To avoid massive build errors, make sure the BOM Revision you follow matches the Revision of the PCB you are building!!
The VP312-VPR Entire Bundle (Variable) is a kit comprised of every single part and component needed (sans opamp) to complete a
VP312 microphone preamplifier. The VP312-VPR is a single stage mic preamp for the 500 series format and is fully VPR compatible.
The preamp is essentially an exact recreation of the legendary API 312 preamp circuit that has become the benchmark for
the API sound. The aluminum faceplate is wet painted and the aluminum L-bracket is yellow chromated.
***Please note: This kit now ships with a
in place of the
To eliminate possible shipping mistakes, I will not be able to substitute or swap these parts. Thanks for understanding.
Following is a list of additional components that are required to complete this build:
(1) 2520 style discrete operational amplifier
PS: DOA's can be added to this kit at the bottom of this page.
This kit ships with a EA2503 output transformer.
Kits with Litz wire 2503's are also available.
This kit includes a Bourns potentiometer for fully variable gain control. Kits using a Grayhill switch are also available for
stepped gain control.
There are only two main differences in the audio path between a vintage 312 card and the VP312. The first difference is the
allocation on the board for a loading resistor, R2. This is often referred to as the Load R and is used in many of the mic preamp
circuits including the VP25
and VP26 preamps. The
input transformer you use will really dictate whether or not you use this Load R as it is optional and not required. With the
EA2622, the circuit sounds great both with and without it. This Load R was typically not found on a stock API 312 card. The preamp
will sound a bit more "open" without it. The second difference is R12 which immediately follows the discrete opamp's output.
I use a 1Ω resistor for R12. I have utilized this R12 for two reasons. The first is as a little protection for the remaining
circuit if there is some sort of rare DOA failure. Hopefully R12 will smoke before any other damage occurs. The second reason is
merely to "jump" over a few PCB traces. Using a 1Ω resistor does not at all alter the sound of the preamp.
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 VP312 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 VP312 is using the popular quadfilar 2503. 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 VP312, 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.