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Pulse Techniques EQP-1A

Analogue Passive Equaliser
By Hugh Robjohns

Pulse Techniques EQP-1A

Is this, as the manufacturers claim, the most authentic recreation of the venerable PulTec EQP‑1A?

As a standard facility of most mixing consoles and DAWs, we all tend to take EQ for granted, even though there are many different types of equaliser with varying levels of sophistication and application. While the true origins of the first audio equaliser are shrouded in the mists of time, two names stand out for me as pioneers of audio equalisation: Peter Baxandall from the UK, and the American, Eugene Shenk. Baxandall was an electronics engineer (and friend of our esteemed Editor In Chief) who came up with a very elegant circuit for an active bass and treble equaliser. He published his design, royalty-free, in 1952 and it has subsequently been employed almost universally in mixing consoles and hi-fi amplifiers, bearing his name as the Baxandall equaliser or 'tone control'. Amazingly, at around the same time in America, Gene Shenk developed a passive design which has become the legendary studio equaliser — the PulTec EQP-1.

The PulTec name is an abbreviation of Pulse Techniques Inc, the full name of the company Shenk established with his business partner Ollie Summerlin at the start of the 1950s. Both Shenk and Summerlin (or Summerland, as many articles claim) were skilled electronics engineers, Shenk having spent 14 years working at RCA on radio telegraphy, while Summerlin had worked as an engineer for Capitol Records and later sold Ampex tape recorders, so brought a good knowledge of the flourishing recording studio industry.

Initially, PulTec made things like adjustable power supplies for valve equipment and audio oscillators, but one of Summerlin's former colleagues at Capitol had moved to MGM to build a mastering studio, and he commissioned PulTec to build a bespoke mastering equaliser. The result was the Model EQP-1 Program Equaliser, which was introduced to the wider studio industry in 1953. The EQP-1 continued to be made in one form or another for 30 years, and even though PulTec were always a relatively small-scale operation, they manufactured around 30 different products in all. Many were equalisers based on the same technology, but there were also microphones and phono preamps. Unfortunately (and amazingly, in hindsight!) when Shenk wanted to retire, he couldn't find a buyer for the business and the factory doors closed permanently in 1981.

The Rebirth of PulTec

With so many vintage audio companies the story could simply have ended there, but 20 years later another American, Dr Steve Jackson, decided he wanted to rebuild the original PulTec equaliser accurately and faithfully, with no component compromises and no gratuitous enhancements. The impetus behind this courageous decision was that because so few original EQP-1s survive without modification, models in good condition from the 1950s and '60s can attract quite ludicrous prices on the rare occasions they come up for sale. And although several manufacturers offer modern reproductions of the EQP-1, there are varying degrees of authenticity and many have also been 'enhanced', moving them even further away from the intentions of the original design.

Amongst the modern hardware recreations or homages of the EQP-1 are models from Manley, Summit Audio, Tube‑Tech, Cartec, Warm Audio, Magnetec, Drip, and various others; several of these have been reviewed in Sound On Sound over the years. For those skilled in wielding a soldering iron, there are many constructional schematics online and comprehensive DIY kits too. And of course the DAW user can employ a digital plug-in emulation from UAD or Waves, amongst many others.

However, Jackson takes the view that none of these recreations truly sound like an original PulTec, because they all use different, modern, components — especially transformers and inductors with inherently different characteristics. So, as an electrical engineer and materials scientist, Jackson spent the best part of 10 years carrying out meticulous research and analysis of each original component to determine its precise specifications. This research was aided by detailed conversations with Gene Shenk himself.

Nowhere was this analysis more critical than in recreating the wound components — the three audio-path transformers, and the multi-tapped inductor which sits at the heart of the HF EQ section. He then had to find companies who could build genuinely accurate copies of these components, using the same materials and constructional techniques. That's something which proved very difficult, as few were capable or interested in reviving manufacturing techniques and materials prevalent in the '40s and '50s!

Nevertheless, this exhaustive and painstaking work paid off in the end, and Jackson set up a new company — Pulse Techniques LLC — specifically to produce and market the EQP‑1A, which he claims sounds exactly the same as the original models from the 1960s. Having achieved that momentous challenge, Jackson went on to remake the closely related PulTec MEQ-5 mid-range equaliser, as well as solid-state versions of both devices using the API 2520 discrete transistor line amp module in place of the original valve amps. Dedicated mastering versions of all these units are also available.

