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CAPI VP312 Microphone Preamp - Used  From Other

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.


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Manufacturer's Description from Other

This kit ships with a EA2503 output transformer. Kits with Litz wire 2503's are also available.

This kit includes a Grayhill switch for stepped gain control. Kits using a Bourns potentiometer are also available for fully variable 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.

Other (Miscellanous Used/Vintage/Consign)

About Manufacturer

Here at Sound Pure we hand-select used and vintage pieces of gear for our inventory. Our used selection always reflects the high standards of quality and craftsmanship that we expect from all of our manufacturers.   We are a small team of dedicated experts, located in Durham, NC.  We buy, trade, and consign instruments that we truly believe in.  Because of this, we have developed a reputation of quality and trust with our customers, located all over the world.   Our best customers are the ones that truly understand our personal touch and commitment to everything we sell, and range from hobbyists and enthusiasts to some of the biggest names in the business.  Our passion bridges the products we love with the real relationships we develop with our customer - whether they are a lifelong musician, or they are just getting started on their musical journey but care about value, quality, and being treated right.  

We vigorously test everything we take in to ensure authenticity, functionality, and reliability.  We are supported by one of the most well-respected luthiers in the Southeast, guaranteeing products that come in are thoroughly vetted, and that products that go out our in premium, performance- ready shape.   We are so confident in our ability to provide the highest quality music equipment in the industry, we even invented our “Try Before You Buy” program where we can ship out a product before collecting actual payment. 

Sound Pure's inventory is constantly changing, ever-evolving, with some items sold before reaching the website.    Looking for something specific? Have questions about this product's condition or history?  Want more photos, or to simply chat with an expert who's had his hands on this actual piece of gear?  It is truly our pleasure to talk with our customers, please do contact us now.