I am seeking some advice about how to properly configure a Roland SC-55 Mk II to work with DOSBox on my iMac. I have no problem with the software settings - this is purely about the hardware setup.

For background, my current setup is as follows:

1. 2019 iMac – it only has one 3.5mm jack (I do not know if it is dual line-in/line-out) and no separate line-in.
2. Bose speakers with subwoofer – an RCA from each speaker into the subwoofer and stereo 3.5mm interconnect from the subwoofer to the 3.5mm on the iMac.
3. Roland SC-55 Mk II connected to the iMac via a UM-ONE Mk2 USB midi adapter.

My problem is that this setup will only allow me to hear sound from either the Mac (i.e. the digital audio from DOSBox) or the MIDI from the SC-55 (by plugging the 3.5mm interconnect from the Bose subwoofer into either the Mac or the front 3.5mm jack on the SC-55) but not both at the same time.

My goal is to mix the output from the iMac and the SC-55 so that I can get both the digital sound and MIDI to play simultaneously through my Bose speakers with maximum quality, but minimum complexity and cost.

I am not an audiophile, but from reading other posts it sounds like I need a mixer in order to link the sound inputs from the SC-55 and the iMac and then plug the 3.5mm cable from the Bose into the mixer’s audio-out interface.

From looking at different mixers online, I gather that most of them only have dual TS jacks for each input and the output as well. If this is correct, then this seems to be what I need:

1. A small mixer with at least two lines in (dual TS inputs).
2. A dual RCA to dual TS cable to connect the SC-55 to the mixer.
3. A stereo 3.5mm to dual TS cable to connect the Bose speakers to the audio output on the mixer. Since the Bose already comes with the 3.5mm interconnect, perhaps there is an adapter to provide it with dual TS output?
4. A duplicate of (3) to connect the iMac’s audio-out jack to the mixer.

Is this correct?

If so, can someone please recommend the best mixer (preferably less than $150) that will provide good quality low-noise output and be resistant to ground loop interference? I noticed when I plugged my Bose into the SC-55 that the ground loop buzzing noise was very noticeable and only went away when plugging the speakers back into the Mac.

If I am not correct in my findings, can someone please point in me in the right direction? If there are multiple approaches I will be happy to consider them all.