The Classic VP26 Rev B printed circuit board is a vintage style, microphone preamplifier circuit. The PCB has been designed and engineered to the popular 500 series format following the VPR Alliance guidelines. This board is also compliant with the GroupDIY 51X Alliance guidelines. The 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. It 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 op amp. The use of series and loading resistors, along with a few decoupling capacitors is also faithfully implemented.
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 designed with the Ed Anderson EA 2623-1 in mind. The circuit utilizes the two secondary windings in series in standard API fashion.
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.