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.

Kevin

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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.

Kevin

 

 

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