# Join independent amp grounds to common headphone ground?

I have mounted a raspberry PI inside an old rotary dial phone. I put an Adafruit stereo speaker bonnet on the Pi. It provides 3w to two entirely separate circuits for true stereo separation. I replaced the mic and speaker in the handset with a pair of 3w speakers, and run the power through the original base <--> handset cord.

It works great!

I would also like to be able to hook it up to a stereo or headphones with a mini jack, instead of using the handset. I got this rj45-mini adapter, but the pins weren't arranged properly out of the box (because when I rewired the phone, I didn't pay any attention to which pins went where.)

Rather than rewiring the phone and handset again, I want to open up the adapter cable, and cross some wires over to make it work with the rj45 wired the way it is.

I am uncertain, though, if I can do that. Headphones share a ground, but the signals on this unit have two separate grounds. Does that matter?

Should this scheme work?

Bonnet L+  ---> Mini Jack Tip
Bonnet R+  ---> Mini Jack ring

Bonnet L-  ---> Mini Jack sleeve (Shared Ground)
Bonnet R-  ---> Mini Jack sleeve (Shared Ground)


I have been trying to dummy up the circuit with alligator clips and stripped wires, and I keep causing the Pi to freeze up. I don't know if that is because I am not getting the right pins connected or because it won't work even if they are connected.

• check if you have DC out of speaker drivers then AC couple – Tony Stewart Sunnyskyguy EE75 Jun 20 '17 at 4:07
• I think it is AC (Because the Multitester shows 0.0 mA DC, whereas the AC mAmps vary with the pulse of the music, up to ~180 mA) – Adam Jun 20 '17 at 4:25
• ok then dont exceed the supply spec or else it will reset. Check regulator in/out – Tony Stewart Sunnyskyguy EE75 Jun 20 '17 at 4:45
• Sorry - I am totally in over my head - this is my first soldering/wiring project ever...Is it the supply spec on the headphones that I should be concerned about? – Adam Jun 20 '17 at 4:48
• I don't see a datasheet of used amplifier IC on this page, but most miniature Class D amplifier chips use bridge output connection. Because of that you should not connect R- with L- in any case. Shorting them you make a short on amplifier's output transistors and this way on power supply. – Todor Simeonov Jun 20 '17 at 5:59

Browsing some Adafruit's products I think that the used chip here is: MAX98357 - datatsheet. Like most of the mini Class D amplifiers this one also has a bridged output. Because of that you should not connect R- with L- in any case. Shorting them you make a short on amplifier's output transistors and this way on power supply.