Gear | 4 min read

Pultec EQ tutorial

A while ago we started a short series of video tutorials for the gear, included in the Rack1 bundle at mix:analog, and the first video was about the Pultec EQP-1A Program EQ.

But perhaps, not all of you are the "youtube tutorial" type of persons, so we've decided to also do a blog post about it. Here, you can get the same useful insights, but at your own pace, with your favourite music for background.

Pultec EQP-1A program EQ
Pultec EQP-1A program EQ

There is a slight mystery surrounding the legendary Pultecs, specifically about their frequency response curves and the interesting interactions between them.

I'm well aware that graphs and theory can seem intimidating at times, but to get the most out of "that Pultec sound", we have to know what happens and why when we adjust their gains and frequencies. We'll take it slow and easy, so don't worry.

Oooh, what does this button do?

Let's be honest - the layout and naming of the parameters on the Pultec EQ is not exactly an achievement in clear design. How many bands there are? What controls go together? And there is no curve display to at least see what happens when you adjust one of them! Horrible.

Audio nerds to the rescue! We read the manual, so you don't have to. And here are the findings:

mix:analog Pultec annotated GUI
A sketch of which controls go together on the Pultec EQ

There are four EQ bands in total - two LOW bands, one HIGH "presence" boosting band and one HIGH attenuating band.

LOW BOOST/ATTENUATE

Both LOW bands are a first order shelving type. That means the steepness of their transition knees derives from a 6dB/octave filter, which is very gentle and does not cause a lot of phase shift.

The corner frequency can be selected, but it is in practice just a little bit different for the boost and attenuate band. But more on that later.

Pultec EQP-1A low attenuate/boost freq. response plot
The graphs on the left show the LOW ATTENUTE filter response at different Frequency settings, and the ones on the right show the LOW BOOST response, again at different Frequency settings.

HIGH BOOST

The HIGH BOOST band is a bell filter and is the most configurable, with the most selectable centre frequencies and continuously variable Bandwidth, or “Q”.

Pultec EQP-1A high boost freq. response plot
HIGH (or "presence") BOOST frequency response graphs for different Bandwidth and Frequency settings.

HIGH ATTENUATE

Lastly, the HIGH ATTENUATE is, like the LOW bands, a gentle 6dB/octave shelving filter with three selectable corner frequencies. The highest Frequency set at 20kHz may look like a low-pass filter, but is in fact a shelf filter, just like the other two. It simply levels out at a higher frequency than 20kHz, at which the plot ends.

Pultec EQP-1A high attenuate freq. response plot
Frequency response plot of the HIGH ATTENUATE band at three different Frequency settings.

The famous "low-end trick"

Now back to the low bands and their weird behaviour. It was most probably just a lucky accident in circuit design that some eager knob-twisting engineer found to actually sound useful, and now it's a classic!

What later became known as "the Pultec low end trick” is a consequence of the fact that the LOW BOOST shelf starts at a just a bit lower frequency then the LOW ATTENUATE shelf. Cool huh?

So when using both boost and cut filters at the same time, perfect filters would cancel out to produce a flat response, but those in the Pultecs do not! The shift in the corner frequencies produces the curve below that, in a way, redistributes the energy from the honky/boxy mid-range to the lowest, fundamental frequencies range.

Pultec EQP-1A freq. resposnse plot, Low Cut/Boost at 100Hz
The combination of both LOW CUT and LOW BOOST filters at a 100Hz Frequency setting.

With the 20Hz or 30Hz setting, kick drums and bass guitars/synths get more weight without sounding stumpy or gaining lots of RMS level.

Pultec low end trick GUI screenshot
A good starting point for a kick drum. For master bus in need of depth, simply go for milder gains between "1"and "2".

Higher frequency selections of 60Hz and 100Hz work well on everything from snare drums and acoustic guitars to vocals and pianos, cutting out boxiness and adding depth.

You haven's heard high boost like that before

Another example of where the Pultecs really shine, no pun intended, is on the HIGH BOOST. No double-filter trickery here, just simple beauty of a passive design. I usually start with very broad bandwidth and play around with the frequency selection to find the value that adds the right kind of air to the source.

In the case of vocals, listen for the "S"-es of that particular singer and try to go with higher Frequency and/or narrower Bandwidth, so the boost won't exactly coincide with them and produce an overly sibilant result.

I rarely use the HIGH ATTENUATE filter, but in a case of a very harsh sounding source, I would probably couple a bit of a boost in the 10-16kHz range and some slight attenuation at 20kHz.

In a way, this is also redistributing the amount of energy contained in the “sub-bands” of the high spectrum, just like what happens in the lows when combining the two LOW bands.

Pultec wizardry

This "energy re-distribution inside spectrum sub-bands" is how I explain the "Pultec wizardry" to myself. But as with all things, you might paint yourself a completely different picture of what this EQ does.

Practice makes perfect and it will settle much better if you try it out for yourself, on the source material that you know. Conveniently, a pair of those EQs is available at mixanalog.com, included in the RACK1 where we offer a free demo to take them for a spin.

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