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.
The Mercury EQ-H1, EQ-P1, EQ-H2 and EQ-P2 are all based on the original Pultec equalizers which were tools developed to deal with the limitations of recorded music. Limitations that most often manifest themselves in the highest and lowest frequencies of the program material. The Mercury Studio Program Equalizers, like the family of original Pultec EQs, are originally designed to bring back the life and musicality lost in a recording. Whether by accident or genius, nothing has been able to do this better in music production than a passive equalizer with tube gain make up amplifier ('Pultec style') equalizer. The interaction of the passive boosting and attenuating shelving EQs (not relying on negative feedback), as well as the transformers, tubes and other amplification circuitry all add to the incredibly musical character of the product. Working engineers try other types of equalizers, but always end up coming back to the Pultec style as the equalizer of choice for those final touches while tracking or mixing and even mastering at times.
Now in this modern age of digital recording, where the source you put into the DAW is exactly what you have on the track, with no rounded edges, no slight compression and none of the "pixie dust" that came with analog tape, the Mercury family of equalizers are needed more than ever to help add the musicality, warmth and tone to your tracks that we have all treasured.