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I'd like to capture sound from a speaker output.
The system seems to be running with a small DC current (bias, so voltage across speaker is about 3 V), so the audio AC signal could be (de)coupled from a load resistor via a small capacitor.

The 10 kOhms resistor drops the 6.5 VDC (open circuit) to 5 VDC. The audio signal is in the order of 10 mVpp.

I would then need to amplify this signal and achieve some sort of galvanic isolation. Probably an opamp and a low frequency audio transformer (<1Vpp according to https://en.wikipedia.org/wiki/Line_level). However, this requires a power supply for the opamp which would be tricky to derive from the system, so I'd need an isolated dc-dc converter.

schematic

simulate this circuit – Schematic created using CircuitLab

Audio quality is not a priority - a simple circuit is (one-off build).

If the "primary" circuit could be eliminated using a high impedance transformer directly, it would be even better. The secondary signal then could be brought to line level with a transimpedance amp.

Maybe a mains voltage relay coil with >10 kOhms could be used creatively (but: core? secondary?)? Is a high sensitivity hall-effect sensor a feasability?

Any ideas appreciated!

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  • \$\begingroup\$ What makes you think you need amplification? What level do you require at the output? What exactly is the source system and how is it powered? What exactly is the destination system and how is that powered? \$\endgroup\$ – Chris Stratton Jun 13 '19 at 15:22
  • \$\begingroup\$ Replacing a low impedance speaker with a high-value resistor means a small signal current which needs to be amplified. I will measure the signal level and update the question. Output level should be the 1Vpp I mentioned. Source system and its power source: unknown. I can only measure at the speaker. Destination system: ADC. Power probably 5 VDC. \$\endgroup\$ – handle Jun 13 '19 at 17:58
  • \$\begingroup\$ On the contrary, if anything you'll probably need to attenuate. \$\endgroup\$ – Chris Stratton Jun 13 '19 at 18:40
  • \$\begingroup\$ What's your mysterious recording device? PC line in/mic in already have load resistors and coupling capacitors. You only need to try to not overdrive it. Maybe a resistive voltage divider is all you need. \$\endgroup\$ – RaphaelP Jun 13 '19 at 18:43
  • \$\begingroup\$ @ChrisStratton Why do I need to attenuate a 10 mV signal (with 5V DC offset)? \$\endgroup\$ – handle Jun 14 '19 at 6:18
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Well, if "Audio quality is not a priority", why don't you keep the speaker and use a microphone for the recording? Choose a quiet room, night time and give it a try. Some post-recording noise removal in a software like Audacity would be a good thing.

It also checks all your requirements:

  • Simple
  • Galvanic isolation
  • No additional power supply
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  • \$\begingroup\$ Thanks, but I want to remove the speaker. \$\endgroup\$ – handle Jun 14 '19 at 6:16

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