Classic Audio LC53A "Love Child"


Background: "The LC53A is basically a very elaborate, hot-rodded 553. The original idea was to emulate the curves and feel of the 550A while using the “swinging input” topology of the 553. While explaining this to Bill Pearson (aka drBill @ gearslutz), he so graciously donned the EQ the “Love Child”. Coincidentally, the acronym has a double meaning as “LC” also describes the inductor/capacitor topology of the 553 style equalizers.

Circuit Design and Topology:
Although the “swinging input” equalizer is not that common today, it was very popular in the early 1970’s. The filter is comprised of a series inductor, capacitor and resistor that are connected between a potentiometer’s wiper & ground, while the “ends” of the pot are connected between the inverting and non-inverting inputs of an operational amplifier. Besides the API 553, many great equalizers used this topology. Like old EQ’s from Sphere, Quad-8, Auditronics and many more. They all have the reputation of being very musical and sweet sounding EQ's. The inherent nature of the filters have a Proportional-Q response much like the 550A, meaning they are more wide at low cut/boost levels and more narrow as the cut/boost increases.

To achieve the different resonant frequency points of the 550A, we are using different values of inductance and capacitance. This is done by using custom wound, multi-tap inductors from the bench of Ed Anderson. I sent Ed the required inductance values for each band and he sent me back a few different inductors to choose from. The compact EP core sounded just as good if not better than the larger and more common RM core inductors.

To achieve the curves I was after, each of the 3 bands had to be isolated or buffered from each other. Without this isolation, there is some interaction that skews each filter. Expected response curves were impossible to predict and varied with the position of the adjacent frequency select switch(s). This is fine and expected in a 553 but not what I was after with the LC53A. So, I went to the design bench and came up with the DTO5 opamp. I use this little opamp between each of the 3 bands. Simple, elegant and effective. Best of all, it’s discrete not an IC.

The EQ contains four of Ed’s custom inductors. Three of them are naturally used for the peaking filters. The fourth one is used for the low shelving filter. I took inspiration from 553’s low band which also appears as shelving but uses an inductor and is actually peaking. The LCR filter is always peaking but if the resonant center point is set very low, the filter will have the response of a typical shelving filter. An inductor used in this way has a very special low end quality and sounds huge!

LC53A 3 Band Equilizer (The Love Child)

Price: $900

I build LC53A'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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