What is the Pultec low-end trick?

The Pultec trick in plain English

The Pultec EQP-1A has two knobs for the low end, one marked BOOST and one marked ATTEN, short for attenuation, and a switch that picks the frequency both of them work at. On most EQs, boosting and cutting at the same frequency would be pointless, because the two moves would cancel and leave you where you started. On the Pultec they don't cancel. The cut reaches further up the spectrum than the boost does, so turning both up gives you a lift in the deepest part of the sound and a dip a little above it.

That shape is useful on anything with a lot of low end. A kick or a bass gets more weight at the bottom, where you feel it, and loses some of the thick, boxy sound in the low mids that engineers call mud. Because one move adds level and the other takes it away, the sound gets bigger without jumping up in volume as much as a plain boost would make it.

You don't need the hardware. The curve is two ordinary EQ moves, a shelf boost and a cut above it, and any EQ in any DAW can draw it. This page shows you how, using a lab that builds the curve from scratch. The EQ lesson covers the band types themselves if you're new to them.

Reading the lab

The lab is the same EQ as the one on the main EQ page, set up with three bands for this lesson. Band 2 is a low shelf playing the part of the Pultec's boost knob. Band 3 starts as a second low shelf at the same 60 Hz and plays the part of the attenuation knob; later in the lesson it turns into a bell. Band 1 is a high-pass filter, switched off until the last step.

The pink line is the EQ curve, with boosts above the middle line and cuts below it. Behind it, the teal shape is the spectrum of the sound after the EQ and the pale line is the original. Hover anywhere on the screen to read exactly how much the EQ is adding or taking away at that frequency, which is how you can check every number on this page.

A/B switches the EQ off and turns the original up or down to the same loudness. That's more important for this trick than for most EQ moves, because a low boost makes a sound louder and louder nearly always sounds better for the first few seconds.

The Pultec EQP-1A

Pultec was the brand name of Pulse Techniques, a New Jersey company that made the EQP-1 and EQP-1A program equalisers from the 1950s. The EQ circuit inside is passive: it's built from coils, capacitors and resistors with no amplifier of its own, so it can only take level away. A valve amplifier after it brings the level back up, and that stage adds a little colour of its own.

The low section has a frequency switch with four positions, 20, 30, 60 and 100 Hz, and the boost and attenuation knobs both work at whichever one you choose. The knobs are numbered 0 to 10 rather than in decibels. The high section works differently. It has its own boost with a choice of frequencies from 3 to 16 kHz and a bandwidth knob, plus a separate attenuation control with its own frequency switch, so the two high-frequency controls don't share a frequency the way the low ones do.

Plug-in copies are everywhere. Universal Audio's Pultec EQP-1A for UAD and Waves' PuigTec EQP1A copy the controls and the valve stage, and Logic Pro has its own Pultec-style EQ called Vintage Tube EQ. Some of them add their own extras, but the low section and the trick work the same way in all of them.

Why the boost and cut don't cancel

Start with a plain low shelf boost of 8 dB at 60 Hz. On the bass loop in the lab it's 4 dB up at 60 Hz and 7.5 dB up at 30 Hz, and by 120 Hz it has faded to half a decibel. The bass gets heavier, and the whole loop measures 2.7 dB louder, because so much of this bass line's energy sits down there.

Now add a second low shelf at the same 60 Hz, cutting by the same 8 dB. On a digital EQ like the lab, a boost shelf and a cut shelf with the same settings are exact mirror images, so they cancel completely and the curve goes flat. If the Pultec's two knobs worked like that, the trick wouldn't exist.

They don't work like that because the Pultec's boost and cut have different shapes. The cut keeps working further up the spectrum than the boost, so above the chosen frequency the cut wins and below it the boost wins. You can build the same thing in the lab by moving the cut's corner higher. With the cut at 5 dB and its corner at 200 Hz, the curve is 2.9 dB up at 20 Hz, crosses zero at about 50 Hz and bottoms out 3.9 dB down at around 120 Hz.

The figure shows the same idea with the two halves drawn separately. Slide the cut's corner up from 60 Hz and watch the pink line, the sum of the two, grow a dip above the boost.

The exact shape on the hardware depends on the frequency switch and on how far each knob is turned. More attenuation makes the dip deeper and eats into the boost, while more boost lifts the bottom further. The dip follows the frequency switch, so you can't move it on its own, but on a modern EQ you can put it wherever the mud actually is.

How to do the Pultec trick on any EQ

The usual way to copy the trick on a stock EQ is a low shelf for the boost and a bell for the cut, because a bell lets you choose where the dip goes and how wide it is. A 4 dB bell cut at 200 Hz with a Q of 1 is almost one and a half octaves wide, so it covers the low mids without reaching down into the boost much.

