- The prototype features Carnhill input and output transformers, a Jensen interstage transformer and an Edcor power transformer. With 3 transformers in the audio path per channel, its clearly a tone machine.
- The prototype filter section is a modification of the Pultec EQP1-A program equalizer. It features independent controls for low boost, low attenuate, high boost, high attenuate and a Q control for the high boost. The prototype has more than twice as many EQ frequency choices as a pultec and with the combination of overlapping frequencies and independent controls, its vastly more versatile. All of the filter caps are paper in oil (of course) and the inductors are my very own hand-wound creations. I even managed to make some of the paper in oil caps by hand.
- The amplifier section uses a single 12AT7 per channel in zero-negative feedback push/pull configuration. I tried many tubes before settling on the 12AT7, which I favored for its wide frequency response and warm "single-ended sounding" tone. Plus, the 12AT7 has just the right amount of gain for this circuit.
- The power supply is onboard in the prototype, but will be external with the production model, due to limited space. The supply puts out 500v through a 6X4 rectifier tube and some very big audio-grade filter caps for DC smoothing.
My prototype passive EQ was first put into service in January of 2010, which means I probably started building it in December of 2009. In the beginning it was a passive EQ filter with no amplifier for makeup gain, no output transformer and the incorrect input transformer, but even then it sounded great. Since then I have occasionally modified, tweaked, added on and taken away from it, until 2013. In '13 I stared hitting it hard with lots of R&D time of which the most was conducted in the first few months of 2014. Yesterday I finally got the EQ sounding so great its breathtaking, which means I'm done. The actual production model will be somewhat different, but it will be strongly based on the prototype.
About the Author
Mike Congilosi II, Inventor/Engineer