CAPI VP312DI preamp with DI, API console style series 500
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- Miss Lin2020-07-10 09:46:19
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Product Details
Condition: | New | Brand: | Capi | Model: | VP312DI |
Type: | Preamp with DI | Country/Region of Manufacture: | United States |
Product Description
Ask me any related questions.Cheers :)"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.
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 opamps output. I use a 1? 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 1? 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 Andersons 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 VP312DI has the very same classic functions as the VP312 with a few very important exceptions:
The microphone preamplifier circuit is exactly the same except for the addition of a high-quality Elna blocking cap after the DOA. This 470?F cap is also bypassed with a parallel Wima MKP .01?F cap (for those audiophile types!). I have found significant DC offset from the DOA depending upon which brand of DOA is being used, especially in Bo Mode. The Mute switch is not very quiet this way and we dont want to harm the 2503. For the die-hard vintage enthusiast, I have utilized a 3 position .1" 3M header and shunt so that these coupling caps can be fully bypassing. This will couple the DOAs output to the 2503s primaries for authentic, vintage 312 operation. A close up of this can be seen below.
Next, we get into the DI information. This is quite extensive so take your time!
First off, the Neutrik slim line jack is auto-switching. When a 1/4" plug is inserted, a relay on a Plug-In Relay PCB switches the source to the 1/4" jacks signal.
Default DI mode or Pre 2622, utilizes a brand new concept developed by myself and Volker Mayer, aka [silent:arts]. We call it the HiZ Plug-In. When in this mode, the Direct Inject signal takes the same path as a microphone. From the HiZ Plug-In thru the mic pad and then directly into the 2622 input transformer. This means the normal mic input pad will be available for use while in HiZ Plug-In mode, if required. This path thru the 2622 is very unique to a 312 style preamp. There is not another in the industry to my knowledge, not to mention the near endless possibilities with the HiZ Plug-In board. So far we have a few passive versions, an IC version and a FET version.
I refer to the secondary or optional DI mode as Bo mode or Post 2622. For this I am utilizing the published circuit created by Bo Hansen. This Bo mode is not the same but is similar to the DI circuit found on the other popular 312 and 512 preamps. The selection for Bo mode and the related options are done via 3 NKK G-Series Ultra Miniature toggle switches. Flipping the top toggle switch to the right, activates a second Plug-In Relay circuit that couples the 1/4" jack signal to Bos circuit and then directly into the discrete opamp, bypassing the 2622 input transformer. Without a 1/4" jack inserted, this relay cannot be activated.
The second ultra mini toggle switch is for a -10dB pad of the DI signal. This pad switch is only active when a 1/4" jack is inserted and the above Post 2622 switch has been flipped to the right. So, no -10dB pad when in Pre 2622 mode but remember, in that situation the standard -20dB mic pad will still work.
The bottom or third ultra mini toggle is only active if the above two switches have been flipped to the right. This switch creates a Low Pass Filter that mimics the "perfect Jaco Pastorius tone". Its another feature that helps make this preamp unique. "
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