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I have an old synth that has single headphones output (but user manual states that it can be connected with line-level inputs). I also have an audio interface that has one combo Mic/Line mono input and one Hi-Z instrumental mono input.

Suppose that there is a Y-cable with single stereo jack on one side and two separate mono jacks on the other side. If I plug stereo end of the cable to the headphones output and plug one of the mono ends to line input of the audio interface while keeping the other branch of Y-cable unplugged, is it safe? By safety, I mean absence of damage to both the synth and the audio interface. For example, I found an information that using stereo cable for connecting stereo output to mono input may result in back-feeding and destroy of stereo output. And what about signal quality? Can a signal be distorted or corrupted with noise by this way of connecting things?

As an alternative, I also can plug the other branch of Y-cable to instrumental input. I guess that gain of this input must be set to very small level. If doing so, is it safe? Is it better or not in comparison with the previous idea?

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It would be interesting to know your source about the back-feed. A Google search shows me http://silentsky.net/wordpress/archives/624 where a back-feed happens when we use of an Y as a mixer. If the impedance of both outputs are too low, the current will have a too high intensity.

Here, the idea is completely different : you connect a channel of the headphones output to a line input of your audio interface, and you connect the other channel nowhere. There are no back-feed issue.

The main risk is that the unconnected channel act as an antenna. I don’t think it can destroy the associated output. The unconnected jack may also hit a ground which is not good… you have to be careful about it.

Note : there are two types of Y : splitters stereo-to-2xmono (ok here), and stereo to 2xstereo (typically to use 2 headphones when a single socket is available). They are not build equally. Edit : There is a third cable HOSA CMP-105 cable mono to stereo cited be the linked article which connect right and left which can damage systems if left/right outputs are shunted. I had never seen such a cable before. I guess the right use is to connect a mono output to a stereo input, and not the reverse.

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  • Thanks for a detailed reply! As for back-feeding, I read about it there: pianodreamers.com/digital-piano-recording-guide in the section "Recording with a Headphone Output". Backfeeding is mentioned there not in context of mixing mono signals, but in context of connecting stereo output to mono input. Oct 30, 2021 at 18:29
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    It is hard to believe the expression « stereo (balanced) cable ». stereo cable and balanced cable are two incompatible usage of a TRS jack. Back-feed is when left and right signals are short-circuited. This doesn’t apply with a splitter designed to separate right and left. Oct 30, 2021 at 19:43
  • Let me clarify what I am talking about. Before I started to think about Y-cables, I used to think that stereo unbalanced jack to stereo unbalanced jack is a good way to connect headphones output of the synth to mono line input of the audio interface. Then I found the article I've referenced above. So, in the original question, example of back-feeding relates to a straight cable, not to a Y-cable. However, I am concerned with other possible issues that are specific to Y-cables. Because I am not experienced in sound recording, I asked are there any problems (not only back-feeding). Oct 31, 2021 at 10:37
  • What you should avoid is to short-circuit an output (with an other output or with the ground). An Y cable (1 stereo - 2 mono) doesn’t do such a thing. The main risk is when the unconnected mono touch a ground connected surface. The recommandation “A simple mono-to-stereo cable or adapter is not recommended for this procedure. Doing so can damage your output device (digital piano).” is to prevent you to use some cables : 1stereo-1mono where each channel of the stereo are shunted in order to mix them and provides a mono signal. Some devices doesn’t like such a shunt (some other are more rugged). Oct 31, 2021 at 22:36

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