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Dusty Wakeman (Mad Dog studios, Dwight Yoakum) discovered this for me in an original 166 back in the mid nineties. Transients would produce a click at start of compression on transients, especially acoustic guitar. Even stock 166's do this. I suspect the xl is having the same problem. The fix was to place a 47 pf cap from the non inverting input pin of the 5534 opamp that follows the current to voltage opamp stage following the VCA to ground. Apparently a current shift would happen that needed to be damped, this fix did that without reducing the 100k hz bandwidth. My hot rodded to hell black 166 with ADA4898-1 opamps and that 2180 vca's works perfectly.
It's a design flaw for sure. Even the dbx design team I talked to about this 13 years ago were unaware of it.
Jim Williams
Audio Upgrades
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