Ricardo's 70s style mic preamp
click to enlarge images
Documentation in progress - in the meantime ask me if you have questions
email me at parenthetical at this domain
This board is an implementation of the 70s style mic preamp posted by Ricardo on GroupDIY in 2016. Group DIY link It has also been discussed on the micbuilders forum.
Ricardo is Richard Lee, an R&D engineer formerly of Calrec and Wharfdale.
Richard's own document about it is also available on the micbuilders group files page here (group membership required): Mic Builders file link
I have implemented the pcb with switches for pad, phase, and phantom power. I have also made pcb holes for Lundahl LL1538, OEP/vigortronix and cinemag transformers, and a mounting hole for transformers that bolt on through the board and connect with wires. You could of course use any suitable transformer and wire it in ad hoc.
The schematic shows 3 transformers in parallel! this is just to create the pcb with options for the transformer, obviously only one transformer is used at a time.
If desired, a 1:1 or 2:1 output transformer could be added after the output, in which case R11 and C4 are no longer required.
I haven't built this pcb yet. More info and pics when I have built and tested one.