I finished my equalizers. Packed in 2u rack case. The sound is amazing, very transparent, tasty, smooth, very happy with the sound. Many thanks to David for making such a project publicly available.
I compared opa134 and DOA. Opa134 is more detailed and crisp, a bit cold. DOA is nicer, warmer, it smooths the sound nicely, but the detail blurs a bit, making the sound more natural. As a result, I left DOA and went along the path C. If you need greater clarity, then it is better to leave opa134. In any case, both options are simply gorgeous.
I wonder if it is possible to replace tl071, and NE5534 with OPA134, and does that make sense? Has anyone already tried it?
Also, my left and right channels are a little out of balance at about 0.4db, is this normal? I can adjust the same balance either on the equalizer or on the bypass.
Ideally, I would like to add an output volume control) Can anyone know where to insert it in the circuit?
And once again many thanks to David) He is a very nice guy, he helped me answer my questions.
In general, the project is not complicated and worth it