roundabout

roundabout is a small patchable synthesizer I’ve been working on.

My goal was to make something small, cheap to produce, portable, self-contained (no need for a mixer or other equipment - just headphones) and that’s fun and exploratory. Something you can just play around with on the couch.

It draws inspiration from other strange standalone boxes from Rob Hordijk, Lorre-Mill and Ciat-Lonbarde and the great series of “Logic Noise” articles by Elliot Williams on Hackaday.

Here’s a noisy “rungler” style patch.

Tech details

  • uses jumper wires and pin headers for patching
  • CMOS-based circuitry
  • 3 oscillators with sync and weighted CV inputs
  • 2 goofy state-variable filters
  • 2 frequency dividers
  • 2 4-bit shift registers
  • 4-bit R2R DAC
  • 2 mults and an extra buffer
  • left and right mixers with weighted inputs
  • stereo output suitable for headphones
  • powered by a 9v battery (rechargeable NiMH recommended)

Update: I’ve put the schematic, BOM and gerber files up on Github, so anyone with DIY skills can produce boards and build one. I also started a rough “manual” with some more details.

If anyone does build one, I’d be more than happy to help with any issues or receive any feedback you might have.

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Woah! This looks awesome. Seems pretty easy to bananify in order to use with CL paper circuits too!

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Really impressive! Love the sounds it makes in the video. I don’t think my soldering skills are up to par yet, but building this seems like a nice goal to work towards.

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Thanks!

Using banana jacks was something I considered at the start, but I ended up finding some creative ways of using the pin headers and jumper cables, which were smaller and cheaper.

For example, instead of “modulation amount” potentiometers for CV to the oscillators, you patch into one of the 4 CV inputs that give increasing amounts of modulation. The output mixer works the same way. (Tom Whitwell’s Startup use the same idea I think.) I even replaced an oscillator “range” switch with patch points.

You can see the CV inputs with the white triangle (showing increasing modulation) here:

I don’t own any CL stuff (unfortunately) so don’t know how well it would play along. You couldn’t easy replace all the pin headers with bananas without redoing the board and adding a ton of jacks (and space) or doing some kind of breakout board.

It could be fun to do something based on this but way stripped down which uses bananas. I did make a bunch of banana cables for cheap when I started this project and don’t have any gear to use them on.

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Thank you! I’m glad you’re liking how it sounds and I’m going to try and put up some more videos.

It’s pretty DIY friendly as in it’s all through-hole components and pretty common/cheap parts. But there is quite a bit of soldering. Here’s a pic of the back of the board.

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Now that I’ve seen the belly, it looks easier to solder than I thought!

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I would have a crack at that for sure - looks like a really fun little device.

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That is a very clever solution to the CV amount inputs! nice sounds, and good job on the filter (often a challenge w/ 9v singleside designs). this looks like a very fun critter, i’ll look forward to the schematics.

Bananification could still be done by leaving off the pin headers and tacking the leads from the bottom of the board and around to the jacks, mounted separately. besides interconnects to other gear, this would also create a little more space for patching, and allow easier access to the controls.

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Thanks @fluxmonkey. I’ve learned a lot from your work! I still don’t really know what I’m doing but fumbling my way through things. I tried to make this as simple as possible, while still being able to get a good variety of sounds out of it.

The filter in pretty strange and hacky- there’s only a resonance pot and you patch into both stages to control the cutoff. So it’s kinda hard to use like a regular filter. And you’re right about the bananification!

Here’s another video showing off a few more patches.

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That second patch in particular sounds so good! What a fun little noise maker @mattkuebrich

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I’ve updated my initial post with the schematic, BOM and gerbers. If anyone decides to build one, please let me know if anything seems missing or unclear.

The PCB is purposefully 100mm x 100mm, so you can get them printed with “prototype pricing” at JLCPCB for like $2 for 5 of them. Shipping is where the cost is. For my last order, the DHL shipping was ~$20 and took 12 days total to get to me (in Chicago, IL).

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Wow great! I’m really exited by this. I’ve actually have the Logic Noise PCB in my backlog but this design appeals to me much more.
I’ve ordered 5 and I’m in the Netherlands if anyone near me wants one send me a PM.