The New Old PulTec

As might be expected, the new PulTec EQP‑1A is a thing of great beauty. It feels solid and robust in that slightly over-engineered way that is typical of vintage equipment. The classic 3U blue-grey front panel is 2mm thick, and the huge rotary controls and switches follow the exact pattern of the originals, including the massive bypass toggle switch. The rear chassis has a hammered paint finish, with an array of valves, transformers, capacitors and filter blocks bolted to the back, along with a pair of XLRs for the input and output connections and an IEC power inlet and fuse (the review model was configured for 117V AC 60Hz mains).

A look inside reveals beautifully laid out hand-wired circuitry.A look inside reveals beautifully laid out hand-wired circuitry.

Removing the top panel reveals the hand-wiring between the front‑panel controls and rear-panel components — the only concession to modernity being the use of tie-wraps instead of waxed string to hold the wiring looms neatly together. Traditional carbon resistors are employed throughout the signal path along with some vintage-style capacitors, all wired directly point-to-point across the bases of the valves, transformers and canned capacitors at the back, and across some of the switches and pots at the front. It's almost a shame that the top panel isn't Perspex, as the internal hand-crafted construction is so elegant and attractive that it really should be on view!

The attention to detail in the things that can be seen here is exemplary, and I'm in no doubt that the same unquestionably applies to all those things that can't be seen — specifically the precise construction of the transformers and inductors — because they play such a critical, albeit subtle, role in the distinctive sound and 'musicality' of the original PulTec equalisers.

In Use

For this review I was supplied with a single EQP‑1A, and although I only had it for a short time (about a week) I thoroughly enjoyed using it. Working with the huge knobs spread across such a large, spacious control panel as this is most enjoyable, perhaps because there's no need to squint at tiny labels or worry about accidentally knocking adjacent controls. The heavy actions of the rotary switches make operating the EQP‑1A a physical, involving act, although the boost and attenuation controls all felt oddly lightweight in comparison.

I connected the EQP‑1A into an analogue insert chain of my Crookwood mastering console, sandwiched between its Mid-Sides matrix to process just the Mid channel. For comparison purposes I set up an identical arrangement in my SADiE DAW with the stock M-S matrix and Universal Audio's PulTec emulation plug-in. The two signal paths were reassuringly alike in the range of sound shaping that could be achieved, and in the close similarity of the resulting effect. For example, setting the LF frequency control to 60Hz with both the boost and attenuation controls at 2 really helped to fill out the low end of a mix without making it muddy. Cranking the HD boost to 3 with the frequency selector at 10 or 12 kHz brought out a lovely high-end gloss. While the effect was virtually identical in both the analogue and digital implementations, the more I experimented the more I developed a preference for the hardware — it had a slightly more involving character and a more natural-sounding high end. It was also much more enjoyable to use!

Pulse Techniques EQP-1A.

Verdict

Although there are many replicas and homages to the Pultec EQP‑1A, including many of very high quality, none are completely faithful copies of the original, principally because the wound components used back then are no longer available. Consequently, while all use high-quality vintage-style transformers, none match the exact specifications or performance character of the original. Steve Jackson has overcome that problem by meticulously analysing the original components and replicating them precisely, using the same materials and techniques as employed in the 1960s. Of course, at around £3800$3500 for a single mono EQ (and £8000 for a matched stereo pair), this kind of pleasure will be restricted to those with very deep pockets, and I think I'll continue to 'make do' with UAD's digital recreation! But where budgets are not restricted and where desire for all the subtleties of the original PulTec EQP‑1A's character are inexorable, I have no doubt at all that Pulse Technique's beautiful offering is as perfectly accurate as it gets. Given the depth of competition for EQP-1-based equalisers, I find it reassuring Steve Jackson has been able to go to such extraordinary lengths to reproduce this legendary equaliser in such precise detail, avoiding all the compromises and conveniences of modern mass production. For me, the Pulse Techniques EQP‑1A is as much a work of art as it is a sound‑processing tool, and whilst few will ever be able to justify owning one (never mind more than one!), the fact that it exists at all is pleasingly inspirational in a world of cheap and pervasive knock-offs.

