# Zynthian V5.1: Hum Noise from DIN MIDI In Connection

**URL:** <https://discourse.zynthian.org/t/zynthian-v5-1-hum-noise-from-din-midi-in-connection/13784>\
**Category:** Hardware\
**Created:** [October 22, 2026, 9:40am UTC](https://discourse.zynthian.org/t/zynthian-v5-1-hum-noise-from-din-midi-in-connection/13784 "2026-10-22T09:40:00Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![cfausto](https://discourse.zynthian.org/user_avatar/discourse.zynthian.org/cfausto/32/12839_2.png) [@cfausto](https://discourse.zynthian.org/u/cfausto)\
**Post date:** [October 22, 2026, 9:40am UTC](https://discourse.zynthian.org/t/zynthian-v5-1-hum-noise-from-din-midi-in-connection/13784/1 "2026-10-22T09:40:00Z")

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Greetings

I am trying to track down an annoying interference issue that I’ve recently encountered while rewiring my studio setup.

When I connect the DIN MIDI output of my Korg SP-200 keyboard to the DIN MIDI input on my Zynthian V5.1 (modded only with the addition of NVMe), I get intense interference on the main audio output, around 200 Hz (so not 1× or 2× mains, 50-100 Hz).

I’ve already tested different MIDI cables and an alternative (external) 12 V power supply for the Korg, but the noise persists. On the other hand, I tested MIDI coming from other devices, such as my Roland SH-201 synth, and there is no noticeable interference. Also, no noise occurs when connecting the Korg MIDI Out to the DIN MIDI input on my Zynthian V2.

Does this ring a bell for any fellow hardware hackers out there? Any hints, aside from using an oscilloscope (which I don’t have)?

Thanks in advance.

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**Author:** ![riban](https://discourse.zynthian.org/user_avatar/discourse.zynthian.org/riban/32/2331_2.png) [@riban](https://discourse.zynthian.org/u/riban)\
**Post date:** [October 22, 2026, 12:59pm UTC](https://discourse.zynthian.org/t/zynthian-v5-1-hum-noise-from-din-midi-in-connection/13784/2 "2026-10-22T12:59:10Z")

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This sounds like a ground loop. MIDI is designed to avoid these by having an isolated current loop. The zynthian complies with this spec with an optocoupler on its input so the MIDI signal is electrically isolated but… there is also a ground on DIN pin 2 which should not be attached at both ends. The purpose of this ground is to allow the MIDI cable to be screened (many are not) to reduce EMF interference. If pin 2 is connected at both ends you can have a ground (earth) loop which can result in hum. (I won’t go into the detail of that - we don’t need to know the low-level detail to know that it exists!) So I would check for a connection between MIDI connector pin 2 (the top centre pin) and ground (e.g. chassis / audio connector sleeve). This should be lifted at the MIDI IN end (recieve, i.e. zynthian for your example).

Some 5-pin DIN connectors have a screen connection to the outer metal sheath of the plug which can also carry the ground. It looks like the connector on my V5 does not have this but my V3 & V4 do not. My MIDI cable does not connect the sheaths so this would not be an issue for me but it is possible that yours does. Check if there is any connection between any of the pins and the sheath.

Another problem may be the opposite - your installation may be missing a good ground and a previous or different configuration may have provided that over the MIDI cable (which is the wrong solution).

Lots of ideas here but nothing particularly concrete. Check you grounds to ensure it is star and not looped.

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**Author:** ![cfausto](https://discourse.zynthian.org/user_avatar/discourse.zynthian.org/cfausto/32/12839_2.png) [@cfausto](https://discourse.zynthian.org/u/cfausto)\
**Post date:** [October 22, 2026, 1:58pm UTC](https://discourse.zynthian.org/t/zynthian-v5-1-hum-noise-from-din-midi-in-connection/13784/3 "2026-10-22T13:58:25Z")

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Thank you so much for your reply, @riban

> [@riban](#):
>
> So I would check for a connection between MIDI connector pin 2 (the top centre pin) and ground (e.g. chassis / audio connector sleeve). This should be lifted at the MIDI IN end (recieve, i.e. zynthian for your example).

Bingo at 1st attempt: my V5.1 has MIDI IN pin 2 connected to ground (unlike my V2 Zynthian!)  
Since I’d have to reach under main board to fix this, for now I opted to create a custom MIDI cable cutting there the pin 2 connection.

I’m aware of the isolated current loop MIDI specification, but testing with different cables and MIDI controllers fooled me. Also, what I measured roughly didn’t seem mains or harmonics frequency interference.

More lessons learned today. Thank you again for taking the time to help me!