3U 8HP 4 Channel Panning Mixer

July 14, 2019 at 7:01 pm (maker, music) (, , , )

As I mentioned in my last post, I used an off-the-shelf 4 channel mixer board in my synth-in-a-box, but I wanted it to be accessible as a eurorack modular panel.  I also wanted it to take mono inputs and be able to set the panning as required to the L or R channels of the mixer.  I managed to squeeze it into one of my 3U, 8HP panels.

Now I didn’t need an on/off switch, and I wanted some space to add a stereo output jack, so I removed the switch and soldered a couple of links in its place as can be seen in the bottom left of this photo.  The plan was to pass the pots through the panel and use leads to connect sockets to the inputs and output.

2019-07-09 19.17.12

The panning circuit was quite simple.  I found it in the book “Make: Analog Synthesizers” by Ray Wilson from MFOS.  In chapter 7 he describes a simple circuit to allow you to hook up your (mono) sound output to a (stereo) PC sound card. It involves a 10k pot and four 2k resistors, with the wiper of the pot connected to ground.  Full details can be found in the book.

For me, I was planning to just solder the resistors directly onto the pots and sockets and then use a short stereo cable to connect to the input sockets of the mixer.  This is all shown in the following photos (complete with my dodgy machining skills).

The four input sockets are mono of course, with the stereo input signal coming off the resistor network.  The output socket is stereo. I soldered the four resistors for each channel together first then “applied” them to the pot and socket.

Then it was just a case of adding the mixer itself and making a simple power cable from the 16-pin eurorack connector to the DC barrel jack.

I used the four knobs that came with the mixer as the pan-pot knobs, as they were nice and small.  Then I used some generic ebay knobs for the volume controls.

When it came to fixing into the rack, I ended up soldering on an additional stereo lead to the output so it can be routed internally straight to the amp.  So in normal use, the output socket isn’t needed, but I can power off the amp and use the output if I wanted to send the audio off to an external amp.

I’m really pleased with how it came out. Not bad for a $15 board and a handful of components.


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Modular Synth in a Box

July 14, 2019 at 2:30 pm (maker, music) (, , , )

Inspired by Morocco Dave who built a small “almost 5U” modular synth case out of a plastic storage box, I have created one of my own.  My goal was to build something that could take Eurorack modules, so looking at around 3U high modules, so for me the best layout was a dual-rack layout with the box standing vertically as follows.

2019-06-16 15.33.08

The box is a common “9 litre” box, with rough external dimensions of approximately 40x25x15 cm.  Mine came from a local discount store.

I’ve just used a few pieces of wood for the cross-bars and covered them with some of that aluminium tape you can buy for patching up cars.  The measurements are taken from the Eurorack standards and based on the instructions from the Synth-DIY Modular Synth Cabinet Howto from MFOS, gives me around 44HP of module space.  Each module has around 10cm height of usable clearance for electronics and

I created two bus-bars following the 16-pin Eurorack power standard out of stripboard and build and connected up a PSU from Frequency Central (£10 for the PCB).  The whole thing is powered using a 12AC “wallwart” power supply via a barrel jack socket on the side.  I drilled out a grid of holes top and bottom to allow air to circulate.

In addition I created a set of USB sockets hanging off the +5V line from the PSU as some of the modules I’m using will be Arduino and similar based, being to power directly from USB will be really useful.  The PSU is probably not powerful enough for an entire rack full of modules, but the idea is to have a platform that allows experimenting and playing around with designs, so that isn’t a major issue right now.

In terms of power bus cabling, I have a whole pile of old IDE cables so I picked up a bulk set of 16-pin IDC connectors and can now make my own bus cables.  The first one was the connector shown in the first photos, linking the PSU to the two stripboard buses.

I wanted a cheap way to make panels for modules, and in the end opted for a supplier on ebay who provides 2m lengths of 2x40mm wide flat aluminium bars.  This particular supplier also included some basic cutting, so for less than £25 I’ve ended up with a whole pile of approx. 260x40x2mm aluminium panels I can cut further as required.

I just use a wire brush to give a “brushed aluminium” finish.  If you want to follow this path, look up “aluminium flat bar” on ebay, and be warned that a cheap supplier will not be giving you accurate dimensions if cutting them for you!  I know 40mm wide isn’t a standard “HP” module width, but as it is almost 8HP, its fine for me.

One thing I was particularly keen to do was have a complete “synth in a box” and by that I wanted to include some basic amplification and speakers.  I had some speakers from an old CRT TV set that seemed pretty good for their size, so then looked around for means of amplification and mixing.  Again basic modules on ebay solved this for me, and I ended up with a cheap 4-way mixer board ($14) and amplifier ($5).  The mixer is based on a NJM3414 low-voltage, high-current op-amp and the amplifier is based on a TDA7297.  Both can be powered from a 12v supply and the amplifier claims 2x15W output.

