P is for the Universal Audio UAD Pultec Passive EQ Collection. I’ve always found it interesting that while there are many “classic” compressors, there are very few classic outboard equalisers. I’m sure this must be because EQ has traditionally been built in to consoles whereas dynamics processing has traditionally been outboard. There are several classic EQ designs from companies such as Neve, API, SSL and Harrison - all mixer manufacturers, but the Pultec equalisers are a notable exception in that they have always existed as outboard equipment. Unlike the classic console EQs of the last 40 years the Pultec design has been around since 1951 and has always been a high end, desirable unit. It is unconventional by today’s standards but that is because it predates today’s standards!
The Pultec EQP-1A is the unit most people think of when they refer to a “Pultec”. Pultec is short for “Pulse Technologies” apparently. It’s unusual design offers a variable width bell filter treble boost which operates at 7 switchable frequencies, a switchable low pass filter, referred to as “atten” which operates at 5, 10 and 20KHz, and the infamous low frequency cut and boost controls which, while sharing a 4 position frequency switch, have independent cut and boost controls. In true engineering fashion the term Hertz isn’t used. Frequencies are quoted in Cycles Per Second and Kilocycles Per Second. White lab coats and Brylcreem are mandatory here!
The passive design and the valve amplification make Pultecs very sweet and at the top end it is difficult to coax a harsh sound out of them, but it is at the bottom end that the EQP-1A has its hidden weapon. The famous low end trick has been an open secret among producers and engineers for a long time but rather than just demonstrate it I decided to look at it in some more detail.
I found this EQ plot on the UA website which goes some way to explaining what is happening. You can see there is a dramatic low shelf boost, the slope of which extends all the way through the lower midrange up to around 1KHz. The corresponding cut doesn’t quite line up and at the most extreme settings results in a dip at around 800Hz. The variety of shapes available at more moderate settings and of course the possibility of different amounts of cut and boost and the 4 available frequencies show how such a variety of results can be gleaned from just three controls.
I experimented to see whether I could replicate the curve using another EQ plug in and how that might sound. I found it very difficult to replicate exactly. I got close using using McDSP's E606 which offers separate peak slope and dip controls on its low shelf filters but as i expected the sound was entirely different.
I suspected phase differences to be responsible and by attempting to null specific frequencies (which would have made the most tedious demo video!) I found that while at frequencies above 1KHz I could null effectively, at the bottom end significant phase shifts made nulling impossible. I was unable to get more than a couple of dB of attenuation regardless of my chosen frequency below about 100Hz.
So the EQP-1A certainly has a touch of magic about it. The midrange MEQ-5 and high pass/low pass HLF-3Cs, while less well known, complement it perfectly and once you are used to them I think its hard to make things sound worse with these even if you try.