How can I wire a recorder cut-in box? Answered
So, a little background: I'm a reporter, and I do a lot of telephone interviewing. There's currently not an app that works reliably for my phone to record both sides of a call, and given that I already have a very nice standalone voice recorder (sony ICD820) that has line in/line out functionality, I'd like to have a way to run the signal from my phone into the recorder, record it, and then pass it on to a headset with a mic on it.
The problem is that the recorder only has 3.5 mm stereo in/out (TRS), with no separate mic channel, so if I connect the (TRRS - mic/l/r/ground) cable from my phone directly into the recorder, then connect my headset to the 'line out' port, it ends up distorting badly, and not recording the mic channel.
As a temporary solution I A) use two "Y" cables (one connected to the phone, and one to my headset) to split the mic from the audio channels, then B) feed the audio channels into and back out of the line in/out ports on the recorder, and then C) use a coupler to mate the two mic channels. This puts the recorder in-line on the audio channel(s) only, and routes the mic channel directly from my phone to my headset (picture attached). This works great, but it's bulky and delicate, and obviously only records the incoming side of the conversation.
How can I wire a box that will give me the ability to "cut in" a recorder on a TRRS source, which is delivering (potentially) stereo audio channels along with a mic channel - ie, up to three channels - so that the recorder is recording all three channells?
(note: it does not matter if some/all of the channels are merged).
WOULD IT WORK TO wire a box that A) summed the Left and Right channels from the phone using resistors on each channel (see the picture in the link), and B) fed the summed audio and the mic lines into the Left and Right pins of a TRS plug, respectively, to give the recorder a stereo source? I have put together a crude diagram of what I think this could look like (note that the diagram includes a DPDT switch to 'cut in' the recorder, but I would likely omit this in the final design for simplicity).
Additional question: I have seen wildly varying info on what size resistors to use on the summing circuit, and also conflicting information as to whether I need a third on the ground (???). The simplest I have seen is just using a 1K resistor on each of the incoming channels. Given that these are just standard headphone/passive mic voltages, what size resistors would I need, and where in the circuit?
Link to the simplest 'summing box' diagram I hve found for this type of application (top image):