The Mercury EQ-H1 features a passive EQ circuit with a single ended gain make up amplifier topology, based on the vintage 'Pultec EQH' circuit. The Mercury EQ-H1 circuit provides a musically satisfying result unobtainable with active and parametric EQs, since it does not rely upon negative feedback (and all its associated phase and dynamic distortions) to achieve equalization. The EQ-H1, like all Mercury products, has transformer balanced (XLR) input and outputs. The only modern additions or changes to the original are a much more powerful and stable power supply and running DC on the heaters, rather than AC, and two additional frequencies to expand the range and make it more flexible.