A Headphone Monitoring Switch
by Rudy Trubitt
I wrote an article on field recording techniques in the November 1997 issue of Electronic Musician. In that story, I promised the plans for a simply do-it-yourself (DIY) headphone monitor switch I use in my work. Here it is! Please note I am not providing a complete, detailed, step-by-step guide to creating this little wonder box. It is not a complicated project, but some previous experience with DIY electronics is required. If you've done this sort of thing before, you'll be able to fill in the details easily.
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).
Figure 1: Wiring diagram of headphone monitoring box
Figure 1 is the wiring diagram for the box. Figure 2 shows a slightly more conventional schematic of the same switching circuit. The common contacts are shown at the far left. These go to and from the left and right outputs of the deck and to the left and right inputs of the phones. Ground connections, while necessary, are not shown. If you build the project into a metal case, that might provide the ground you need. I ran a wire between the sleeve contacts of the input and output connectors of the box.
Figure 2: Schematic of headphone monitoring box
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.
I also put a 1k Ohm linear taper pot across the output. This gave me a knob for setting the level on my Sony D8, rather than their inconvenient push-button volume control the deck provides. If you do this, you might find, as I did, that it is more convenient to leave the deck's switch in "line out" position, which gives you maximum level from the deck without having to ever wonder where its volume level is set. Then just turn down the pot on the switch box to a comfortable level. Of course, this is at the expense of battery consumption, but since I'm using an Eco-charge battery, this hasn't been an issue.
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.
c. 1998, Rudy Trubitt
From Rudy Trubitt's site. (Republished with permission.)