Hello, I am using Behringer UMC204HD as an external audio interface. It has two inputs. I want to connect M:C and TD3 at the same time. Is it possible to connect M:C’s main outputs into one input of the UMC. If it is, which cable I should get? I would appreciate a link of the cable.
I just checked the M:C manual, and the outs are dual TS/TRS.
And Sweetwater says your interface has Two TRS ins.
Welcome to the forum by the way.
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This connects a stereo output to a balanced mono input. You won’t hear mono parts of your signal. 
What you need is the sum of left and right channel. You could achieve this the simplest way with a Y-cable with 1 kOhm resistors soldered in both signal wires. No need to do this unbalanced.
You got it backwards. It sums two mono (unbalanced) outputs, which the M:C is set up to output
To one stereo (balanced) input which goes to his audio interface.
It is the right cable, as I checked the manual.
Number 5
He want’s to connect two stereo devices to his two mono inputs of the interface, if I understood correctly. So he needs to sum the stereo outputs to mono.
Like described here:
http://silentsky.net/wordpress/archives/624
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These are balanced TRS mono inputs.
I have tried to connect it with this cable before Kozmos KCL-317-3M Stereo TRS 6,35 - 2 x Mono TRS 6,35MM 3mt Kablo Fiyatı, Ãzellikleri | Zuhal Müzik | Zuhal Müzik it didn’t work, sound was low and it gave lots of noise. I was thinking to try this cable Hosa 1-4'' TRS (M) Dual 1-4'' TS (M) Insert kablo, 1 mt. STP-201 Fiyatı & Özellikleri which says not mono to stereo but mono to mono.
Thank you 
They are definitely mono inputs
TRS is balanced mono in this case. Read about balanced and unbalanced signalling. XLR/TRS inputs are always mono.
eq:
AS you described, you won’t hear the mono signal part with such a cable. Neither with the cables you posted nor with the one BLKrbbt posted (which are the same). You need to SUM the signal. You cannot buy such a cable.
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so do you think this cable would work? Hosa 1-4'' TRS (M) Dual 1-4'' TS (M) Insert kablo, 1 mt. STP-201 Fiyatı & Özellikleri
No, its the same as the others. If you’re not sure about soldering like in the first link I posted, I think you need to take a simple audio mixer.
You got any recommendations for the mixer ?
This would be the right cable. But you’d have to solder 1 kOhm (or 470 Ohm) resistors into both signal wires.
Any 4 mono input mixer with pan pots will work. There you route both mono parts of each device to either left or right output which is connected to your two mono interface inputs.
I understand now about the inputs being balanced but not stereo.
The documentation I was referencing didn’t have the whole story.
I was wrong.
Maybe it might be time to upgrade to 4 inputs.
Sorry I was so headstrong.
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Yes, in the end 4 mono inputs (or two stereo) in the interface would create more flexibility. Or maybe even more inputs for more synths to come - which would be the best point to me to say: Welcome to the forum. 
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Thank you! I am considering to get a mixer at this point 
And for now I want to use M:C only but not at the same time with TD3. I have two more questions to be sure: To connect M:C main outs to 2 inputs of the external audio interface, I need two 1/4 TS cables? And is it possible to connect M:C main outs to RCA input of a mixer like this
with this cable maybe? Kozmos KCL-329-3M 2 x 6,35mm Mono - 2 x RCA 3mt Kablo Fiyatı, Ãzellikleri | Zuhal Müzik | Zuhal Müzik
This cable should do the trick. Unlike the others that have been posted in this thread, this cable sums a stereo pair carried on a single TRS connection to a mono signal on a TS connection.
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@ark Oh yes, already with built-in resistors. Great (even if not available ATM). You can use the headphone outputs with the stereo plug.
To the vestax DJ mixer:
You can connect and mix your hardware with this item to a stereo output. The question is, how will you get one synth to the left channel (1) and one to the right channel (2) of your interface? You can do this with the panning knob in a 4x mono mixer. Know: balance and panning knobs do not exactly the same thing!