This is a followup to Injecting a UART TX output signal to an audio input pin, without messing up the audio?
The preferred solution for TX was a tristate buffer, which makes sense, until we recognize that the buffer's output can see negative voltages (audio). The high-Z is not high-Z when there's a negative voltage, it conducts (probably across internal body diodes).
Is there a clever solution to this? I don't have dual voltage rails, and haven't found an analog switch or buffer that can handle a negative voltage on its output, unless I have a negative rail. I'd rather not add a negative voltage rail just for this, or a relay, if it can be avoided.
The "dirty" approach is to put a large-ish resistor at the output of the buffer, hopefully much larger than the output resistance of the audio source, which will minimize current flow on the negative wave, and perhaps distortion. I'm not crazy about this, as it also drops the UART TX voltage some due to the series resistor and the audio input circuit forming a divider. A workaround might be to output a 5V TX and let the system divide this to 3.3V.
Is there a clever way to solve this problem robustly?