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Heider® FD312 H2 Mic / Line Preamplifier
Heider® FD312 H2 Mic / Line Preamplifier
US$899.00
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VP25 Entire Bundle (Stepped)
[VP25-Stepped]
 US$261.81 
At least 1 component of this Bundle is out of stock

Click to enlarge

Click to enlarge

To avoid massive build errors, make sure the BOM Revision you follow matches the Revision of the PCB you are building!!

The VP25 Entire Bundle (Stepped) is a kit comprised of every single part and component needed (sans opamp) to complete a VP25 microphone preamplifier. The VP25 is a single stage mic preamp for the 500 series format and is fully VPR compatible. The aluminum faceplate is wet painted and the aluminum L-bracket is yellow chromated.

***Please note: This kit now ships with a CA2622 in place of the EA2622. 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 Grayhill switch for stepped gain control. Kits using a Bourns potentiometer are also available for fully variable gain control.

The VP25 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. The only exception being the change to the larger 2503 output transformer. The rest of the circuit 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 opamp. The use of series and loading resistors, along with a few decoupling capacitors is also faithfully implemented.

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 VP25 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 VP25 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 VP25, 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.

This product bundle contains the following items:
VP25 Partial Parts Kit» 1 x VP25 Partial Parts Kit
   168 pieces in stock
  US$87.72
CA2622 Input Transformer» 1 x CA2622 Input Transformer
   96 pieces in stock
  US$48.00
EA2503 Output Transformer» 1 x EA2503 Output Transformer
   43 pieces in stock
  US$36.00
Grayhill 1x12 Fixed Stop, Shorting Rotary Switch, Round Shaft» 1 x Grayhill 1x12 Fixed Stop, Shorting Rotary Switch, Round Shaft
   4726 pieces in stock
  US$26.99
VP-Gainswitch Resistor Set» 1 x VP-Gainswitch Resistor Set
   3020 pieces in stock
  US$1.50
Bourns 600 ohm T-Pad Attenuator» 1 x Bourns 600 ohm T-Pad Attenuator
   551 pieces in stock
  US$31.00
C&K SPDT Toggle Switch» 1 x C&K SPDT Toggle Switch
   605 pieces in stock
  US$5.96
Toneluck 2PDT Pushbutton Switch» 3 x Toneluck 2PDT Pushbutton Switch
   2797 pieces in stock
  US$2.10
C&K Black Bullet Switch Cap» 2 x C&K Black Bullet Switch Cap
   19124 pieces in stock
  US$0.38
C&K White Bullet Switch Cap» 1 x C&K White Bullet Switch Cap
   10113 pieces in stock
  US$0.38
DOA PCB Socket Set» 1 x DOA PCB Socket Set
   3608 pieces in stock
  US$4.50
CAPI 5-50-1A Control Knob» 1 x CAPI 5-50-1A Control Knob
   65 pieces in stock
  US$5.60
CAPI 5-62-1B Control Knob» 1 x CAPI 5-62-1B Control Knob
   1052 pieces in stock
  US$6.10
VP25 or VP26 Components» 1 x VP25 or VP26 Components
   This product is out of stock
  US$1.00

Cost of separate items: US$261.81

Available Add-On Options for the VP25 Entire Bundle (Stepped)
Selected Discrete Op Amp:
 CA-0252-DIY Single Kit» CA-0252-DIY Single KitAdd-on to cost » US$19.50  
 CA-0252 Discrete Opamp» CA-0252 Discrete OpampAdd-on to cost » US$75.00  
 CA-0251 Discrete Opamp» CA-0251 Discrete OpampAdd-on to cost » US$37.50  
 No DOA» No DOAAdd-on to cost » US$0.00  
 gar2520 Full Kit Bundle» gar2520 Full Kit BundleAdd-on to cost » US$17.50  
 gar1731 Full Kit Bundle» gar1731 Full Kit BundleAdd-on to cost » US$17.31  
 gar2520 - Assembled» gar2520 - AssembledAdd-on to cost » US$60.00  
This product was added to our catalog on Tuesday June 09, 2015.
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Classic Audio Products, Inc. is not affiliated with API. Customers and fans should not refer to Classic Audio Products, Inc. as "Classic API." API is a registered trademark of Automated Processes Incorporated. Classic Audio Products, Inc. has no affiliation with Automated Processes Incorporated.
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