In my former life, before I was a full time artist and composer, I was a product design engineer and I worked in a variety of fields including professional audio.
Below are several designs that I created for my own recording studio.
The downloadable PDF files have complete instructions for building these units.
I offer this information for individual and small studio use not for commercial production.

Plate Reverb unit
complete plans for construction
PDF

Solid state version of the classic LA-2A Leveling Amplifier
PDF

Converting an inexpensive 10 watt Chinese amplifier into a, high quality, single tube, unity gain, output stage
PDF