I received the boards from China and built a couple to test. They seem to work well. With a 9V supply and low level input the output can run up to 4.2V Pk-Pk before any distortion appears. Frequency response is flat from about 100Hz to 12KHz. Board size with connectors is 40mm x 50mm. There’s an on-board 5V regulator so the supply can be 8-24VDC. Gain is set such that 20mV input is 1V output. Trimmers on the input and output allow attenuation of the input and output. It’s a text-book non-inverting op-amp circuit using a dual low-voltage rail-rail op-amp.
The pan was for the DC power jack to be +ve pin but it turned out to be -ve pin. A result of working off the brief and partly illegible data sheet before having connectors in-hand. I wasn’t sure about the DC jack so I included the 2-pin power connector as an option. Probably more reliable long term than the DC jack anyway.
The picture show a stereo to mono version, but with a few extra parts on-board it’s stereo. I may put one between my main PC and the Logitech amp/speakers so I don’t have to run the Line-Out at near 100% for many sources.