Since you mentioned it, I used Wilco for the low and mid bands, Fastron for the high :grin:. Wilco would work great for the high band as well, but resistance might have to be added to keep the curves from being too sharp to be useful. The DCR of the Wilcos is very low.
Analag, the curves I posted on page 5 are actual measurements of the "ghetto prototype", not simulations. I'm glad you think they look better than the Pultec, but that's certainly in the ear of the listener. The Pultec has been well-loved for a very long time, so it must have something going for it :wink:.
All: thanks for the words of encouragement and the suggestions. They're all good ideas; I particularly like Paul's idea of creating a "feedback pair" PCB. It'd certainly be a versatile product for many applications if it conformed to his recommendations. I'm thinking it'd be cool to make it narrow enough to mount on its side in a 1RU enclosure. I've never taken the time to learn any PCB layout software, but I'll mess around with some hand-drawn layouts just to get a general idea of how I'd do it.
As for the passive EQ, it's been one heck of a journey. It all started almost exactly two years ago when I looked at the Langevin EQ251A and the Cinema 4031B and thought, "how can I build something like that with fewer parts and no hard-to-get switches?" Within days, I had drawn up a workable idea. This was followed by two years of very sporadic tweaking on-paper--the design was put on the back burner more times than I could count, as I lost interest and then regained it again.
Recently, inspired by seeing Jaakko's completed version, I decided to stop putting it off and build some sort of prototype, even a junky-looking one, just so I could feel like I'd accomplished something with the design. I fully intended to post the latest version of the schematic here, as I always do. And that was still my intention when I started this thread.
But with a working unit--and one that works well, at that--in hand, a bunch of realizations came flooding into my mind. I don't believe there's any passive EQ out there right now (for studio use) that's not a Pultec clone of some sort. And I'm pretty sure that nobody is offering a completely passive EQ (in the general style of the Langevin, Cinema and other old-school "program equalizers") with no built-in amplifiers, allowing the operator to use the amplifier of his choice for gain makeup.
Although the principles of L-C filters are well-known--and I certainly hadn't invented a whole new type of filter or anything like that--I had hacked together an implementation that was different from any commercial program EQ I've ever seen. And it uses about half the number of parts of the old classics. In other words, it's definitely not a clone of any product I'm aware of.
For whatever reason, many people seem in love with the idea of passive equalization these days, and I had here a circuit made of available, non-exotic parts, that could be built at a reasonable cost, which could fill a unique niche in the market. And that's when I realized at last that I'd be an idiot if I just put it out there for anybody else to exploit. I spent many hours over the course of two years refining this simple circuit to be "just so", and nobody is going to make me feel guilty for wanting to try to get some sort of return on my time and effort.
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