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Topic: Replacing mic with phone output (Read 2826 times) previous topic - next topic

jack wp

If you are worried about burning out the board, then it seems to me you should not add a powerful op-amp, or a battery. Just start with resistors which will reduce burnout rather than promote it. Use the scope to adjust the resistors. And of course, the volume control on the phone will be a critical adjustment.

buffer_overfly

You won't be loading it, you'll be replacing the load of the microphone with the load of a resistor.

If it has a 1.2k resistor from 3V, and the voltage on the microphone is about 1.5V, then replacing it with a 1.2k resistor will be exactly replacing it.

Your resistor Rx from Vm to MIC is changing the circuit.
I've tried adding an opamp but it only increases noise. I'll stick to the voltage divider and no DC bias. The original electret mic was powered by 0,5V and that is enough.

However I'm concerned about preventing the DC from the mic socket to flow into the phone jack. And viceversa, if the phone had DC offset (doesn't have, only a few mV), I'd like to prevent it from flowing into the mic. This is the simplest circuit that works:

Code: [Select]

Phone jack ----- 10k ---- 10K
 (0V DC)                   |---------------- Mic + (500mV DC)
                           |
                           1K    
                           |---------------- Mic -
                          GND

This circuit is basically a -26dB pad.
How could I isolate the mic from the phone? Would adding a cap between Mic+ and the 10K resistor do? Should I add another cap between the phone jack and the firs 10K resistor?
The competent programmer is fully aware of the strictly limited size of his own skull...(E.W.Dijkstra) (dammit only 140 chars?)

jack wp

#17
Aug 07, 2015, 05:53 pm Last Edit: Aug 07, 2015, 05:54 pm by jack wp
"How could I isolate the mic from the phone?"
Are you hooking the mic to the phone, or replacing the mic with the phone.  I believe you had earlier indicated the later.

The phone output may need a load resistor (or may not). It will require some trial and error, but maybe for starters, something like this.

Phone jack --+--- .1uf --
 (0V DC)        |              |---------------- Mic + (500mV DC)
                   10k           |
                    |             1K            Mic input (replace mike with phone )         
                  GND          |---------------- Mic -
                               GND      

buffer_overfly

I've found this page on line to mic pads.

The -20dB pad for electrets is the one I've tested. It has a 10uF capacitor but with the positive terminal towards the mic positive terminal. Since the mic is the only DC in my case, this cap is the most important for me.

Now to isolate the mic from the phone DC offset (negligible for the phones I tested so far), I guess I'd have to add another cap but with the positive terminal pointing to the phone jack. But would this be possible? Maybe line-level is guaranteed to be centered at 0V?

The competent programmer is fully aware of the strictly limited size of his own skull...(E.W.Dijkstra) (dammit only 140 chars?)

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