Where does the sound of a compressor come from? Likely it is primarily the variable gain element, but this is further influenced by the many things that surround it: valve source characteristics determined by the input transformer, valve load characteristics influenced by the interstage transformer, maybe control signal impedance, definitely time constants, and of course the power supply. All these elements must be tuned and combined in just the right way to achieve the sonic goal of musicality.

The original RCA BA6A was developed in Camden, NJ. The dual channel BH6A is faithful to its BA6A heritage from input through interstage magnetics and was originally conceived just down the road from Camden in New Jersey. It upgrades the makeup gain and output power amplifier with a high-performance equivalent that is reliable and a better match for modern interface characteristics.

The time constants match the original and add a new third choice of variable attack and release. A flexible side chain enables linking via variable frequency coupling, plus an adjustable low pass filter and a fixed frequency high pass filter with two selections.

A safe, modern solid state power supply efficiently creates DC for filaments and high voltages.

Calibration uses the clever method developed by RCA in the 1950s. It is simple and accurate, and results in a perfect balancing of the valves. Even with 6SK7 pairs that have been matched using valve testers, balancing nulls the differences that may arise from actual in-circuit loads and biasing. With the original BA6A, this required lowering the front panel and bravely reaching into the abyss with skill and hope. The BH6A moves these controls to the front panel and even includes the adjustment tool that might be hard to find in some modern facilities.

The meters are highest quality Simpson movements that match the originals, packaged in a smaller enclosure. LED backlights are driven from the regulated bipolar supply and serve the dual purpose of illuminating the meters and giving a quick verification of this part of the power supply.