Special Pages


Utility LFO

Courtesy/Thanks to: Ken Stone

Description: This board 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.

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 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 suppled 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.


This module will work on 12 volts, though the output swings will be reduced by 20%.
PCB info: 1" x 6" with four 3mm mounting holes 0.15" in from the edges.
Please email me if you find any errors.

Parts list

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

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

Part Quantity
47n 2
100n 3
10uF 25V 2
1k 14
2k2 2
4k7 2
10k 2
47k 2
100k 6
150k 2
1N4148 4
TL072 1
TL074 2
BC547 2
Ferrite Bead (or 10R resistor) 2
0.156 4 pin connector 1

Article, art & design copyright 2004 by Ken Stone


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   Power rail decoupling for the Utility LFO.
   Picture of the finished Utility LFO
   Picture of the finished Utility LFO
   The schematic of the Utility LFO
   The component overlay and wiring for one of the two LFOs. Wiring for the second LFO is identical.
   Wiring in two DPDT switches as shown will allow for the variable outputs of the two LFOs to interact
   Cat Girl Synth Parts Supplier Information page
Ken Stone
   CGS (Cat Girl Synth)
Parts used
   High speed Switching Diode
   Low-Noise JFet-input Operational Amplifier
   Low-Noise JFet-input Operational Amplifier
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