LFO Rack
Three waves, three jacks, one knob · A TL074 on twenty strips · 8 HP · By Onion Madder · Mess O' Pedals
The Utility LFO on the stripboard shelf makes a triangle, a square and a sine from one TL074 around a half rail, because a 9 V box has no negative voltage to swing into. A rack has a real ±12 V, so this is the same four op-amps with every reference on ground and every wave landing at about ±5 V, each on a jack of its own. Rate, a range toggle, an LED. Eight HP, and nothing on it you have to explain.
What you'll need
tap to expand / collapse
On the board, counted from the layout file:
- 1× TL074 (IC1)
- 2× 100k (brown-black-yellow; R5, R13)
- 1× 10k (brown-black-orange; R11)
- 4× 1k (brown-black-red; R6, R8 to R9, R15)
- 1× 22k (red-red-orange; R10)
- 3× 47k (yellow-violet-orange; R1 to R2, R14)
- 1× 68k (read the bands; R12)
- 2× 100n ceramic disc (104; C7 to C8)
- 1× 1u box film (105, or a small film cap; C1)
- 2× 10u electrolytic (C5 to C6) - spot it: the stripe marks the negative leg
- 1× 10u NP electrolytic (C4) - spot it: the stripe marks the negative leg
- 3× 1N4148 signal diode - spot it: a tiny glass body, black band toward the cathode (D3 to D5)
- 2× 1N5817 Schottky, the reverse-polarity diode - spot it: a fat black body, band toward the cathode (D1 to D2)
- 12 wire links (J1, J1b to J11) - solid core, tinned
- 1× 2×5 pin header, plain, 0.1″ - the Eurorack power connector. spot it: ten square pins in two rows. Not the shrouded kind; a shroud is 20 mm long and would cover the neighboring strips
- Stripboard, 20 strips × 23 holes or larger
Off the board
- 1× B1M 9 mm Alpha pot, the RATE knob, wired as a variable resistor: lug 3 to SQ, wiper to RATE_W, lug 1 empty. The 47K floor it needs is on the board
- 1× sub-mini SPST toggle, RANGE, between TRI and RANGE_C: closed, C4 sits across C1 and everything is ten times slower
- 3× Thonkiconn PJ398SM 3.5 mm jacks: triangle, sine and square out. Every sleeve to GND_P
- 1× 5 mm LED, the rate light, long leg to LED_A. Its resistor and the diode that protects it on the negative half are both on the board
- 1× 8 HP Eurorack blank panel, one knob, and a Eurorack power cable
What it is
Op-amp B integrates and op-amp C compares, which is the whole oscillator: C's output drives the RATE pot into B's summing node, B ramps into C1, and when the ramp crosses C's threshold C flips and the ramp turns round. R2 against R5, 47K over 100K, sets how far the ramp gets before the flip - about ±4.9 V, which is why the triangle goes to its jack through nothing but a 1K. The square out of C is the chip's full swing, about ±10.5 V, so op-amp D scales it, 100K in and 47K back, to ±4.9 V and the right way up: high while the triangle rises. Op-amp A makes the sine: the triangle through 22K into two 1N4148s on ground comes out with its corners rounded to about ±0.6 V, and A gives it a gain of 7.8, 68K over 10K, so it lands at about ±4.7 V beside the others.
Two things about the shape of the board. The Schmitt's feedback resistor is the one part a quad op-amp will not let lie flat, because OUT, IN- and IN+ are three consecutive pins and the resistor wants two of them: so the trigger point is carried down to a strip of its own (TRIG, strip t) and both R2 and R5 land there. And the rails stop at column 5, so the four holes to their left on strips a to g are free strips - the sine jack sits on one of them, an inch from the op-amp that makes it.
The sine shaper is the least certain thing on this page. Where two diodes start to bend a triangle depends on how hard R10 drives them, and 22K is a starting value from the datasheet curves, not from a bench. Anything from 22K to 100K is worth trying with a scope on pin 1; the gain follows whatever R10 ends up at. R2 and R5 are not to be swapped: 47K on the triangle side and 100K back from the output is what makes the trigger window narrower than the swing, and the other way round it never starts.
The layout file is the guide
As on every stripboard guide here, the build document is the file itself: ↓ Download LFO_Rack.json - import it into Copper Bottom and the editor gives you the board picture, the placement walkthrough, the electrical checks and a printable build sheet, all from the same file. No coordinates are written out on this page on purpose: the editor is the viewer, and a number typed twice is a number that can drift.
