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I can't see how there has to be any crosstalk in that circuit (unless it's mono'ed)?
Right. In the most common mode, plain stereo, there is no issue.
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my console requires to be in normal stereo mode... I see no reason to go mono or monitor incoming signal while "at the air".
OK, then it is OK, and its simplicity is very good.
But some of us must check "on air". As Dave says:
you should be able to switch them any which way while you're online without disturbing your program signal. Depends on what you do, of course.
Look what happens in the
worst case: mono and both faders full-up:
Ignore the "500ohms" for now. Note that ALL sources have some output impedance inside the box, so I've shown 50 ohms for each source. Note that your recorder is also on these points, sharing the 50 ohm resistor with the monitor control.
In plain stereo, the 50 ohm source and the 5K fader causes a 1% signal drop to the recorder, 0.09dB, both channels, negligible even if you switch it in and out. (Actually the loss will be higher due to other loads, but still negligible.)
In "L to L+R" or "R to L+R", the loss rises to 0.17dB on one channel. Probably negligible.
In the "L+R to L+R" full-mono position,
IF the faders are ever all the way up (red line around switches and faders), the two sides of the source are
shorted together (blue dotted line). The signal to the recorder becomes full-mono. It is mixed in the two 50Ω resistors. And if it was stereo at all, the two source outputs with slightly different signals will "fight" through their 50 ohm output impedances. Since they are typically only rated to drive 600Ω loads, any large stereo difference will distort badly.
Now consider the 500 ohm resistors. In full-mono, each recorder sees a mix of 50Ω and 1050Ω. The two source outputs see each other through 1100 ohms, so any "fight" is not going to distort. Separation is about 26dB. (Actually about a dB better due to fader loading.) Most clients would hardly notice if separation fell to 26dB. The downside is about 1dB loss of maximum level to the monitors.
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I never use level control all the way up. It's more like at 45% of its travel.
Then assuming a audio-taper pot (or attenuator), what you have is like my drawing, except the "500Ω" is more like 9KΩ. In that case separation is better than 45dB (excellent for proper stereo), but loss of maximum monitor level is 20dB. Gain is now fairly cheap and we often have 20dB to throw away. So your system works fine as long as you never use the top half of the pot range while "on the air".
But why use just half the pot? Spread out. Put about 30KΩ where I show "500ohms". Now "all the way up" gives about the same monitor level as you had at "half up". The bottom half of the pot becomes useful for soft listening. Separation with on-air mono-check is now 67dB.
And you can put in a switch:
500Ω = high gain, noise-check, but degrades stereo
33KΩ = normal
100KΩ = dim, for using the phone while keeping an ear on the program
open = Mute
