Roswell Pro Audio mini K47 KD

Roswell KD Circuit Design
Roswell developed a new circuit for the KD, to accommodate two characteristics not typically demanded of large-diaphragm condensers: the ability to withstand high-SPL sources without clipping downstream devices (e.g., the preamp), and to capture subsonic bass frequencies.

The simple solution to reducing output would be to add a big capacitor to the mic's input circuit; this would indeed increase headroom and decrease output. High-value capacitive pads can add distortion, though, which is not part of the desired sound for this model.

Instead, we changed the circuit topology to reduce the output level without adding distortion. We adjusted internal voltages to raise headroom without artificial attenuation. And we changed component values to reduce low-frequency filtering.

The result is a mic purpose-built for kick drum, both internal and external placements.
Component Details

The demand for high SPL capability affects JFET choice. Some transistors are capable of higher headroom than others.

Roswell individually "bias" every JFET for the KD circuit; this configures the transistor for its max SPL and minimum distortion.

The main signal capacitor in the KD is a vintage component, no longer made and increasingly difficult to find. It provides a uniquely transparent sound, and fits a large value (capacitance) in a small package. We use another similar part as the final DC filter capacitor on the capsule power supply.

This modified circuit topology and the combination of components gives us the clarity and punch we want, with plenty of headroom and dynamic range.