I am new to analog audio interface design. I am trying to get a line level output from a amplified headphones level output. I want both of the output to work at the same time in order for the user to be able to listen through headphones while recording the line level output.

Would something like this work ? (coupling capacitors not included):

A circuit to get line level output from headphones level output

I1 being the output of my headphones amplifier, R4 or R5 being a pot to adjust the line level output.

My guess is that the high-Z input of the opamp will not disturb the headphones output but I also wonder if the headphones amplifier can fry the opamp if the headphones are not connected.

I could have taken the line level output before the amplifier but the input of the amplifier will most likely be differential audio and it makes it a bit more complicated for me.

I would like to have advice on whether or not this can be a suitable circuit and also what are the best practices and/or things to avoid when it comes to analog audio interface.

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  • \$\begingroup\$ What is producing the headphone output? That pretty much sets the maximum voltage you would expect to see. You could always build in a couple of protection diodes if you think it may be a problem. \$\endgroup\$ – Finbarr May 2 '19 at 9:08
  • \$\begingroup\$ It is a 25 mW audio amplifier, the TPA6132A2. I am still not very familiar with impedance matching, etc. And I am not sure how the output of this amplifier will react if I connect it only with the high-Z input of the opamp and not at all with the low impedance of the headphones. How would be implemented protection diodes for this application ? \$\endgroup\$ – J. K. May 2 '19 at 9:45
  • \$\begingroup\$ At low power like that you're only looking at about 2V maximum signal levels, so you won't have a problem. You could always add a resistor across the input to simulate the headphone load. \$\endgroup\$ – Finbarr May 2 '19 at 10:19
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    \$\begingroup\$ the matching of impedances will cause 6dB drop in voltage; do you want that? Some public-address (airport?) Audio systems have the option of a 70 volt power distribution system, with speakers hanging off that and pulling small amounts of power into each of many small local loudspeakers. No matching is done; this is like the 60Hz power distribution within your home; the outside power transformer can provide 12,000 amps at 120 volts, thus Rsource is 120/12,000 = 0.01 ohm; none of your appliance loads are that low an impedance, but the system works just fine despite the impedance mismatch. \$\endgroup\$ – analogsystemsrf May 2 '19 at 12:33
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    \$\begingroup\$ Just make it significantly larger than your headphones or only plug it in when the headphones aren't there. 470 ohms, say, wouldn't cause any issues either way. \$\endgroup\$ – Finbarr May 2 '19 at 12:35

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