I'm looking at the second board, the NEUTRAL, and it looks like it is a version of this http://www.forsselltech.com/JFETMP1.PDF but with single ended output (2x2SK289, OPA2604, JLM99V as out driver). Am I right? Did you use DC servo for the output circuit? (somewhere under the JLM99V a chip?)
I'm very interested because I started to design a board for something similar, but in this case I would preffer to buy from you.
Give me please more details about the NETRAL.
The BA Neutral (BAN) is for building several versions of the Graeme Cohen Mic pre topology like Amek/SSL/Forssell/Green mic pres use but redesigned to run on one 48v to 62volt power rail. The PCB can take LM394 dip or can, MAT02, 2 x 2SK389, 2 or 4 x BC550C etc as its discrete front end. Working around a OPA2604A, NE5532A dual dip etc. The output stage can be JLM99v, Hybrid, 990, 992, 993 etc with output transformer if you want fully floating balanced output or OPA2604A, NE5532A etc for electronically balanced output stage. Has input stage trim pot to better balance transistor/fet match which is adjusted simply with a multimeter held between the two TP1 and null for 0v. Also a output swing trim that centres the output of the output stage opamp to half volts to give best maximum +/- swing. This is another 0v null trim while holding the multimeter on the 2 TP2 points. There is no servo as there is always half the supply rail volts on the output so a output cap is always needed and the trim pots do the DC offset trimming.
Whats the difference between the dual 99v and the dual BA?
Difference is BA Dual (BAD) it designed to run on one 48v to 62v power rail and has no servo options on the PCB, no remote relay switching and no A and B output for switching between single and dual mode. Otherwise is it similar. It can with a JLM14 and JLM111DC output transformer do 86dB of gain which allows you to insert a passive EQ in the trim/fader/insert point on the BA dual and makeup the 16dB loss of the EQ and still have a 70dB mic pre. or same goes for inserting a opto comp etc.
Below is a small Power switch and LED PCB that comes in the full BA everything kits now to make that even simpler.