audiomicrophone
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?
The power connections above certainly seem to match the USB pinouts which is the most important thing to get right for a start. Maybe once constructed you could test that the charger is working with a cheaper device with a micro USB port to make sure you've got the pin ordering right.
I haven't verified the other information you've researched on Google Glass in particular but even if the device is in USB data mode I think it's relatively unlikely that placing headphones across the D+ / D- lines to ground would cause any damage. If you wanted to be cautious you could place a 1K resistor between the headphone ground and USB ground on the first test to limit the maximum possible current down to the sort of range used during normal USB communication. I'd expect that should be barely audible just for an initial test.
Finally I'm not sure how criticial it is for that device but to ensure that it is in the right mode it probably is worth shelling out for something closer than a 470k resistor. Most hobby type electronic shops won't sell them but 499k is the closest E192 value. Here's one at Farnell / Element 14 in the UK. I noticed the delivery charge is quite high but they probably have some other stuff you need or you may be able to pick them up (not sure how that works in the UK).
If you want both the left and right channels to provide audio you need to make a little mixer.
Two resistors (around 10KΩ) connected to the left and right inputs (pins 2 and 5), then joined together, will make a simple mixer. Then you link that to the LM386's IN + through a capacitor (around 10µF is usually OK).
Pin 1 links direct to the IN -
You ignore pins 3 and 4 - they are used to pass the audio through to another destination (internal speakers) when there is no plug in the socket.
simulate this circuit – Schematic created using CircuitLab
Best Answer
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.