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I want to connect an old phone to a Bluetooth module. I first thought to use the original speaker and microphone, but the quality is quite bad, so I decided to rip apart an old PC-microphone and cheap headphones.

The microphone is a simple electret condenser, the headphone speaker (don't know exact model) is two-wire and measures 31 ohm, and I use the BTM510 Bluetooth module. The Bluetooth datasheet states that it can drive speakers with a minimum impedance of 16 ohm directly.

The problem is that the old phone horn uses a common ground for the (original) speaker and microphone, so there are only three wires running up the cord. From what I found on the internet, not all amplifiers can have their audio- connected to ground, but I would really like to keep the original three-wire cord.

Is it possible to connect the speaker and microphone to the module with the speaker negative side connected to ground, like in the following manner (with Vcc being about 3.8V). Would this work properly? Would I loose half the power of the speaker?

microphone-speaker-bluetooth schematic

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Do NOT tie any of the speaker pins of the BT module to ground! This will likely damage the module.

You can wire the speaker and microphone as shown, but as you say, you'll lose half the drive voltage.

As an alternative approach, you could use a 1:1 audio transformer; connect one side to the two speaker terminals of the BT module, and connect the other side between ground and your speaker.

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  • \$\begingroup\$ If I understand correctly I could use the schematic I have drawn, without the wiring to the GND. Does this also mean that part of the microphone signal is fed directly to the speaker (although it is probably incredibly weak), and conversely, would the speaker signal not propagate to MIC_AP_C (causing echoing on the other end?). \$\endgroup\$ – Arvest Sep 24 '12 at 19:42
  • \$\begingroup\$ @Arvest: No, sorry if I wasn't clear. I just mean that you should disconnect the SPKR_A_N pin from ground (leave it unconnected); you can still have everything else connected to ground. \$\endgroup\$ – Dave Tweed Sep 24 '12 at 19:48
  • \$\begingroup\$ That actually makes a lot of sense now I think about it.. Thanks Dave! \$\endgroup\$ – Arvest Sep 24 '12 at 20:00

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