Everyone knows you can boost and cut the low-shelf on a Pultec to clear out muddy frequencies, but I wanted to run some quant to see what that’s actually doing to frequency and phase.
I measured a Pultec EQP-500X (500-series solid state) using pink noise and a transfer function. These tests are for knob setting of 30Hz, “3” boost, “2” attenuate.
Let’s start with the boost with knobs set at “3” and 30Hz. Looking at the broad curve we have a +6 boost at 50Hz easing down to a +1 boost at 500Hz, and roughly flat by 1kHz. If you use that on base it ends up boost some 250-500Hz, which can sound full but also could get muddy.
Now here it is with the attenuate knob set to “2.” The resulting curve is now more like a +4 50Hz boost, crossing zero at 250Hz, a -2db dip at 500Hz, and a +2db boost at 6kHz.
Looking at it another way; we can overlay the two, and normalize so both are a +6db bass boost. There’s a bit of a steeper slope on the low shelf of the attenuated version which now zeros out at 500Hz, and then a +2.5db upper-midrange lift. And as for the phase shift, it’s not bad. If we think about what this would be doing to a bass, it’s +12 degrees at 25Hz and -11 degrees at 70Hz. So yeah, we can now see why this has become famous on low end instruments…
I’d say the way to think about the Pultec low shelf attention knob is as a midrange-dip control which is operating 3-4 octaves above the frequency selected by the low shelf knob. So with knob settings of 30-100Hz, you can also dip between 500Hz and 2kHz. It’s actually a four band EQ; low shelf boost, low-mid dip, high-mid peak boost, high shelf cut. Also, because of the way things interact, the resulting curve depends on how much you boost/cut. With bigger cuts I was able to pull the dip down to 250Hz.
To conclude, here are the curves with the low shelf set to 30Hz, 40Hz, 70Hz, and 100Hz, using boost “3” and attenuate “2.”