Music and Synth DIY

Effects, Eurorack, Schematics, SMD, Synth DIY, Through hole

Star trails – a gated “effects send” loop for Eurorack (finished)

Say you’ve got a bassline or a lead line, and rather than muddy it up by adding delay to the whole thing, you just want to put some dubby echos on one or two notes.

Think of it like this…this…this…this…

Rather than this…rather than this…rather than this.

Usually, it’s either effect the whole thing or nothing. You can’t just gate the delay, as the delayed notes need to ring out until the feedback is done.

You could use a mult, duplicate the audio, send it to the effects via a VCA with a fast but not instant attack envelope to get rid of the “pop” or “click” as the gate opens, send a gate to the A/S/R envelope generator feeding the VCA, and mix the returning audio and the original audio with a mixer – or you could use this simple 4hp eurorack module to do it for you.

Rear of main board

Star Trails duplicates the audio, sends it through a vactrol out of the module, through and effect of your choosing, and back into the module to be mixed with the original audio.

In a nutshell, it’s a gated send/return loop, inspired by the schematic for Tom Whitwell’s fantastic Vactrol Mix expander for the MTM Turing Machine (which was, according to the MTM site, in turn inspired by Grant Richter’s Cadavre Exquis voltage controlled mixer). Thanks Tom, in the unlikely event you’re reading this šŸ™‚

Jacks & pots board

There’s a manual gate button – plugging in a gate signal (5v high, 0v low but overvoltage tolerant) overrides this. An LED indicates when the gate is open. There’s a switch for the L & R channels to invert the phase of the returned audio.

Finally there’s a blend pot to mix in the dry signal with the wet.

This is a 99% through hole build, with a handful of 0603 caps thrown in to save space on 1u poly caps.

Want to make one or use it as the springboard for something else? The Kicad files & gerbers are here.

If you want a gold panel, use the ENIG process with JLCPCB or whichever fabricator you’re using – but it looks nice enough in silver if you want to save a few quid.

If you build one and run into any issues, or you’re got any feedback, or you just want to send me a pic or video of it out in the wild, drop me an email or comment here. Hopefully a demo video to follow shortly if I get the time.

Creative Commons License
This work is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License
Main board


  1. Hi! Found your site by searching for a MIDI Muso CV-12 eurorack adapter, but you’ve got a lot of great stuff here! This one is a sure-fire build for me, looking forward to the release of the files when you’re ready šŸ™‚

  2. doctea

    ..and, having looked more closely at some of your PCB screenshots, I must say that your work here is definitely worthy of laudation!

  3. Dave

    Iā€™d love to see the design files when you are ready