The EQP-1 Design

The original Pulse Techniques EQP‑1A was designed principally by Eugene Shenk, although it is clear that his business partner also playd a key role in its early development. Many reports claimed the whole EQ was his own original design, explaining that references in the original user manual to a license from Western Electric actually referred to the arrangement of negative feedback employed in the amplifier — something which had apparently been patented earlier by Harold Black of Western Electric.

In the 1950s, 'line equalisers' were all passive and thus inherently 'lossy' devices, and when set to give a flat response the output signal was typically around 20dB quieter than the input. In the telecommunications industry these losses simply added to those of the telephone lines themselves which were already being addressed with external line amplifiers. However, in the burgeoning recording industry the need for separate amplifiers was inconvenient, and so Shenk and Summerlin decided to incorporate an amplifier stage at the output of the passive equaliser such that the combination provided unity gain overall (in the flat response condition). Today, that adaptation may seem very simple and obvious, but it represented an important step forward in practicality for the recording and broadcast industries at the time.

The original PulTec EQP-1 is rather odd in many ways. For a start, the low-frequency band has separate controls for boosting and cutting and, although the original user-manual said they shouldn't be used simultaneously, it's not only possible — they are usually used this way! The result is that, because the boost and cut circuits are not exact reciprocals of each other, the combination results in some complex but musically very beneficial EQ. Typically, the result is a low-frequency boost with a cut about an octave higher, and that characteristic lends a welcome clarity to low end instruments by reducing the muddiness of a mix.

An important factor in the EQP-1's sound character is the use of so many wound components. Not only is the HF section based on a multi-tapped inductor, but there are three separate transformers in the signal path, each adding their own unique harmonic distortions and phase shifts. And then there is the sound of the fully balanced and wide-bandwidth valve gain stage itself.

Two of the three transformers are required because the equaliser circuitry is unbalanced, so the input transformer couples the balanced input signal into the unbalanced passive EQ section, with a nominally 600Ω matched-impedance interface (although it can be configured for 250, 150 or 40 Ω load impedances if required). Instead of taking the output of the passive EQ into a simple unbalanced amplifier stage (an approach which was used in some other PulTec products), the EQP-1 features an inter-stage transformer to re-balance the signal, which is then passed to a symmetrical valve output amplifier to restore the overall gain. Both of these transformers were originally made by the Triad company, while the output transformer — which is an integral part of the amplifier section — was custom-made by Peerless.

This output transformer isolates the amplifier's circuitry from the outside world and provides the matched-impedance (600Ω) interfacing, which was standard at the time. The amplifier itself is a pretty simple symmetrical push-pull arrangement using two dual-triode valves, the two halves of each working on the two sides of the balanced signal. The first gain stage is built around an ECC83 (12AX7), while the output transformer is driven by an ECC82 (12AU7), and overall negative feedback is obtained from an additional secondary winding on the output transformer, which is fed back directly to the cathodes of the input valves. This neat configuration compensates for any transformer distortion as well as the effects of output loading. The amplifier benefits directly from Shenk's earlier work on radio telegraphy at RCA in having an unusually wide bandwidth.

Many argue that another important aspect of the EQP-1's sound was due to its linear power supply, based on a 6X4 valve rectifier, which provides a smoothed HT power rail of around 325V DC. This is then reduced by high-wattage series resistors to about 290V for the output valve anodes (via the transformer primary's centre-tap), and 140V for the input valve anodes (or plates, for US readers!).

PulTec EQP-1 Variants

Over the years there were several variations of the original PulTec equaliser. The very first EQP-1 was housed in a 3U rackmounting chassis with a distinctive blue-grey front panel, and it offered simultaneous LF boost and attenuation at 30, 60, and 100 Hz (marked as cps or cycles per second, of course), plus HF boost at 3, 5, 8, 10, and 12 kHz. There was also a separate variable-attenuation low-pass filter fixed at 10kHz.

A compact version housed in a 2U chassis was subsequently released in 1956, called the EQH 2, although this employed a different valve amplifier design, and had slightly different EQ curves which proved quite popular. Consequently, some of these EQ revisions were subsequently imported into an upgraded version of the EQP 1 towards the end of 1960. This revised model, still with a 3U chassis, was labelled as the EQP‑1A and it featured more frequency options for both the LF and HF boost sections, as well as adding 5 and 20 kHz options to the low-pass filter.

