I'm designing an audio mixer and took good care to create a relatively low THD signal path. At the final output stage I'm measuring a a THD of 0.06%, (10 Vpp, sine @ 1 kHz.)
This is measured directly at the op-amp output. After that I added a resistor to protect the output, a 360 Ω 0805 SMD resistor.
When I measure after the resistor I found that the THD jumped up to 0.7%.
First I thought it was because of a bad resistor, so I replaced it with a high-quality Susumu thin film (RG2012P-361-B-T5). This did not fix the issue, unfortunately.
Lowering the resistance does make it better, I'm at 0.15% with a 51 Ω resistor, but I would really like to add some output resistance.
The output after the resistor is floating when nothing is connected. There is a nominal 10 kΩ - 100 kΩ input impedance in the next device.
I'm really scratching my head as to what could cause this.
Turned out to be a small non-linearity by a component in the signal path I initially overlooked.. Moving the RHS of R2 to the other side of R34 fixed the issue (ie zero output impedance config), as proposed by danmcb.