Classic Audio 553F


"This equalizer is basically a fairly faithful recreation of the vintage API 553. The 553 has always been know as a great sounding, limited featured equalizer. My original intention was creating the LC53A. In working on that, I had the skewed idea that simultaneously whipping up a simple 553 would be a breeze. Nothing is ever that easy especially when working on multiple modules at the same time.

I decided it would be nice to add a HP and LP filter array in the same fashion as the Missing Link. While each filter has only 3 selectable frequencies, I have retained the handy -6dB/-12dB per octave switchable options. These filters are buffer by the simple 3 transistor discrete followers used 4 times in the 550A.

The 3 equalizer bands are faithful to the vintage counterpart. Low band is shelving only at 100Hz. Mid band is a wide peaking filter at approximately 2.7kHz and the high band is shelving only at 10kHz. It would have been nice to add more flexibility by adding a few switchable frequencies. In order to keep the bandwidths consistent though, the inductance values would need to be changed along with the capacitance values. Things would get too complicated, expensive and move farther away from the original 553 not to mention, I am already doing this with the LC53A.

Inductor selection was a long process for this EQ. The low and mid bands both require inductors. In discussing the requirements with Ed Anderson, he felt it would be cool to stay with a toroidal style inductor like the vintage 553. I had expressed the desire to try a Molybdenum Permalloy Powder (MPP) core inductor. After some time, Ed sent me a pair of MPP core inductors as well as a pair of more common ferrite core. He said the MPP tested very well but did not know if the increase in price would be worthwhile. While my goal is always to keep the kit prices low, sound and function do come first. That said, I decided to let the ears be the judge. With 2 complete modules in hand, I went up to my good friend’s Fat Recording Studios for some listening. Always nice to get my creations out of my cave and into the real world! The ferrite core version sounded very nice. An excellent choice much along the lines if the vintage 553. Expecting something similar, we were both blown away by the magic of the MPP core version, especially in the mid band position. Quite euphoric indeed. More expensive yes, but a few bucks very well spent in my opinion. I am excited for folks to start wrapping their ears around this one.

Last thing to mention, I will have 2 PCB’s available for this build. The first is a fully variable version that utilizes 10% tolerance, center detent Bourns pots. This version will have the traditional cut/boost of 15dB and will be a little more cost affordable due to a few less components as well as no Grayhill switches. The second version is fully stepped. The basic kit will have cut/boost increments of approximately 1.25dB up to 7dB of boost and a hidden cut setting of -9dB. In my opinion, this EQ is a sweetener not a fixer."

Price: $600

I build 553F's on a per order basis. Lead time is usually around 1 to 2 weeks, but depends on parts availability.  For a time estimate or to place an order please contact me

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