Designing a 500-Series Pultec-style EQ Pt. 1 March 24, 2016 12:21

Today's post is by Joel Cameron of Rascal Audio, who's collaborating with us on a new EQ kit.

I love DIY! I got my start building gear almost two decades back by scouring the internet (a much smaller internet back then) for schematics of classic gear in hopes of building the stuff I couldn’t afford to buy. This was long before sites like DIYRE came along, of course, and I had to figure out how to do things pretty much from scratch. Along the way I made a lot of mistakes, of course: bad grounding (“hummmmm.....”), popped caps, burned up power supplies, toasted transistors and opamps, etc. But each lesson learned was invaluable, and after a while I figured out not only how to make great gear, but I began learning how circuits worked and what it was made these old designs so great. I eventually began to come up with circuit ideas of my own...

Well, I’m pleased to say that my latest idea is one specifically aimed at the DIY community—a 500-series Pultec EQP-style equalizer!

Drawing Inspiration from a Classic

For more than a decade now the filter topology used by Pulse Techniques (aka “Pultec”) in their EQP variants (EQP-1/1R/1A/1A3/1S3 and EQH-2) has been my absolute favorite EQ circuit. Appropriately referred to as a ‘program EQ’ these units paint with broad, deeply enhancing strokes that make them a proper choice for both tracking and mixing. No surgical maneuvers here... this is all about tone! Of particular interest (especially for those working in the DAW environment) is the inductor-based HF boost band which can add clarity and sheen while remaining entirely sweet without any hint of harshness (try that with most EQ plugins!!!). And the LF controls can add immense, unflappable fullness to your low end as well as tame the unwanted mud and weight from bottom heavy sources. And because the Pultec filters are passive, they do all of this while sounding totally natural. In fact, its effect feels so natural you need to be careful not to overuse it.

Recently I put together a mix room in my house, and I’ve been craving a few more Pultec-style channels for processing stems (I mix out of the box). Wanting to save rack space and eyeing the empty slots in my 500 rack, it hit me: I need to build some Pultec-style EQs to fill those slots.... and (lightbulb!!) what a perfect project for the DIY community! 

The basic EQP-type filter circuit is actually quite simple, requiring surprisingly few components, so I just needed to add high-quality gain makeup and I/O and we’d have a powerfully musical device that anyone can build. 

I contacted Peterson at DIYRE to see if he had an interest in a project like this, and he was game, so I made some drawings and sent them on to him. I also breadboarded the initial concept and sent the contraption on to Peterson and the gang to get their stamp of approval. 

The first prototype. It sounds much prettier than it looks.

As of now we have a tentative, proven design that sounds amazing, though before we commit to a final product we wanted to run the overall idea past DIYRE’s loyal readers to see if anyone had some thoughts they wanted to toss in to make this truly killer. 

What We Have So Far

The EQP5 (its working title... “EQP” for obvious reasons and “5” for 500 series) will feature an enhanced version of the Pultec EQP-type filter (‘enhanced’ in that it has four independent bands, not three as original EQP’s do). The original design uses a single control to select the frequencies for both the LF+ and LF- sections simultaneously. But these two sections really are separate in the circuit, and the single control of the original is a 2-pole switch, so... we’re separating these into separate switches, so you can boost at one frequency and cut at another, dramatically increasing the usefulness of the thing.

A four-band Pultec!

Each of the four bands has a pushbutton switch for selecting one of two available frequencies per band. This circuit is a broad brush, and the frequency selections are broadly musical over a variety of source material including individual tracks and complete mixes. The use of pushbutton switches keeps the project affordable and the build simple (and also keeps the front panel from becoming too cluttered for big fingers!

The stock design features an IC-based gain make-up (the passive filter has about -16dB loss for which we need compensation) and electronically-balanced I/O. There will be an option to have the gain provided by a discrete opamp driving an output transformer. Any discrete opamp compatible with API’s 2520 footprint can be used (including the RED-25, ML2520 and others available from DIYRE). 

Spot the 2520-style opamp and output transformer in the prototype.

Little known fact: the last Pultecs made were had solid state gain makeup provided by an API 2520 discrete opamp driving an output transformer, so this approach is definitely the way to go for a more vintage vibe. It adds a more three-dimensional fullness that seems to reach beyond the speakers, directly engaging the listener. A jumper is included with the optional output to allow the selection of either output topology, so you won’t lose the option of a cleaner signal path if that’s what you want for certain applications.

Questions for You, Dear Reader

Okay, all of this has been tested and sounds fabulous. Here is where we really would like some input: I originally intended the pcb to provide four frequency options per band with any two of them user-assignable (via jumpers) to the front panel pushbuttons. The only concern for doing this is that it might offer additional confusion to newer DIYers, plus it would preclude any ability to silkscreen the front panel with chosen frequencies (which can be disconcerting to some users). To keep things simpler we could simply choose the stock frequencies ourselves, two per band, and have them screened on the panel like normal. And then for those who are more adventurous we could make faceplate available that has no frequency labels along with a chart of alternate capacitor values, so users could experiment to their hearts’ content.  

Frankly, those of you who have used Pultecs know how odd the stated frequencies are—how often do you see 20Hz or 30Hz on any other equalizer design? The truth is that these given frequencies affect content well into the midrange, so the labels can be a bit misleading; you really have to trust your ears more than a frequency printed on a faceplate. As such, I think that a faceplate without screened frequencies along with giving the user the ability to program their own choices via jumpers is a useful idea, but what do you think?

Should frequencies be marked on the front panel?

Should there be multiple frequency options?

Any other thoughts you’d like to share?

Thanks for reading. We’ll keep you updated on our progress!