What would the difference be to build it with the 78L05?

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Awesome, I’m excited to have you try it!

I was torn between including the 5V regulator or not, but ultimately think it should be there. It helps with battery life and limits the volume from blowing our your eardrums with raw waveforms. I’d say the only reason not to use it would be if you’re using headphones with it directly and are feeling it’s too quiet.

Edit: I should also mention that the output levels run pretty hot. When using the 5V regulator, the output can get reach up to 4.2V p2p. Without it - around 6V p2p.

The Norns inputs, for example, have a max limit of 5V p2p (as far as I know). Something to keep in mind.

Ordered some boards for this - my first time ordering PCBs! Now to gather components :slight_smile:
Forgive the ignorant question (i’ve only previously built kits with all the right parts supplied, hah): is there a particular wattage requirement for those resistors etc? 250mW?

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Oh nice, thank you! For general synth DIY projects 1/4W resistors work just fine. I get them cheap at Tayda. They have an easy ordering page, so you don’t have to search each value individually. Please let me know if you have any other questions and refer to my Github page, as I keep updating it with more info.

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excited to get mine built up soon!
So many projects :smiley: :smiley:

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made one!

For something with so many little parts this was not so hard, thanks to the clear layout and instructions.

a few thoughts on the build which might be useful for people as inexperienced as me:

  • it’s not super easy to see which way to orient the LEDs on the board. on the schematic it’s pretty clear that the ‘flat’/short leg side goes northwards, but it doesn’t translate too well to the pcb print, just something to watch out for
  • try and get the three caps running down the centre nice and straight cos they’re pretty tight up against the chips
  • i accidentally bought whopping great 470uf caps - will pay more attention to component dimensions in future!
  • also bought a volume pot which was way too long, whoops
  • this might just be my lack of experience but i’m not sure how to interpret the ‘designator’ on the BOM in terms of where things go on the board. I’d like to experiment with some of the alt options eg. putting 68nf in C2 and C3 rather than 100nf but not sure how to identify which ones are 2 & 3?

I’m guessing it would be fine to use a battery clip adaptor to power from something like a Ripcord rather than a battery?

going to make another one and see how they play together! :slight_smile:

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Wow, looks great! Hope you’re having fun with it. Thank you so much for all the helpful notes.

I just made a note in the BOM about the LED orientation. If I do another rev, I’ll make that more clear on the PCB. I also added a note about the power filtering cap (470uF). It should be the smaller 16V ones.

Ah yeah the designators aren’t really helpful since I didn’t put them on the PCB. I just updated Github with an interactive BOM. You can search by designator with the “Ref lookup” search field. Or click on individual components (if you have the “ungrouped BOM” button selected) and see individual component info.

C2 and C3 (100nf on the PCB) are the caps for Filter 1.
C6 and C7 (47nf on the PCB) are the caps for Filter 2.

They are the caps on the top-right corner of the board. If you’re building another board, definitely play around with them to get some variety.

I haven’t tried powering it from something like a Ripcord, but if it’s outputting 9V, I assume it should work. Let me know if you try. I’d be interested if there’s any additional noise, although the power filtering cap should take care of that.

It looks like you are using the 5V regular. I put some thoughts about pros/cons of using the regulator here. It’s worth trying it both ways.

Don’t forget the standoffs to bring the little guy off the ground. :slight_smile: Please let me know if you have any more questions or observations as you keep exploring it.

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The interactive BOM is brilliant! thanks, that’s a big help.
definitely going to try a few variations including one without the regulator. might as well, i had to buy 5 PCBs :sweat_smile:


update-

just three friends hanging out.

3rd version: decided to get a bit experimental; cut a 4-way header down to 3 and soldered it in where the 5v regulator goes, now i can choose between jamming the regulator in or bridging with a wire for full power mode! Took a similar approach to the spots for the filter capacitors too, to make it easier to try out different values. Definitely has a bit more ‘heft’ to the sound at 9v - i’m going to test out powering it directly with a crazy chain and battery snaps, if that works without issue maybe convert them all to 9v permanently.

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Whoa, this is so awesome to see!! I haven’t even built 3 of them yet. I’d be interested to hear how well they all play together. I actually ended up doing the same thing with the regulator on my build.

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