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PostPosted: Tue Nov 21, 2017 10:10 pm 

Joined: Tue Nov 21, 2017 7:59 pm
Posts: 1
Location: shenzhen, , CN
Recently, I'm building an adapter to connect an RJ9 headset output to a 3.5mm jack output(TRRS, mic included). I have designed it so far to use DG409 (4:1) analog multiplexers to choose what lines are connected to what (for example Pin 4 on the RJ9 connector will be connected to the Sleeve of the 3.5mm jack (TRRS).

Furthermore I am using RC4558 op-amps to buffer the speaker signals and ground line. I needed to buffer ground to lower the impedance seen on the ground line (I was getting a lot of 50hz-hum harmonic noise on the speaker and microphone signal lines without using a buffer but after using it the noise diminished significantly - I assume that this was due to the ON-resistance of the analog MUX when passing GND through it).

The design for two of the sections is as follows and details multiplexing a SPEAKER signal and a MIC signal.
So what your seeing here is half the circuit design basically. The other 2 pins outputs (of the RJ9 connector) are connected in the same way and have the ground line switches turned on instead.

From my experimentation, it seems that the SPEAKER signal passes through fine but my MIC line experiences some odd static-like noise that is quiet (recorded voices can be heard clearly) though is reoccuring (buzzes every 1 second interval).

I understand that for the MIC signal section the buffer op-amp has an open-circuit at its input and that this is most likely what is generating this noise. But when I add a pull-down resistor at the input as shown below...
...No sound seems to be recorded.

I was wondering if there's any solution to this odd noise problem that still allows for flexibility in having any of the 3 signals/ inputs (SPK, MIC, GND) be passed to any of the RJ9 connector pins.

Would adding a feedback loop (with a resistor) to the positive input of the buffer amplifier be a solution? I still want the Pin 1 section to be able to pass the SPEAKER signal or GND if needed.
Thanks all advice, so appreciate.

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