BT50

500 Series Equaliser

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.