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

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


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:

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:

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


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.