To avoid massive build errors, make sure the BOM Revision you follow matches the Revision of the PCB you are building!!
The VP312DI-VPR-Litz Entire Bundle (Stepped) is a kit comprised of every single part and component needed (sans opamp & HiZ Plug-In)
to complete a VP312DI microphone preamplifier. The VP312DI-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 DI version of the VP312 has very flexible and complete Direct Inject options built
in. 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
A HiZ Plug-In Kit or two
PS: DOA's can be added to this kit at the bottom of this page.
How to add one or many HiZ Plug-In kits to your VP312DI Kit:
There are many HiZ Plug-In options that can be used with a VP312DI preamp...passive as well as active. Too many choices to have
dedicated kit bundles for each one of them. Instead, it is easier to view them as ala-carte kits that can be purchased separately,
along with the DI preamp. You can select one, two or all of them if you so desire. The HiZ Plug-In subcategory can be viewed
This kit ships with a Litz wire 2503 output transformer.
Kits with EA2503's are also available.
***Please note: The Litz 2503-L wire colors are different than the EA2503. To use the Litz
2503-L on this build, please follow the table on the bottom left of this
This kit includes a Grayhill switch for stepped gain control. Kits using a Bourns potentiometer are also available for fully
variable gain control.
Rather than be totally redundant and have a product description that is a mile long, I will try and simplify by saying that the
VP312DI has the very same classic functions as the VP312 with a few very important exceptions. You can read up on the VP312 by
visiting any of the product pages
The microphone preamplifier circuit is exactly the same except for the addition of a high-quality Elna blocking cap after the DOA.
This 470μF cap is also bypassed with a parallel Wima MKP .01μF 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 don't 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 DOA's output to the 2503's 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" jack's 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. The operating voltage for the HiZ Plug-In is switchable between 16V and 24V. A 51x rack will be required for 24V
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 Bo's 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". It's another feature that helps make this preamp unique.
***Please note: For 15-pin VPR use, the PCB must be slightly and permanently modified by cutting off the extra gold finger
connectors, as seen in the pic below.