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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
The BT50-Litz 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 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
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
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:
Cost of separate items: $471.81