I could use a splitter, but then the (attenuated) DTMF tones might propagate to the mic and make it vibrate. Would this be harmful, or produce sound?
Run the two signals thru two 10K resistors, feed the junction into the LM386 input.LM386 is basically a power amp, not an op-amp where you would use negative feedback from the output to create a gain stage for example.
But there's something I don't understand. If there's this rule of not connecting 2 outputs together, then why is it allowed at the resistor junction?
I have built a summing amplifier, and it works fine. This is what I've done:Electrect mic (V1) -----> ------------ | | | | -----> Vout -------> Phone mic | |Line out (V2) -----> ------------Although not depicted, R1 is 100 Ohm, R2 is 6.7 kOhm, and Rf (from the opamp virtual earth to Vout) is about 100 Ohm.I've tested it with only V2, and it gets correctly attenuated to 40mV peak to peak centered at 0V.I assume V1 will get correctly mixed as well. However, I now have two additional problems:Problem #1: when wiring an electrect mic in V1, nothing from this mic is heard in Vout.I've never worked with electrect mics, so I'm probably doing something wrong.I've found that when connected directly directly to a phone, the mic DC offset is between 1.2V and 1.5V. Does an electrect mic does need external power to work? I thought it was just like a speaker but inverted, transforming air vibration into a very small voltage. My mic at V1 does not have power, and although I think it might generate a small amplitude signal when approached to a loud sound source, it probably is about 10mV in amplitude and my scope is not able to show it as it gets mixed with its own electrical noise. Do I need to add external power to this V1 mic to get a strong enough signal added to Vout??Problem #2: Vout is correct but when connected to the phone mic, nothing can be heardThis is probably the same problem as the above one. Remember that my Vout signal is centered at 0V. I plugged it directly to a phone mic input, and I was able to hear it briefly, but then the phone switched to its own mic ignoring the audio jack input. To begin with, I don't know if it is a good idea to connect the output of an opamp to a DC source like the phone mic input. Can this DC offset get into the opamp and cause trouble? Or should I add my own offset to Vout before connecting it to the phone mic input?How can I solve these two problems?
You feed voltage to the mic through a resistor and the resistor allows the signal to ride on top of the voltage. (If you connect the mic to power supply or battery without the resistor, the low impedance of the power supply will "short out" the audio signal.)