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.

  • \$\begingroup\$ check if you have DC out of speaker drivers then AC couple \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 20 '17 at 4:07
  • \$\begingroup\$ 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) \$\endgroup\$ – Adam Jun 20 '17 at 4:25
  • \$\begingroup\$ ok then dont exceed the supply spec or else it will reset. Check regulator in/out \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 20 '17 at 4:45
  • \$\begingroup\$ 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? \$\endgroup\$ – Adam Jun 20 '17 at 4:48
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    \$\begingroup\$ 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. \$\endgroup\$ – 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.

Internal short circuit protection limits the curent so that the amplifier is not fried, but this excessive current can make a power glitch on Raspberry's supply and make the CPU freeze or reset.

On this product picture: enter image description here can be seen that both positive and negative outputs go trough SMD ferrite beads (right next to the 4-pin connector) which confirms that they are HF switched.

So the conclusion is that this amplifer board cannot be used with headphones with common ground wiring.


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