The VP26 preamp is essentially an exact recreation of the legendary preamp circuit found in the vintage API consoles of the mid to late '70's. The 528/536 input modules have precisely this same exact circuit. The rest of the circuit is slightly different than the ever popular 312 schematic or what is found on the 312 cards themselves. You will find a few more components in the audio path like coupling capacitors on both sides of the discrete opamp. The use of series and loading resistors, along with a few decoupling capacitors is also faithfully implemented.
The love-hate relationship with the input pad is loyally carried out on this board. The simple voltage divider or U-pad resistor network follows the exact method used on the original module. This method is historically and mathematically correct, unlike many pads implemented into the 312 type circuits. Notes on the schematic give the builder a choice of attenuation levels, with the standard and original version being -20db. You can choose from a few options calculated to represent similar source and load impedances to the ones that exist in the circuit before the pad is engaged. While some of these will provide as little as half of the attenuation, the source and load impedances may be a slight compromise, changing the interaction between microphone and preamplifier. Maybe for the better, maybe for the worse. Experimentation required. Let your ears be the judge!
The mic input transformer is Ed Anderson's EA2622. This transformer mimics the inherent flaws of the vintage AP2622 that we so enjoy and works perfectly in the VP26 circuit. A Zobel network and loading resistor are required and are implemented exactly as the original.
Of course, the discrete operational amplifier is the heart of this preamp. The industry standard 2520 footprint is utilized here.
For the balancing output transformer, the VP26 is using a trifilar wound 2623-1. The circuit utilizes two of the secondary windings in series in standard API fashion. Although the 2503 is the most recognized output transformer for an API mic preamp, it should be noted that the console input modules made use of the smaller AP2623-1 for this duty, not the 2503.
The last and possibly most unique feature of the VP26, is the use of a custom, Bourns 600 ohm T-pad output attenuator. The 3-module attenuator gives the user the ability to drive the preamp harder, and simply turn down the output level. The insertion loss of the t-pad is very minimal with a near perfect transfer of power, from input to output. This is the exact same attenuator as used for the input of the popular 1176 compressor builds. The t-pad can be skipped if so desired. The PCB is clearly marked and requires only moving two of the flying leads from the output transformer.