Banana to 3.5mm utility box

[beedoola]

I wanted to build one of these for use with a synth that uses Banana connectors to make it work with Eurorack gear. The ones I've seen like what is shown below. The female banana connectors don't have a ground lug so I'm assuming the lone banana jack for the ground just connects to the sleeve of the 3.5mm jacks, right?

[Pixel]

I have no idea what you are doing but it seemed interesting so I googled and found this https://modwiggler.com/forum/viewtopic.php?p=1220668

[Jazz P Bass]

Yeah, I do not see a dedicated ground.
Adding one would help.

[Mick Bailey]

My modular synth uses 4mm banana plugs and all of the grounding is internally connected to the 0v rail so that only the hot connection is required. If using an external connection that uses a regular 1/4" or 3.5mm plug the synth ground may need to be connected to those socket grounds, but it all depends on what is being connected (DC levels, waveforms or signals) and what you're connecting to. Looking at the box pictured I'm in agreement with your suggestion that the single black socket is the ground connection. This may connect to the case or to the individual socket sleeve connections as per Jazz P Bass. I prefer to connect to the sockets - a little additional wiring but more reliable ground connections.