On the bass loop, an 8 dB shelf at 60 Hz with that bell keeps 6.4 dB of boost at 40 Hz, where the lowest notes of this bass line sit, and takes 3.9 dB out at 200 Hz, right on one of this bass line's biggest peaks. The loop comes out 1.9 dB louder against 2.7 dB for the shelf alone, so you keep nearly all the weight at the bottom for less level overall.

1. Find the low end firstSweep a narrow bell boost between 40 and 120 Hz until the kick or bass gets loudest. That's roughly where its fundamental sits, the lowest frequency in each note. Then set the bell back to 0 dB.
2. Put a low shelf boost thereStart at +4 to +6 dB with the corner at, or just above, the frequency you found.
3. Add a bell cut a few times higherStart about two octaves above the shelf, so 200 to 300 Hz for a shelf at 60 Hz, with a Q around 1 and about half the gain of the boost.
4. Sweep the bell to the mudMove it slowly between 150 and 400 Hz and stop where the sound gets clearest without going thin.
5. Add a high-pass under itA shelf boost keeps lifting everything below it, rumble included. A high-pass at 20 to 30 Hz stops the boost spreading into rumble below the lowest notes.
6. Compare at the same levelSwitch the EQ off and on with the levels matched. If you can't hear what improved, take some boost off and listen again.

Choosing the frequency

On the hardware you get four choices, and each one moves the boost and the dip together. As a rough guide, 20 and 30 Hz are for the very bottom of a full mix or a deep sub, 60 Hz is the usual setting for kicks and bass, and 100 Hz suits kicks and bass parts whose weight sits higher up.

The frequency has to suit the sound, because an EQ can only raise what's already there. The drums loop in the lab has very little below 100 Hz, so a 6 dB shelf at 100 Hz changes less than you'd expect. Move the bell up to 300 Hz and it lands on the loop's strongest low-mid peak, which leaves the drums clearer in the low mids and about 1 dB quieter overall. A boost under a kick with no sub in it mostly brings up noise and rumble.

Starting settings for common sounds

These are places to begin on a stock EQ, with the Pultec frequency that does the same job. Sweep both bands around them and let the A/B decide.

On the mix bus and in mastering

On a whole mix the same curve needs to be much smaller. A 3 dB shelf at 60 Hz and a 2 dB bell cut at 250 Hz, with a 24 dB per octave high-pass at 25 Hz underneath, peaks at 2.4 dB around 37 Hz and dips 2 dB at 250 Hz. On the full loop in the lab the mix gets a little deeper and a little clearer, and it measures about 1 dB quieter, so the A/B has to turn the original down to match.

Keep the headroom in mind here. Low frequencies carry most of the energy in a mix, so a sub boost can push the peaks up by more than it seems to add in loudness, and a limiter later in the chain then has to work harder. If you're mixing into a limiter, check the peak meter before and after.

Common Pultec trick mistakes

Boosting far too much

A 14 dB shelf with a 9 dB bell cut is 13.6 dB up at 20 Hz and 8.8 dB down at 200 Hz. The bass booms underneath, goes hollow in the middle and comes out 5 dB louder, with most of the extra level in sub that laptop speakers and earbuds can't play. Check big boosts on headphones and on a small speaker before you keep them.

Judging it louder

The boost adds level and louder sounds better at first. Compare with the EQ switched off at matched loudness, either with the A/B in the lab or by pulling the output gain down in your DAW until the two versions sound equally loud.

Doing it on every low instrument

Pultec-style boosts on the kick, the bass and the drum bus each sound good in solo. Together they pile up in the same octave, and the low end of the mix turns heavy and vague. Give the deepest octave to one part and cut a little there on the others. The kick and bass lesson covers how to split the low end between those two.

Putting the dip where the body is

The cut is there to take out mud. If the bell sits on the frequency that gives a part its warmth, the sound goes thin and the boost below can't bring that back. Sweep the bell and stop where things get clearer, before they get thinner.

Forgetting the rumble

A shelf boost has no floor, so it lifts everything below its corner, including rumble at 10 or 15 Hz you'll never hear. Put a high-pass at 20 to 30 Hz under it, as in the last step of the lab.

Train your ears on low-end boosts

The quiz under the lab hides the screens, boosts one range of the sound by 9 dB and asks you which, with the levels matched so tone is the only clue. Learning to tell a bass boost at 100 Hz from a low-mid boost at 350 Hz is most of what this trick needs from your ears, because one is the weight you're adding and the other is the mud you're taking away.

Questions people ask

Keep going