Balancer and De-Balancer on a TRS Jack to upgrade unsymmetrical gear

[Janus Audio]

If you ground lift the balanced output from your mod boards from a synth this may solve the problem - but not necessarily always! Decoupling MIDI via optical seems to be the only cure in some cases - I have had this issue with synths that have balanced output, lifted the grounds and still got noise from the USB MIDI connection - (may have been on a Kurzweil PC3X IIRC). Using normal MIDI no issues. If the ground lift works for your synths you could always implement a ground lift DIP switch on the Jack board.

I already have a solution for that:
The shield connections (sleeve of the jack) on my boards is connected via a 100nF cap to the ground (basically a low cut). So I don't get any ground hum with my boards anymore. And I also have a solder jumper to bypass the 100nF in case I want I hard ground connections for the line cable shield.

The ground hum from the MIDI over USB I only have on the synthesizers that aren't modded yet.

 

[RoadrunnerOZ]

I already have a solution for that:
The shield connections (sleeve of the jack) on my boards is connected via a 100nF cap to the ground (basically a low cut). So I don't get any ground hum with my boards anymore. And I also have a solder jumper to bypass the 100nF in case I want I hard ground connections for the line cable shield.

The ground hum from the MIDI over USB I only have on the synthesizers that aren't modded yet.

Any MIDI HF noise from controller movements etc?

 

[RoadrunnerOZ]

There used to be an issue in the early 2000’s with some Yamaha keyboards and synth MIDI modules like the Kurzweil KME1 Micro Ensemble with grounding as well - if pin 1 was connected on the MIDI cable the module would not receive MIDI and simply not work - snip pin 1 at one end and all worked fine, I think there were some Roland modules with the same issue as well - seemed to be when the keyboard and module were both running off a wall wart power supply.

 

[Janus Audio]

Any MIDI HF noise from controller movements etc?

I never had that in any of my devices. Sounds like it is not a problem with the unbalanced connection, but crosstalk from the digital MIDI signal into the audio path. This can have many reasons like poor separation of digital and audio ground for example; or not enough power decoupling.
But I am pretty sure it's nothing you can solve with a balanced line connection at the output of the device.

 

[RoadrunnerOZ]

It could be from ground differences between two unearthed external power supplies each providing its own ground - leakage differential of HF noise from switch mode supplies?? It’s there without the USB MIDI when there are only 2 devices (audio interface/laptop) but gets worse when the USB cable connects two devices grounds via the USB cable (synth-keyboard/laptop) in a 3 device system with a USB audio interface already having ground to the laptop, coming via the audio interface analog inputs then USB ground.
It happens with just an audio interface and laptop both with external power supplies - disconnect the laptop supply and no noise.