CAPI BT50 500 Series EQ

The BT50 is Jeff Steiger's take on the very first edition of the legendary 550. 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 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.

The original 550 only had three frequencies on the high and low bands. CAPI has expanded those to the typical five frequencies found on a vintage 550A. They also 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, the BT50 has flipped 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. A shunt jumper near the output transformer can be used to make up the 6dB of loss 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.

- 3 Band Parametric EQ with proportional Q
- 50Hz HPF and 15kHz LPF
- Hard bypass switch
- Dual opamp topology (gar2520, gar1731, and other options)
-HF and LF have selectable shelving or peaking options

All units are 100% VPR Alliance compliant.