First post, so please go gentle.
I am simply looking to take a digital, mono amplified speaker signal (3w @4 ohms at 5v) and feed a Bluetooth transmitter, so I need to make sure I don't mismatch signal/voltage levels. Also, the speaker or Bluetooth option would probably be switchable.

Would someone more adept than I be able to scribble something up that would show me the correct circuit, resistors and values? Something along the lines of this, but the values aren't appropriate.

Example Circuit
Some additional info I was given:

  1. There is no "speaker gnd". Both sides of the speaker connection go up and down. That may or not matter depending on what you're plugging in to though. The solution is simple: isolation capacitors
  2. The amplifier is class D. That means that the output from the amplifier is digital and it needs something to smooth out the signal. Normally this happens inside the speaker itself. In your circuit you would need to have a coil or a capacitor to smooth out the output the right amount.

So does anyone know how to set this up? I found some posts on here that were close, but I do not have a big Home Theater amplifier here, just this little output on a small Arduino based sound board.

These are the amp and transmitter in question. I appreciate any help at all, thanks.
Amplifier Blutooth Transmitter

  • \$\begingroup\$ There is no "speaker gnd" ... gnd is just a reference point that you choose ... the negative probe of the voltmeter or the ground probe of an isolated oscilloscope would usually connect to that reference point \$\endgroup\$ – jsotola Apr 7 at 3:31

Your circuit looks reasonable, using ~1:1, 600 ohm (Ω) transformer. I'd add a small ceramic capacitor, perhaps 10 nF, across the transformer and 200 Ω resistor, in case the Bluetooth transmitter has trouble with Class D high-frequency harmonics.

Also, change one 10 kΩ resistor to 1 kΩ and replace the other 10 kΩ resistor with a 100 kΩ potentiometer, to provide a volume control. Set the sound volume of the speaker to a reasonable level, pair the Bluetooth device, and adjust the pot for similar output from the Bluetooth speaker.

No isolation capacitors are needed, since the output circuit has a far higher impedance than the speaker, and there should be no DC offset on the speaker in a well-designed device.

Schematic based on Question author's diagram

| improve this answer | |
  • \$\begingroup\$ Thank you. Something like this then? i.imgur.com/xkpQ2CZ.jpg \$\endgroup\$ – Brian Conner Apr 7 at 3:35
  • 1
    \$\begingroup\$ No, the capacitor goes in parallel with the transformer, i.e. bypassing RF. \$\endgroup\$ – DrMoishe Pippik Apr 7 at 4:07
  • \$\begingroup\$ Thank you I see \$\endgroup\$ – Brian Conner Apr 7 at 4:35

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