In any recording situation, monitoring is critical to make sure you’re getting what you want on tape. This is just as true in field recording, but in most cases, one’s monitoring options are severely limited–stereo headphone is the only choice.
Since I often use dual-mono mics, hearing a stereo feed of the two is not always convenient. I wanted the option to hear JUST the left mic in BOTH ears, or just the right mic in both ears, as well as a normal stereo signal. This is simple enough to do with a big rotary switch. When completed, you can create a little box that your headphones plug into, which in turn is plugged into the stereo phone output of your deck. Then, by turning the knob on the switch box, you can hear normal stereo, left-only mono, right-only mono, left+right mono and even left-right reversed stereo (or normal stereo again).
Note the use of summing resistors in the left+right mono section. This was an attempt to prevent the two outputs from “fighting” each other if there were very different voltages in left and right outputs. I used 8 ohm resistors here, but a higher value might be better. Maybe ~20 ohms? Also, I initially decided to put normal stereo on both ends of the switch’s travel so I’d always be able to find it without looking. However, I sometimes wish to have left-right reversed. If you’d like to try this, simply swap the leads on one of the “normal stereo” connections.
One final caveat: The left only/right-only mono positions are -6dB down, since only one half of the deck’s headphone amp is driving your phones when the switch is in those positions.
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