UNVERIFIED. This module is drawn and checked in Copper Bottom - the copper agrees with the netlist below, and the checks come back clean. It is the standard rack mixer, drawn fresh: the Bug Mixer's job, but on a bipolar supply the half of that board that made a half-rail reference is unnecessary, so nothing of it is carried over. 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.

Mixer Rack

Four in, one out, nothing clever · A TL072 on eighteen strips · 8 HP · By Onion Madder · Mess O' Pedals

Every other module on this shelf is a bug board with a rack block bolted on. This one is not: the Bug Mixer ran from a single 9 V and spent half its op-amp holding a half-rail reference to swing around, and a rack has a real ±12 V, so that half is free to do something useful. What is left is the mixer every DIY rack has: four level pots, four 100Ks into a summing op-amp, and a second op-amp to turn the result back the right way up. Eight HP, one channel per row.

What it is

Each input jack's tip goes to its level pot's lug 3, lug 1 to ground, and only the wiper comes to the board - W1 to W4. A 100K from each wiper meets on SUM, the inverting input of op-amp A, which holds it at 0 V and turns the total current into a voltage through another 100K: unity per channel, inverted. Op-amp B, 100K in and 100K back, inverts it again, so what leaves through the 1K is the mix in phase with what came in. No input caps, because a rack signal is already centered on 0 V and DC through a mixer is a feature - it mixes control voltages just as well as audio.

Why it is taller than its chip. A chip pin's strip has two neighbors and both are other pins, so a resistor between two op-amp pins cannot simply stand between them. Every pin the panel needs is extended to a strip of its own by a vertical link - SUM to strip 12, the inverter's input to 14, the two outputs to 15 and 16 - and the parts go there. That is the whole board.

The layout file is the guide

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

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

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 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

An op-amp summer is a see-saw held level: the inverting input never moves, every channel pushes its own current in, and none of them can feel the others - which is the difference between this and four resistors twisted together. The feedback resistor decides how much output voltage that current becomes; 100K over 100K is one to one. The second stage exists only because the first one turns everything upside down, and a mixer that inverts is a nuisance the day you mix a signal with a copy of itself.

Things to try

• Six channels. One more strip pair, a pad, a 100K. The chip does not care
• Gain. 220K for R5 makes it a mixer that can also make things louder; 47K, one that can only make them quieter
• An inverted output too. Op-amp A's output through its own 1K to a second jack gives the mix upside down for free - handy for CV

The circuit is the four-channel inverting mixer in every op-amp application note and every DIY Eurorack mixer since there were DIY Eurorack mixers; the MFOS mini mixer is the same thing. The layout is Onion Madder's, drawn in Copper Bottom.