If you're reading this then you probably know what it is. But if you don't... it's an excellent high pass filter for tightening up signal - and rugged enough for live use. HPF-Pre, updated September 26, 2017 - hpftechllc Note that I asked Francis to *reverse the order of the IN and OUT jacks so when the knobs are facing forward the IN jack is on the rear right and the OUT jack is on the rear left - so it plays nicely with other pedals on a pedalboard. Velcro on the top easily holds another pedal. James 917-362-7042