The MIDIThru. An Open Source MIDI Hub.

  • Based on open source circuit designs from Peter Kvitek
  • 5VDC operation (with 9V centre pos adapter)
  • Provides 5 copies of the MIDI input source

A few builds ago, I introduced you to the CVPal, a utility to convert USB MIDI signals to CV and Gate signals for older analog gear. The MIDIThru is another utility, that can be thought of as a MIDI splitter. But it is more accurate, to refer to it as a MIDI signal duplicator.

A MIDI duplicator does exactly that, it takes the MIDI messages from the source and duplicates it 5 times, and sends it out 5 MIDI ports, simultaneously. The main purpose of the MIDIThru is to allow you to connect 5 hardware synthesizers or devices, to a single controller or computer. Now you may ask yourself, why would I want to do that? There are three major reasons why:

1) Propagation delay. Most synthesizers have a MIDI input, and MIDI output and possibly a MIDI thru. Most synths can also make a duplicate copy of the midi input, and send it out the output or thru ports. It is quite possible to connect the midi controller to the first synth’s input, then take a midi cable from the output or thru channel on the first synth, and connect it to the input of the second synth, then take another midi cable from the output or thru channel on the second synth, and connect it to the input of the third synth… and so on, and on.

The problem is, each synth may implement the duplication of the midi signal differently. Some implementations and components run at different speeds, and can cause very slight delays. As the MIDI signal flows from the first synth through the second and through the third, a noticeable delay between then the first synth plays a note and the third or fourth synth plays, occurs. This delay is called a “propagation delay”, as the delay is induced as the signal propagates though the entire chain. The longer the chain, the worse the effect. In studios, or large home or stage setups, you want an easy way to use your controller or keyboard or Ableton laptop with multiple synthesizers, so you can switch between them, without having to replug all the stuff in an out constantly. Alternatively, you make want to share a common clock signal across 5 synths, so they can all be at the same tempo.

A good midi hub uses components to ensure speed, and makes 5 copies simultaneously. This reduces any chance of propagation delay in the system.


2) Electrical Isolation.  Plugging analog synthesizers, which usually have higher voltages and much larger currents, can cause severe damage to digital equipment (which runs at lower voltages and currents), if grounding and power supplies are not working as intended. You friend maybe using an expensive vintage synth, but when you connect it to your laptop magic blue smoke appears and your laptop is no longer functioning properly. Most MIDI implementations use devices called ‘Opto-isolators”, which electrically isolate one midi connection, from the hub power supply, and output channels. Having said that, you can’t guarantee nobody messed with that vintage synth, or if it even used opts-isolation in it’s design. I have seen some implementations where the MIDI was sent directly out of the processor, and down the channel………. Pfffffffft! Magic Blue Smoke. The mid hub ensures everything is isolated, from everything else. Which brings us to number 3…


3) Noise Reduction. Digital power supplies are inherently noisy!!! Many PCs, digital hardware synths and controllers use switching power supplies, which can be heard in the output amplifiers, mixers and recording if grounding is faulty or if there is a ground loop somewhere in the mix. Your synth may sound awesome with your controller, until you connect it to a USB or midi cable and the the PC. while it is also hooked up to the mixer. You get a hum sound, oscillating at 60Htz, or weird screeches, ringing… all sorts of noise. My laptop and workstation cause a buzzing in my mix, every time the hard drive does a seek thingy. It is annoying as hell. I always use isolation techniques first and foremost. I always use mid hubs of some type, and I never get noise induced from ground loops… now, bad cables on the other hand…..

The build

I downloaded all the files from Peter’s MidiSizer site, and sent the files to Osh Park to make me 3 printed circuit boards. You on the other hand, can order one PCB directly from Peter’s site for $15 US. The board will be much better quality too. This design is awesome, all parts can be purchased for under $20. The build is very simple, and actually makes a great project for those starting in DIY synth building.

Like the CVPal, I just needed a small plastic box to place everything in, so I repurposed an old home network router box, and found a 9V adapter in a garage sale (I buy tons of these).