Designing a 500-Series Pultec-style EQ Pt. 2 September 14, 2016 15:35

Today's post is part 2 of a series by Joel Cameron of Rascal Audio about designing a new EQ kit.

Howdy, folks! 

Okay, so back in March I wrote about the early development of a 500-series DIY Pultec EQP-type equalizer which we have subsequently dubbed the ‘EQP5’.  At that time, we had the initial working prototype that I had built into a small cookie-type tin, and we received a lot of encouraging interest and feedback from you guys (thanks so much!).  Since then Peterson laid out proto pcbs in 500 format, built a set up and sent the unit to me for testing and troubleshooting.  Well, we’ve tested, tweaked, cut traces, soldered jumpers, etc, have generally played with this thing for a while now, and have settled on the final circuit design.

The Final Circuit Design

The final circuit, barring any last minute changes, is the ‘four-band’ EQP type filter with two frequencies per band:  30Hz and 60Hz for both LF+ and LF-, 10kHz and 16kHz for the HF+ and 10kHz and 20kHz for the HF-.  Adventuresome builders will be free, of course, to tweak frequencies to heart’s content, and alternate component values to assist them in doing so will be included.  The input and output of the basic kit include electronically-balanced I/O with an optional discrete opamp/output transformer output option as well (for a bit more girth, dynamic imaging and overall vintage vibe).  

I gotta say, this thing sounds wonderful!  I am really pleased with it.  I’m confident this little guy will find a welcome home in 500 racks across the globe, from the small, high quality home studio setup to heavyweight, well-appointed facilities alike (after all, who couldn’t use a few more Pultec-style channels for tracking and mixing? And one could load a 10-slot rack full of these for less than $3000, including the rack!).  It’s great on everything.

So you wanna hear it??  I have made some audio samples for y’all to check out:

HF- CCW (off)
HF+ 10k @ 12:30
LF- 30Hz @ 11:30
LF+ 30Hz @ 11:00

Electric GTR:
HF- 20k @ 11:30
HF+ 10k @ 1:30
LF- 30Hz @ 12:00
LF+ 30Hz @ 8:30

HF- 20k @ 11:00
HF+ 10k @ 2:00
LF- 60Hz @ 11:00
LF+ 30Hz @ 10:30

Custom Transformer/DOA Output Stage

I’m also pleased to report that the transformer used in this prototype is a new trifilar design that we developed specifically for this project (and others forthcoming...).  It is essentially a trifilar version of the more typical quadfilar API 2503-type output used in similar circuits.  This steel-core output transformer provides the same 1:2 connectivity as is common for such discrete op amp driven outputs without the waste and expense of the fourth, generally unused winding.  If you prefer to use a quadfiler design (in case you already have an API 2503 or Cinemag CMOQ-2, etc. that you wish to put to good use.) the pcb is designed to readily accept those units in the same 1:2 configuration, so you’re good either way.

On a similar note, this new output transformer will be right at home in any API 312-type build, providing the same 1:2 output, again, without the expense of the fourth, unnecessary winding.  Just wire the colored leads the same way you would a 2503-type, and you’re good to go.

This transformer, when used with a discrete opamp, provides greater dimensionality compared to the stock output.  Transient signals, such as drums, percussion/loops, strummed acoustic guitars and thumping bass, particularly benefit from this treatment, appearing to lean forward from the speakers with less actual level needed.  And, as the audio samples demonstrate, the stock, electronically-balanced output is no slouch either, showing remarkable musicality with a touch more clarity while preserving the original dynamic content of the material.  Both outputs yield a wonderfully organic result.


What Makes This Design So Musical?

So why, exactly, is this filter so musical on such a broad range of sources?  What is it about this design that has placed it among the most desired and coveted of all EQ’s in the history of recorded audio?  Well, two things certainly contribute:  for one thing the filter itself is entirely passive, and secondly, it’s curves are broad and musical.

With regard to the passive nature of the Pultec EQP-type filters it is useful to note that, in theory, two equalizers that share the same transfer function will sound the same regardless of whether one is active and the other is passive.  (Google ‘transfer function’ if you want to know more about that).  In practice, however, it certainly appears the passive designs I have had the pleasure of using (such as passive models from Spectra Sonics consoles of the early 1970’s, Langevin’s EQ251, and yes, the beloved Pultecs, among others) do yield a smoother, more natural characteristic (particularly when boosting hi-mids and highs) than their modern/active cousins.  I mention this, because simply saying “passive eq’s are smoother than active eq’s,” isn’t really a true statement, though in practice this often seems to be the case, and smoothness of this filter, in particular, is quite lovely.

In addition to the natural quality common to many passive filters, the EQP filter in particular (originally designed by Pulse Techniques co-founder, Eugene “Gene” Shenk back in the 1950s), provides broad, gentle curves that are great for overall sweetening duties.  The low frequencies may be labeled “30” and “60” Hz, but their impact reaches well into the midrange.  The high frequency curves are similarly broad, making them very powerful for balancing the frequency spectrum.  In fact this filter design was intended for balancing overall audio spectrum on program material (i.e. buss outputs or whole mixes) rather than honing in on problematic frequencies within individual sources the way more current EQ designs are intended to do.  As such the EQP-type circuit is uniquely capable of enhancing your audio in a way no other can.

The result is euphonic – you’ll want to put it on everything!

So check out the samples, feel free to ask questions or make comments, and we’ll keep you posted on progress!

Thanks for reading.