Circuit and PCB notes

First prototype PCB: PCB_plusminus_000.gif
		     made 29.01.2019 - gwr
                     corrected faults: loose ground problem solved with wire bridge
                     wrong microphone size: pins are only 1.9mm apart
                     9V1 zener diodes added
Improved design: PCB_plusminus_001.gif
                     made 02.02.2019 - gwr 
                     no zener's, using voltage divider.
                     cut off frequencies for the preamp changed
Version 3.0: Using BUF634P buffers on the OP27's.
                     made 05.02.2019 - gwr
Version 3.1: made for boards 2 - 7, laserprinted PCB
Version 3.1: TIF file photofilm made at Polo
             used for boards 7 - 18
             Up to board 17, soldered and finished 10.03.2019