1U GT2: Build Document
The Foxfield Instruments GT2 is a 1U Dual Gate-to-Trigger converter. Given a gate at an input, it will spit out a very brief trigger voltage from the relevant output. The first input is normalised to the second, meaning that a gate at the left input will emit a trigger from both left and right outputs.
GT2 is based on a design by Ken Stone; see Credit for detail.
Like all Foxfield Instruments' tiles, the GT2 is specifically designed for DIY assembly.
The GT2 is designed using surface-mount components (“SMD/SMT”). It all may look small, but with a little patience you’ll be fine. That said, we definitely do not recommend this as your first DIY kit. You should be reasonably confident with a soldering iron before moving onto SMD kits. If you’ve built a few kits and know your way around a soldering iron, though, you should be fine.
FOXFIELD TILES ARE NOT COMPATIBLE WITH INTELLIJEL 1U SYSTEMS. Apologies. We write this in bold quite a lot. There’s more in a footnote if you’re interested why.
Table of Contents
- Specification
- What’s in your kit
- What you’ll need to build your kit
- Before you begin
- How to solder SMD components
- Building your kit
- Connecting your tile to power
- Usage ideas
- Credit
- Appendix
Specification
- Format: “Pulp Logic 1U” - NOT COMPATIBLE WITH INTELLIJEL 1U SYSTEMS
- Power connector: Pulp Logic “Tile Tail”.
- Width: 6HP
- Power consumption: TBD
What’s in your kit
Inside the paper bag are some small plastic bags. To avoid confusion around resistor and capacitor values, we’ve separated out components that might otherwise be confusing. If there are identical looking components, count the number on a strip; if they have markings, read those markings. We separate things to avoid confusion, but go slowly and take your time.
We would not recommend tipping all your bags out into the same space; you will get confused. Remove components as you need them. Similarly, do not remove components from their tape until you’re ready to attach them - the components are very small.
| Quantity | Part | Marked/Description | Location |
|---|---|---|---|
| 5 | 100k resistor | `1003` | Bag A |
| 2 | 10n capacitor | yellow 1206 part, no numbering | Bag A |
| 1 | 100n capacitor | yellow 1206 part, no numbering | Bag B |
| 1 | 10u aluminium capacitor | Small metal can | Bag A |
| 1 | Fuse | 1206 part with a letter (`B`) | Bag A |
| 1 | 15k resistor | `1502` | Bag B |
| 4 | 1k resistor | `1001` | Bag B |
| 3 | Diodes | small black block with metal legs | Bag A |
| 2 | 47k resistor | `4702` | Bag B |
| 1 | LM358 op amp | Black chip with eight legs | Bag A |
| 4 | 3.5mm mono jacks | Including nuts | Mechanical Bag |
| 2 | 3mm LED | Mechanical Bag | |
| 1 | PCB | Main bag | |
| 1 | Panel | Main bag | |
| 4 | M3 screws | Mechanical Bag | |
| 1 | Tile tail | Paper bag |
What you’ll need to build your kit
Foxfield Instruments kits may use surface-mount components, but they’re designed to be assembled by humans with hand-tools.
We list the tools we recommend that you’ll need on our DIY page.
Before you begin
- Read through these instructions to get an idea of where you’re going.
- Dig out all the tools and equpiment you’ll need above. Having things ready is better than diving into doors with a busy workbench.
- Clear your workspace.
- Get your soldering iron up to temperature.
- Whilst you’re doing that, make a cup of tea. Don’t keep it too near the workbench, but tea always makes DIY go easy.
- OK, let’s begin!
How to solder SMD components
If this is your first SMD kit, you might want to go over our up-to-date guide to soldering SMD components. We’ve collated some videos and guides from around the web, which should prove instructive.
Building your kit
Throughout this build document, we’ll refer to the front and back of the PCB. The front is where the jacks and pot go: it’s got the outlines of them on it. The back is where the majority of the components are, and where the Foxfield logo is.
We’re going to start with the back of the board.
1. IC (LM358 op-amp)
Solder the op-amp first.
Much like through-hole, we solder SMD components in height order - lowest to highest. However, with our tiles, we make one adjustment to that build order: solder the IC first. It’s the hardest thing to solder, and it’s in the middle of the board - if we put it on now, we’ll have the most room to work.
The IC must be fitted in the correct direction. There is a clue to its directionality: the angled edge on one side of the chip. This matches the thick line that intersects the pads facing the upper edge of the board (towards the power connector). The slanted edge shouild match this.
2. Bottom Passives and Diodes
Solder the 1206 passive components into the correct places
Now you’ve got the IC in, attach all the 1206-sized resistors and capacitors, along with the two diodes on the bottom.
The resistors' have their resistance marked on the board, and the build guide will confirm what the marking on top of the component translates to.
The three 1206 capacitors are not marked with their values; be careful not to confuse the two 10n resistors with the 100n resistor.
The resistors and capacitors can be mounted either way around - back-to-front, upside-down, it doesn’t matter - although we’d strongly recommend keeping the resistor values visible just in case you have to debug.
The diodes are marked with a triangle bewteen the pads. Diodes must be fitted in the correct orientation - they will not work other wise. If you take the diodes and look at them in good light - or shine a cellphone torch on them - you’ll see a line printed on the top, towards one end. This should go towards the pointed/narrow end of the triangle silked on the board. Imagine the line ‘completing’ the Diode symbol:
3. Topside: flat components
Solder the SMD components onto the top of the board
Flip the PCB over; we’re going to build up the top (the side with the jacks on it). Make sure you have a way to firmly hold the PCB steady that isn’t your hands - a PCB vice is really helpful, although a cutting mat would do.
