I want to connect this microphone amp to my audio interface:
Please note the stereo output Jack socket (J1) connections: only the two inner connections are used, leaving open the external one. In this way the two earpieces are wired in series, allowing mono operation and optimum load impedance to Q4 (64 Ohm). Source
Is this possible? J1 has no GND connection (differential audio?). Can I just connect pin3 form J1 to GND to make this work?
Ok, so what's happening here is that they're expecting you to connect a pair of earphones to that socket with each earphone having an impedance of 32 Ohms. These earphones are to each have one of their connections to a common line (usually grounded in most circuits).
What this device does, instead of grounding that common line, is present a signal between the other two lines, leaving the common line unconnected. In this case, a current will flow through one earpiece, to the (floating) common line and then back to the second earpiece ("in reverse", if you like) and back out the other signal wire. This means that the two earpieces will be mono (and out of phase, yuk!) and in series.
Don't worry about the lack of ground. One of the inputs is tied to +V which is effectively a ground for the output. Don't connect the common line anywhere. It's a pretty grotty circuit if you expect HiFi, but it will work. Also, don't expect this unit to work connected to other audio equipment rather than headphones.
