What made this crossover unique was its ability to defeat the Humpty Dumpty effect. Put a signal in, recombine the output, and you get an exact replica. Symmetrical.
The circuitry to do this was clever enough.
In most crossovers, a separate high pass and low pass function is designed, each with (hopefully) the same characteristics. But, of course, that never really happens—each a little different if just from parts tolerances if nothing else.
What Curl did was make a derivative crossover. That means that one of the two functions (high pass or low pass) is derived from the output of the other.
In the case of the Symmetry, John designed a variable low pass filter (varied from a knob on the front panel). He then took the output of this filter (which hasn't any highs) and popped it into one input of a differential amplifier. To the other input, he placed the original musical signal. The output of this differential amplifier is the exact difference between what we started with (a full range musical signal) and the output of the low pass filter (all lows, no highs).
A perfect high pass filter.
Brilliant.
The Humpty Dumpty Effect
What made this crossover unique was its ability to defeat the Humpty Dumpty effect. Put a signal in, recombine the output, and you get an exact replica. Symmetrical.
The circuitry to do this was clever enough.
In most crossovers, a separate high pass and low pass function is designed, each with (hopefully) the same characteristics. But, of course, that never really happens—each a little different if just from parts tolerances if nothing else.
What Curl did was make a derivative crossover. That means that one of the two functions (high pass or low pass) is derived from the output of the other.
In the case of the Symmetry, John designed a variable low pass filter (varied from a knob on the front panel). He then took the output of this filter (which hasn't any highs) and popped it into one input of a differential amplifier. To the other input, he placed the original musical signal. The output of this differential amplifier is the exact difference between what we started with (a full range musical signal) and the output of the low pass filter (all lows, no highs).
A perfect high pass filter.
Brilliant.
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