Let’s do the flatter components first.
First, the fuse. This is marked F on the PCB and easily fitted like any other passive. It does not have a correct direction.
Next, the diode. Again, this must be fitted in the correct orientation. See above for details of how.
4. Topside: 10u capacitor
Solder the capacitor onto the top of the board.
Now we can fit the 10u capacitor in the middle of the board. This must be fitted in the correct direction. Fortunately, that direction is outlined for us on the board: the base is asymmetric, and the black line on the top of the cap should match the black line on the board. They might seem a bit fiddly to solder on, but trust in the flux and you’ll be fine.
5. Topside: through-hole components
Solder all the through-hole components, being careful to align them with the board.
Now we’ll fit the jacks and LEDs. This will probably be familiar from many other through-hole builds, but as a reminder:
- remove the nuts from the jacks.
- put the LEDs in their marked hole and do not solder them. The flat edge of the LEDs are marked on the board (the flat edge is on the inside of the board for both LEDs).
- Place the jacks into their marked positions and do not solder them.
- Take the front panel of the board and mount it onto the jacks. It should fit neatly, becoming parallel to the PCB, and pushing down until it’s at the top of the jack bodies. When you’re satisfied with its positioning, screw the jack nuts to hold them to the front.
- Confirming that the components are still flush to the board, flip the PCB over and solder the jacks in place.
- Now manoeuvre the LEDs so they sits in their hole correctly. Solder them in place and trim their legs.
6. Tile tail
Fit the power connector.
Finally, we need to fit the power connector. To do this: remove the front panel.
Separate the three wires at the end of the tail and strip them with your wire strippers.
Next, tidy and tin the ends: twist stranded core of each wire together and tin it with your soldering iron. Do not think you can get away without tinning wires. It will make your life way easier.
Insert the tile tail into the board from the underside. Tile tail power is configured as follows:
- black - GROUND
- red - +12V
- white - -12V
These are marked on the rear of the PCB - the ground is the square pad at the top corner of the board. Attach the wires so that they go through from the base, and you solder it on the top-side of the board.
Trim the soldered ends, and re-mount the panel.
7. That’s it!
You’re done. You’re ready to power up your tile and test it.
Connecting your tile to power
At Foxfield, we’d always recommend powering up a new DIY build as the only thing connected to a power supply.
If you’ve got a bench supply or testing supply, that’d be ideal (you’ll need a dual-supply +/-12V for the LFO tile).
Of course, you don’t have to - and many people don’t - but we’d hate to see your modular come to grief through DIY errors.
Tile-tails are keyed; you can’t power them up the wrong way. Just connect the connectors so they’re firmly linked, and coloured cables match up.
When turned on, you wont see anything until you insert a signal into a GATE input - for instance a square wave from an LFO. With a signal in the left GATE, you should see trigger pulses from the left TRIGGER output; if nothing is connected to the right GATE, you will also see pulses from the right TRIGGER output. With a signal in the right GATE, you will only see pulses from the right output. The LEDs reflect the state of the output.
Enjoy your new tile!
Usage ideas
- Turn a long gate from a sequencer, MIDI converter, or other note source into a short pulse, ideal for plucking Low Pass Gates or striking other percussive sound sources.
- Note that you’ll need a hard-edge signal to trigger the GT2: a gate, or square-wave oscillator/LFO should work, but a triangle wave or sine wave is unlikely to be able to trigger it.
Credit
The Foxfield GT2 is based on Ken Stone’s original CGS24 circuit. Permission has been given to re-implement this circuit in the 1U format.
Appendix
“Why don’t you support Intellijel 1U?”
Space, and personal design choices.
Intellijel 1U doesn’t just have a smaller frontpanel: it also has a smaller space between the rails for a PCB. We didn’t feel we could fit enough components onto our parallel PCB that would fit comfortably into Intellijel’s format; we decided against multi-stacked boards. The Foxfield 1U series fit what they can onto a single PCB.
Currently, pretty much all 1U DIY seems to be focused on the Pulp Logic (and Erthenvar/Synthrotek) format, so we thought we’d stick to that.
There’s also a philosophical distinction: 1u “tiles” focus on small, single-purpose, 6HP or 12HP objects; Intellijel’s 1U modules focus on wider, sometimes multi-functional units.
We appreciate this is a shame given the likely popularity of Intellijel 1U; we also suspect that Intellijel will be making a handsome and useful range of their own 1U modules, that many purchasers will be entirely satisfied with.
Needless to say: we may revisit this distinction in the future.
“What’s the different between Intellijel 1U and Pulp Logic 1U?”
The difference is: how you measure the 1U.
Pulp Logic style tiles (such as Foxfield tiles) are designed for use with lipless rails, and measure 1U from the top of the top rail to the bottom of the bottom rail.
Intellijel 1U modules are designed for lipped rails, and measure 1U from the top of the top lip to the bottom of the bottom lip. As such, the module panel, and gap between the rails, is smaller than Pulp Logic.
In addition: Intellijel 1U modules use standard 10-pin Eurorack power connectors. Pulp Logic uses “tile tail” connectors.
Eurorack doesn’t really have any standards; it has conventions. This is a case where two conventions emerged separately.