Hi there, I’m using some old-style MIDI cables to connect some older synths, however I’m finding that when I plug in the MIDI cables it causes a hum through my speakers, which I suspect is due to a ground loop. I know you can sometimes disconnect the screen wire on certain cables and this eliminates the problem, is it possible to do this on a MIDI cable?
SOS Forum post
It’s possible to lift the ground of a MIDI cable to break a ground loop — but it’s probably not the first thing you should try!
SOS Technical Editor Hugh Robjohns replies: The short answer is, yes, you can — but given that your MIDI cables may be revealing a pre-existing ground loop rather than causing a new one, I’d start by taking all the standard precautions to avoid ground loops.
First, make sure that all your connected audio equipment is powered from a single mains wall socket in a sensible way — which means using just one single or double wall outlet and distributing the mains to the equipment via a star-arrangement of plug-boards. Next, consider the audio connections and think about using line-isolating transformers or pseudo-balanced cabling if you need to connect unbalanced equipment with Class-1 (grounded) mains power supplies to other devices. (Any double-insulated or Class-2 mains-powered equipment doesn’t have a mains safety earth connection, and so is unlikely to cause or suffer from ground loops.)
If all of the mains and audio wiring is up to scratch and ground-loop free, then examine the wiring of your specific MIDI cables. Although the MIDI input device uses an opto-receiver to isolate the two data lines (connected between pins 4 and 5 of the DIN plugs), the cable is normally screened and that screen is typically connected to the equipment grounds at both ends via pin 2, which is where the potential for a new ground loop occurs.
Sometimes a sneaky ground loop is formed because the DIN plug’s metal body is connected to the cable screen inside the plugs, so check for that and remove those connections at both ends, if present. If you still have a problem you may finally have to resort to cutting and isolating the cable screen’s connection to pin 2 (the middle pin), but ideally only at the MIDI in end. However, be aware that this solution can disable the power supply return path if the MIDI receiving device also expects to be powered over its MIDI connection — that’s unlikely, but still something to bear in mind.
Published May 2016
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