I’ve built the mixer into a panel and added some simple panning “front ends” to each input, but I’ll leave details of how I did that for another time.  For now, here is the basic case with built-in stereo speakers and amp.

Being able to just unplug the power and pick the whole thing up is great.

My physical construction skills are not particularly great.  I don’t have the patience to do a really good job, and don’t have the skills, tools or experience for anything approaching any kind of professional finish.  But for a homemade “just for me” project,  I’m really please with the results.




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Codebugs, Glowbugs and DIY Tails

February 20, 2016 at 11:21 pm (computers, kids, maker) (, , , , )

We got a couple of Codebugs just before Christmas and have been having quite a good time playing with them so far.  You write code for them using an online IDE based on Google Blockly and the device itself has two buttons, a 5×5 grid of LEDs and 4 GPIO ports with nice croc-clip friendly contacts on it so you can straight away start playing with linking code to the physical world.   It also has an expansion connector (the ‘Tail’) and when you connect it to a PC via USB it comes up as a mass storage device and you drag code across to it to run it.

We’ve done the obligatory “scroll your name” across the LEDs, hooked up fruit to create a fruit ‘touch’ keyboard and have started exploring some of the projects available via the online community.

My only slight gripe is that the mechanism for getting code on it isn’t quite as intuitive as it might be.  We’ve largely got the hang of holding buttons down whilst powering it up.  although, when trying out code, it tends to stay plugged in, so we use the ‘reset’ then ‘hold’ technique designed for when it is powered by battery most of the time.  However sometimes the buttons aren’t held quite long enough for it to register.  Also, every time it comes up in mass storage mode, the previous programme is erased.  If you are not careful you end up with lots of “compiled” Codebug programmes lying around your browser’s download directory.  Seeing as all projects are managed by the online IDE it would be nice if there was a neater way to send the code straight to the Codebug without copying between directories.

That aside, its a great device and has generated lots of interest with the kids and I’ve recently purchased a set of Glowbugs.  These are WS2812 based RGB LEDs with simple croc-friendly (of course) contacts that you connect to the Codebug using the GPIO, configure and off you go.

For our next project however, we quite wanted to use all 4 GPIO as inputs and I know that the Glowbugs can be driven directly via the expansion header, so I set about seeing if I could create a DIY ‘tail’ connector to breakout the CS, +5v and GND connectors, as used by the Colourtail, to something that would accept croc clips.

I also have a cheap, purchased from China, 24-LED WS2812 pixel ring with the same +5, GND, DIN, DOUT interface as Neopixels and the Glowbugs that I wanted to use.  I’ve already connected this up via the Codebug Tail, but again wanted something croc-clip friendly that the kids could use.

So armed with a 20x80cm prototyping board and some right-angled headers, I set about making a simple DIY tail adaptor and connector for the LED ring.  Warning – massively dodgy soldering coming up.


As a major goal was to make this easy for the kids to use, I wanted the connectors to break out in the same order as the Glowbugs – so (with all boards face down) this means from left to right, GND, DIN, +5v.  However the tail connectors are CS, GND, three not required for this application, and VCC, so I needed a wire link to get CS (for the data) from the left-most pin to a central spot.


With a small portion of board cut and smoothed ready for headers, I decided to use crude solder blobs as a simple way to create croc-friendly pads and connections, as can be seen via the very dodgy soldering going on in the following!  In case you can’t quite make it out, the ‘circuit’ is highlighted in the last pic.


So after adding another three ‘pads’ on the other side, the final thing looks like this.


In order to be able to easily use the LED pixel ring, I decided on a similar approach to add some ‘pads’ to the ring.  Once again I wanted the pads in the same order as a Glowbug.  I could have added both an ‘in’ and ‘out’ connector, but decided for simplicity only to create an ‘in’ – so the ring will always be the last thing in the chain.  In the following, the wires are coloured as follows: green is GND, blue is DIN and red is VCC.


The biggest problem with just connecting the pixel ring directly to the Tail connector was the poor physical connection of the wires to the ring itself, so this time to give it a degree of kid-robustness, I used a hot-glue gun to stick the pads to the ring and protect the solder links.


With a final blob of glue over the top of the solder connections to the ring, everything is ready to go.


So to use the Glowbugs connected via the DIY tail, you have to use the configuration blog to enable the ‘Colourtail’ rather than ‘Glowbug’, but otherwise, everything else is just the same.  And of course, the ring is just treated as a set of additional 24 Glowbugs added on the end of the chain.

So crude, and soldering that will definitely not be winning any prizes, but it works, and passes first contact with the kids.




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