I have a circuit board that has a built-in speaker with two wires going to the speaker as pictured below. I want to remove the speaker and connect a stereo 3.5mm jack so that I can use that 3.5mm jack as an input to an amplifier. Could I use the ground wire which I assume is the wire that's fully white and then merge the left and right channel on the stereo jack with the pink/white wire?

 

Merging L and R signals is better done at levels lower than speaker levels.
If the IC 'U8' is the audio output amplifier, look at its data sheet and see if its L,R input pins can be handled and joined (no DC on them) wired to your proposed additional external monophonic audio amplifier.

If you mean the reverse; using that single speaker level signal on the image to feed a stereo audio amplifier, yes, it can feed both input channels. Use an attenuator as an any-value potentiometer.

SPK+
\
/
\
/
\<-------------- to stereo amplifier L+R inputs
/
\
gnd

Imagine that as a vertically drawn potentiometer, any resistance, 10K will work.

 

The white wire is probably not at ground potential. Most small amplifiers use a bridged output so both wires will be active with signals 180 deg. out of phase. You could use the output from pink wire, referenced to the real circuit ground with a resistive load connected in place of the speaker. I would suggest that you attenuate the signal out if you are going to input it to another amplifier.
Regards,
Keith

 

This is a telephone PoE handset. So another option I have instead of take the output from this speaker is to take the output from the red and green Rj9 port wire. With that i connected the green to the 3.5mm jacks ground and I merged the red to go to left and right on the 3.5mm jack. I would have preferred to go straight to the loud speaker to connect to the amp.

 

h john,
I did a similar modification.
Connected a Class D audio output of a Solid state recorder module, to a 5+5Watt audio amp.
Disconnected the speaker from the recorder module, connected one end of a 100n cap to one of the speaker output pads.
Connected the other end of the 100n to the top of a 10k trim pot, bottom of the trim pot to 0V common.
Wiper of the trim pot to both inputs of the 5+5W audio amp input,
Works OK.

If your speaker output requires a load, add a 10ohm resistor across the speaker pads.

E

 

The white wire is probably not at ground potential. Most small amplifiers use a bridged output so both wires will be active with signals 180 deg. out of phase. You could use the output from pink wire, referenced to the real circuit ground with a resistive load connected in place of the speaker. I would suggest that you attenuate the signal out if you are going to input it to another amplifier.
Regards,
Keith

KW is right. If the amplifier uses a bridges output, connecting either side to common will at best not work and at worst cause some destruction of parts that you can not get a replacement for. Use a series resistor and capacitor in each lead to your stereo output jack. You may need to adjust the resistor value to avoid overloading the amplifier input.