At the start of the 1970s, the EQP‑1A was slimmed down to occupy a 2U rackmounting chassis, and this version was identified as the EQP‑1A3 — but the control switch options and internal circuitry all remained absolutely identical to the earlier EQP‑1A.

A little later again, the front panel was changed from blue-grey to silver, and as valve technology started to be perceived as old-fashioned PulTec embraced solid-state electronics! In the mid-1970s the EQP‑1A acquired solid-state rectification in the amplifier power supply, and then a fully 'transistorised' version was introduced with a discrete transistor-based operational‑amplifier module instead of the valve gain stage. This model was christened the EQP‑1A3SS — SS standing for 'solid-state'.

Other variations included the rare EQP‑1AS and EQP‑1AS3 models with slightly different filter frequency options, and there was also a complementary mid-band equaliser unit called the MEQ-5.

PulTec EQP-1 Audio Precision Plots

This plot shows the separate bass shelf responses available at maximum boost or maximum attenuation, selected independently at each of the four frequency options. It is clear that while the boost curves comply with the frequency knob values, the corner frequencies of the attenuation curves actually start roughly half an octave higher. This plot shows the separate bass shelf responses available at maximum boost or maximum attenuation, selected independently at each of the four frequency options. It is clear that while the boost curves comply with the frequency knob values, the corner frequencies of the attenuation curves actually start roughly half an octave higher. 

A comparison of the treble boost responses at maximum, with the bandwidth control set fully broad or fully sharp. The maximum gain is 9dB greater when the bandwidth is set to narrow, compared to the broad setting. It's also interesting that there is a 1dB loss through the complete equaliser at low frequencies.A comparison of the treble boost responses at maximum, with the bandwidth control set fully broad or fully sharp. The maximum gain is 9dB greater when the bandwidth is set to narrow, compared to the broad setting. It's also interesting that there is a 1dB loss through the complete equaliser at low frequencies.

A plot showing the HF attenuation curves at maximum, with the three different corner frequencies.A plot showing the HF attenuation curves at maximum, with the three different corner frequencies.

This is the classic benefit of the Pultec style of EQ. The bass Boost and Atten controls have both been raised to 2.5 on the scale, which results in a gentle +3dB bass shelf combined with a mid-band cut of -2dB. Adjusting the frequency selector moves the whole curve left or right, moving the centre of the mid-band cut between about 250Hz and 2kHz, with the corner of the shelf moving between about 100 and 750 Hz. This combination is very useful for adding warmth and low-end weight without low‑mid muddiness or congestion. The 20Hz setting works particlarly well with kick drums, and the 30Hz mode with bass guitars.This is the classic benefit of the Pultec style of EQ. The bass Boost and Atten controls have both been raised to 2.5 on the scale, which results in a gentle +3dB bass shelf combined with a mid-band cut of -2dB. Adjusting the frequency selector moves the whole curve left or right, moving the centre of the mid-band cut between about 250Hz and 2kHz, with the corner of the shelf moving between about 100 and 750 Hz. This combination is very useful for adding warmth and low-end weight without low‑mid muddiness or congestion. The 20Hz setting works particlarly well with kick drums, and the 30Hz mode with bass guitars.

This plot illustrates the steady progressive rise in second harmonic distortion with increasing input level. All other harmonics remain pretty much constant regardless of input level.This plot illustrates the steady progressive rise in second harmonic distortion with increasing input level. All other harmonics remain pretty much constant regardless of input level.

Pros

  • A genuinely accurate replica in the true sense, based on a decade of critical materials science research.
  • Employs precisely crafted wound components, built with original materials and techniques.
  • It looks and feels wonderful.

Cons

  • Such fanatical accuracy is hugely expensive!

Summary

The most sublime and precisely accurate construction of the EQP‑1A — as perfect a replica as it is possible to make.

information

£3798 including VAT

KMR Audio +44 (0)20 8445 2446

sales@kmraudio.com

www.kmraudio.com

www.pulsetechniques.com

$3505.

Pulse Techniques +1 888 478 5832.

info@pultec.com

www.pulsetechniques.com

Published February 2019