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CAPI 511_9-11-VPR Filter Parts
CAPI 511_9-11-VPR Filter Parts
$16.00
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BT50 Entire Bundle
[BT50-Bundle]
 $449.99 
Number of Product Bundles in stock: 29

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!!

***Please note: Installing the multi-deck Grayhill switches can be a difficult task, especially with the 6-deck parts for high and low bands. Patience and care must be taken. Classic Audio Products, Inc. cannot be held responsible if a builder damages one of these switches during the process. Purchase and proceed at your own risk! On that note, I have built many of these and had zero issues.


The BT50 Entire Bundle is a kit comprised of every single part and component needed (sans opamps) to complete a BT50 equalizer. This module is fully VPR compatible.

Following is a list of additional components that are required to complete this build:
(2) 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 here.

The BT50 is my take on the very first edition of the legendary 550. The 550 was the very first equalizer offering from API so therefore the very first 500 series EQ module ever! It uses a somewhat unique Bridged-T topology with boost filters placed in the feedback loops and cut filters in between the discrete opamps. The 550 was introduced in the late 1960's and was basically an enhanced version of the Melcor GME-20. Soon thereafter, API released the "improved" 550A. A decade and a half after that came Paul Wolff's 4-band version, the 550B. Like its vintage counterpart, the BT50's filters are Proportional Q.

Those who have had the pleasure of using the original 550's know that they are special. They have a meaty thickness to them that the rest of the family doesn't have. The rest are still great, don't get me wrong but the original goods 550 is euphoric.

The original 550 only had three frequencies on the high and low bands. I have expanded those to the typical five frequencies found on a vintage 550A. I have split apart the "bandpass" filter so they can be used individually. LP is 15kHz and HP is 50Hz. For reasons involving the Grayhill switch configuration I have flipped the controls. Cut/boost is via the upper clear concentric knob and frequency select is via the lower blue knob.

The input is historically unbalanced. This can sometimes lead to a 6dB loss depending on the output stage of the device driving the BT50. I have included a shunt jumper near the output transformer that will make up the 6dB, at the output stage. The module was originally designed to drive a 600 ohm load at unity gain. This will produce an approximate 1.3dB increase in gain when driving a modern high Z load. A shunt jumper to reduce this gain is also placed near the output transformer.

Pictorial Build Guide and Support Documents

Additional information and photos:

Click to enlarge
Click to enlarge

This product bundle contains the following items:
BT50 Partial Parts Kit» 1 x BT50 Partial Parts Kit
   35 pieces in stock
  $390.53
EA2503 Output Transformer» 1 x EA2503 Output Transformer
   29 pieces in stock
  $36.00
DOA PCB Socket Set» 2 x DOA PCB Socket Set
   4069 pieces in stock
  $3.84
CAPI 5-50-1A-S Control Knob» 3 x CAPI 5-50-1A-S Control Knob
   334 pieces in stock
  $5.60
CAPI 5-62-Blue-B-S Control Knob» 3 x CAPI 5-62-Blue-B-S Control Knob
   202 pieces in stock
  $6.10

Cost of separate items: $469.31

Your savings: $19.32
Available Add-On Options for the BT50 Entire Bundle
Selected Discrete Op Amp's:
 No DOA's» No DOA'sAdd-on to cost » - - -  
 (2) SL-2520 Red Dots» (2) SL-2520 Red DotsAdd-on to cost » $130.00  
 (2) gar2520 Full Kit Bundles» (2) gar2520 Full Kit BundlesAdd-on to cost » $35.00  
 (2) gar1731 Full Kit Bundles» (2) gar1731 Full Kit BundlesAdd-on to cost » $34.62  
 (1) gar2520 & (1) gar1731 Full Kit Bundle» (1) gar2520 & (1) gar1731 Full Kit BundleAdd-on to cost » $34.81  
 (2) gar2520 - Assembled» (2) gar2520 - AssembledAdd-on to cost » $120.00  
 (2) gar1731 - Assembled» (2) gar1731 - AssembledAdd-on to cost » $120.00  
 (1) gar2520 & (1) gar1731 - Assembled» (1) gar2520 & (1) gar1731 - AssembledAdd-on to cost » $120.00  
 (2) BB2521 LME49710 Full Kit Bundles» (2) BB2521 LME49710 Full Kit BundlesAdd-on to cost » $19.16  
This product was added to our catalog on Tuesday November 22, 2016.
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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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