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 Switchboard, which I have built and heard, with the growl toggle and the starve pot I added on the bench; strips 0 to 13 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.

Switchboard Rack

Four voices, tone and rest, beepy to growl · The Switchboard plus a rack block · 8 HP · By Onion Madder · Mess O' Pedals

The Switchboard is four fixed 40106 oscillators and a 4051 that picks one at a time, walked by a 4040 counter on a fifth gate's clock: tone, rest, tone, rest, with a slow sixth gate swapping which pair of voices plays. On the bench it sounds beepy and clicky, and the growl toggle turns that into a robot and a fuzzy growl. In a rack the same board takes a clock from anything that ticks, and comes out at rack level.

What is the Switchboard, and what is new

Strips 0 to 13 are Switchboard.json hole for hole, growl pads and all; every net of the Switchboard is re-derived from the new copper before this page is written. Off came D1 and the +9V pad, the old reverse-polarity diode, because the header brings its own two, and one link, J22, which carried the clock down to the 4040. The rack block sits on ten new strips under the board rather than beside it, because the board is already 38 holes long and a module's board has to fit behind the panel between the rails.

Clock in. J22 is now a normalled jack, the Spider Rack's way. The 40106's clock leaves on the CLK_N pad to the jack's switch contact, and the jack's tip comes back on CLK_T through 100K (R8) to the 4040. With nothing plugged in, the jack's own switch joins the two and the rate knob runs the steps as before; plug in a rack clock or gate and it takes over, the 100K limiting what the chip's input diodes see. The growl toggle still works on the board's own clock line, whichever clock is counting.

The starve. The 78L05's output does not go straight to the chips: it leaves on the +5V pad to the starve pot, and comes back on VCC. So the module needs that pot, or a wire from +5V to VCC in its place, to make any sound at all. I chose 20K for it at 9 V; at 5 V the chips will give out sooner in the turn, and the bench will say whether 20K is still right.

Level. C7 and R7 stay, so the level pot is wired exactly as on the box, but its wiper now goes through 47K into the TL072 with 100K feedback, a gain of about two. The mux puts out a 0 to 5 V square, which after C7 is about ±2.5 V, so the top of the knob is about ±5 V.

The layout file is the guide

As on every stripboard guide here, the build document is the file itself: ↓ Download Switchboard_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

25 pads leave the board. Ten of them are the power header, and two are test points or carry nothing off the board.

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.

The rack block, the same on every module

The bottom ten strips of this board are the rack block every module on this shelf carries, drawn here under the board instead of beside it. 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.

Check the cuts under the chips 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 an op-amp that puts pin 4 and pin 5 on the same strip, which here means the negative rail a whisker of copper away from ground. 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 - a track cutter leaves a hair at the edge of the hole more often than it looks.

How it works, in one breath

Four gates of the 40106 are four oscillators on fixed resistors and 47n caps, four pitches. A fifth gate is the clock, on the rate pot. The 4040 counts it, and two of its outputs drive two of the 4051's address pins, so the mux steps through four of its inputs: two are voices and two are ground, which is the tone-rest pattern. The sixth gate is a slow drift on the third address pin, swapping which two voices take the tone steps. The growl toggle joins the drift oscillator's timing node to the clock line, or straight to its own output, and that is where the robot and the growl come from. In the rack all of that runs at 5 V through the starve pot, and the TL072 doubles the mux's output to rack level.

Things to try

• Clock it from the Spider. The Spider Rack's CLK OUT into CLK IN and the two step together; its CV OUT does nothing here
• Audio-rate clock. Patch an oscillator into CLK IN and the steps blur into a tone of their own
• A deeper starve. A bigger pot in place of the 20K reaches further toward the point where the chips give out. Untested; the 20K is the one I have heard, at 9 V

The Switchboard is the layout of that name from this site: a 40106, a 4040 and a 4051 in the Lunetta way, with the growl and the starve from my bench build. The rack block is the standard DIY Eurorack power entry and an inverting op-amp stage. Drawn in Copper Bottom.