The netlist
Quoted verbatim from the layout file. It was written from the board, so checking the
board against it can only pass - what it is for is carrying the circuit somewhere else, and
telling you what each strip is supposed to be doing while you stare at it. TRIG is the
comparator's trigger point, where R2 and R5 meet; SINE_NODE is where the diodes live.
The off-board wiring
20 pads leave the board. Ten of them are the power header.
| Pad | Goes to |
|---|---|
| SQ, RATE_W | The RATE pot. Lug 3 to SQ - the raw square, about ±10 V, which is a control signal here and not a jack - and the wiper to RATE_W. A variable resistor: lug 1 stays empty |
| TRI, RANGE_C | The RANGE toggle. TRI is the triangle itself, RANGE_C is C4's free leg; the switched wire carries the op-amp's output, not its summing node, so a panel wire here is harmless |
| TRI_J | Triangle out, jack tip, through 1K. About ±5 V |
| SINE_J | Sine out, jack tip, through 1K. About ±4.7 V, and the number on this page least likely to survive the bench |
| SQ_J | Square out, jack tip, through 1K. About ±5 V, high while the triangle rises |
| LED_A, LED_K | The rate LED. It is driven from the scaled square through 1K, with a 1N4148 back to back across it so the negative half puts no more than a diode drop of reverse voltage on it |
| GND_P | Ground for the panel: the three jack sleeves. One pad, one wire, daisy-chain the rest |
| H1 … H10 | The power header, 2×5, pins down the strips. H1 and H2 are -12 V and take the cable's red stripe; H3 to H8 are ground; H9 and H10 are +12 V |
The panel
8 HP, which is 40.6 mm of a 128.5 mm blank - Amplified Parts sells reversible black-and-aluminum blanks in every width from 2 to 20 HP. The template is drawn for 9 mm Alpha pots (7 mm holes), Thonkiconn jacks (6 mm), sub-mini toggles (6.5 mm) and 5 mm LEDs, with the rack's rails kept clear top and bottom and the four M3 mounting holes where Doepfer puts them. I/O along the bottom, controls above, as everything on this site does it.
↓ lfo-rack-panel.pdf
Nobody has drilled this one yet - the holes are the module's pads, where they sit is the format applied to them.
The rack block, the same on every module
The top right of the board is the same idea as every module on this shelf. The 2×5 header takes the rack's power cable, mounted with its pins running down the strips so each pair of pins lands on one strip: -12 V, then three grounds, then +12 V. That is the only way it can go on stripboard - turned the other way, the -12 and ground pins would share copper. The red stripe on the cable goes to H1 and H2. A 1N5817 stands in series with each rail so a cable put on backwards does nothing instead of everything, and a 10 µF and a 100n sit on each rail. No regulator: there is no CMOS here, and a TL074 is happy on the rails as they come.
The IDC socket on the power cable overhangs the header by about two strips at each end. Nothing tall stands there - the generator that drew this refuses to put anything standing under the socket - so keep it that way if you move a part.
Check the cuts under the chip before the first power-up, with a meter. A DIP on stripboard puts pin 1 and the last pin on one strip, pin 2 and the one before it on the next, and so on down - each pair held apart by nothing but the chip's own cut. On a TL074 that puts pin 4 and pin 11 on the same strip, +12 a whisker of copper from -12. It is the normal shape of every stripboard op-amp build and it is fine when the cut is clean, but a cut that did not quite go through is a dead short across the supply with no resistor anywhere to survive it. Cut them, then measure each one.
How it works, in one breath
An LFO is two op-amps chasing each other: one ramps, the other decides when the ramp has gone far enough and sends it back, and the speed of the ramp is the rate. Everything else on this board is housekeeping - one stage to make the square the same size as the triangle, one to turn the triangle into something rounder. On a filter cutoff the three shapes are three different feelings: the triangle sweeps, the square jumps, the sine breathes.
Things to try
- Change C1. 100nF puts the fast end up near audio; 4.7 µF makes it a tide. It sets the whole range
- An inverted triangle. There is no op-amp left, but a second jack on the raw square through 10K and a 10K to ground gives a second, hotter square for free
- A rate CV. A 100K from a jack into B's summing node beside the pot adds voltage control of the rate - crude, not exponential, but it works
- Feed it back to the stripboard shelf. The triangle into the Dragonfly Rack's 1 V/oct input is a siren; into the Mixer Rack with audio, a tremolo
The core is the two-op-amp triangle-and-square oscillator in every op-amp book; the diode sine shaper is the classic one. The scaling stage and the layout are Onion Madder's, drawn in Copper Bottom, the same evening as the Utility LFO it comes from.