a 500k pot for speed instead of the 100k pot.
Method 2: Increase the value of the 47n between pins 1 and 2 of the TL074. You may need to reduce the value of the 4k7 resistor in series with the speed pot to get your maximum speed back.
This is a guide only. Parts needed will vary with individual constructor's needs.
|Ferrite bead (or 10R resistor)||2|
|0.156 4 pin connector||1|
- PCB is 1" x 6" with four 3mm mounting holes 0.15" in from the edges.
Readers are permitted to construct these circuits for their own personal use only. Ken Stone retains all rights to his work.
- Utility LFO for music synthesizers (archived) by Ken Stone, 2004, with permission of the author
- CGS58 BOM at BOMpiler
- CGS Synth discussion group, for discussion of locating parts, modifications and corrections etc.
- CGS58 Utility LFO, revision 1.1, Elby Designs