UNVERIFIED. This module is drawn and checked in Copper Bottom - the copper agrees with the netlist below, and the checks come back clean. The board it grew from is the Grub Divider, which I have built and heard; rows 0 to 17, columns 0 to 19 of this layout are that board hole for hole. The rack block has not been built. Every number on this page is a datasheet number, not a measurement. Build it once, fix whatever fought back, and this banner comes off.

Grub Rack

Seven octaves under anything you patch in · The Grub Divider plus a rack block · 10 HP · By Onion Madder · Mess O' Pedals

The Grub Divider makes no sound of its own: give it a square wave and a CD4024 halves it seven times, and seven toggles choose which of those octaves-down get mixed back out. In a rack that is a sub-oscillator with a clock input - patch the Dragonfly, the Hive, anything square, into CLK IN, and the Grub puts the floor under it. The same board, a jack where a wire was, and a level knob into an op-amp instead of into a cable.

What is the Grub, and what is new

Rows 0 to 17, columns 0 to 19 are Grub_Divider.json hole for hole; every net of the Grub is re-derived from the new copper before this page is written. Off came C2 and R10, the old output cap and its 1K, and the output jack pads. The seven weighted resistors, the seven SWn_A pads, the BUS pad and the volume pot's three pads all stay - the pot is now the level control, its wiper going through 1 µF and 47K into the TL072 with 100K feedback, a gain of about two, which takes the divider's biggest swing to about ±5 V.

Clock in. The CLK_IN pad still takes any 0-to-5 V square straight in, as the guide says. The new CLK jack reaches the same node through 100K (R11), which with the chip's own input diodes clamps a ±5 or a 10 V rack signal to the 5 V rail at well under a tenth of a milliamp; R8's 100K pull-down keeps the input quiet with nothing plugged. One link inside the Grub's own area does the carrying: J3, from (4,4) to (3,4), puts the clock node on strip 3, which was empty end to end and now runs into the block. The reset pad MR_PAD is unchanged - a wire to any SWn_A for odd divisions and looping, exactly as before.

The layout file is the guide

As on every stripboard guide here, the build document is the file itself: ↓ Download Grub_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. The bug board's nets keep the names they have on its own page.

The off-board wiring

28 pads leave the board. Ten of them are the power header.

The panel

10 HP, which is 50.8 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.

The rack block, the same on every module

The right-hand end of every board on this shelf is the same idea drawn six times. 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. A 78L05 then makes a 5 V rail for the chip, and a TL072 on the real ±12 V does the rack-level work.

Five volts for the CMOS, not twelve, is what makes these rack-polite for free. A CD4000-series input switches near half its supply, so a +5 V gate from a sequencer clocks a 5 V chip directly but would never reach the ~7 V a 12 V chip wants; a pot across 5 V makes exactly the 0 to 5 V control voltage every rack sequencer speaks; and a hot or negative rack signal into a CMOS input needs only a 100K in series, the chip's own input diodes doing the clamping at well under a tenth of a milliamp. It is also a change from the 9 V these boards were built at - every chip is rated down to 3 V so the circuits are fine, but the oscillators will sit at slightly different pitches, and that is one of the things the bench is for.

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.

How it works, in one breath

The 4024 is seven flip-flops in a row, each halving what it is fed, so its seven outputs are the clock at one, two, four, eight, sixteen, thirty-two and sixty-four times slower - an octave apart each. The Grub mixes them through resistors that weight the deep ones heavier, into the level pot. That mix used to go through a cap and a 1K to a jack; now the pot's wiper goes through a cap into an inverting stage that doubles it, which is what a 0-to-4.5 V swing needs to become a rack's ±5. The chip's clock input is a Schmitt trigger, so a slow or ugly square still clocks it cleanly.

Things to try

• A reset jack. MR_PAD through its own 100K to a jack would let a rack gate reset the count. Not drawn - one resistor and a pad's worth of work
• Seven outputs. Each SWn_A pad is one octave; a jack on each, before the toggle, makes the Grub a seven-way sub-octave source at 5 V logic level
• Louder or softer. The 100K feedback resistor is the gain: 47K for unity, 220K for a lot

The Grub is the Grub Divider layout from this site, a CD4024 with weighted outputs. The rack block is the standard DIY Eurorack power entry and an inverting op-amp stage. Drawn in Copper Bottom.