In April of 2006, Eisen Audio released the first-ever Do It Yourself kit for 500 Series. At that time, only two third-party manufacturers were producing mic preamps for API’s popular modular format. We found 500 Series to be a sensible platform for auditioning different mic preamp designs, and the EXILE tech shop where Eisen Audio was headquartered contained lots of old parts to try. DIY500 (and later DIY500 mkII) was essentially a PCB containing dozens of superimposed transformer footprints (for input and output balancing), as well as a discrete op amp for variable gain, and the standard mic preamp feature set. Now discontinued, these were sold as minimal kits through 2010, including the custom metalwork and standard hardware, switches, pot, and connector required. You can read a Tape Op review.