One of the most iconic and desirable hardware units in studios the world over, the Pultec® (shortened from Pulse Techniques) EQP-1A passive equalizer has been making a name for itself ever since its release in the 1950s!
History & Founding of Pulse Techniques
Pulse Techniques, the company that would go on to produce a whole line of legendary tube equalizers, was founded by Eugene Shenk and Ollie Summerlin in Februrary 1953. Operating out of a 1200 square foot facility in New Jersey, the two had just one other full-time employee, Peter Menti, for the entire 29 years it was officially in business!
Shenk and Summerlin had been planning to go into business for several years by the time they opened their doors, with Shenk’s 14 years of experience working for RCA - along with a considerable background in radio repair and amateur electronics and an education from New York City’s RCA Institutes - forming the foundation for their designs. He had particularly specialized in circuit design involving pulse techniques in electronics, which the two planned to focus on in their new line of products.
It was several years and a few hit-and-miss designs before Pultec would find success with their first major product: a program equalizer for broadcast that they felt was superior to the current offerings on the market. It won the approval of MGM’s chief engineer, Clair D. Kepps, and soon after, the Pultec Model EQP-1 was born. Though it went through several revisions, most notably the EQP-1A and later EQP-1A3, this product would remain the backbone of Pultec’s line through their entire history as a company.
What Is A Passive Equalizer?
A passive equalizer circuit, by definition, can only affect a given frequency band or set of bands by cutting those frequencies. It does not use “active” (i.e., powered) circuitry on a per-band basis to boost frequencies, as most modern designs do. Instead, passive designs like the Pultec use a separate line amplifier to compensate for signal loss: in other words, when you “boost” a frequency in a passive design, you’re actually cutting everything around it, and the line amp is making up the difference in gain so you perceive that band to be boosted.
There are a number of advantages to this approach in the world of professional audio. For one, the use of simple inductors to affect frequency bands omits the need for additional circuitry in the EQ section that may introduce phase distortion or otherwise degrade the signal. For another, the separation of makeup-gain line amplifier and EQ circuitry means that different types of output circuitry can be used in a given passive design. While the most classic Pultec designs like the EQP-1A and MEQ-5 use tube amplifiers for this section, later solid-state versions used opamps like the API 2520, which naturally have a “faster” sound than tubes - a quality that can be very useful in some cases.
Beyond the EQP-1A
One of the most popular Pultec designs besides the iconic EQP-1A was the MEQ-5 midrange equalizer, which contained three bands focusing on midrange frequencies - two boosts with a cut in the middle. This is, of course, the unit we recreated in the MEQ-A, our second product ever. Those who think shoehorning a mid band into the EQP-A circuit is essentially the same thing are missing out on the excellent single-ended line amp unique to the MEQ-5, which is worth the price of admission alone! (There are, of course, other technical reasons to be skeptical of this two-in-one approach - we’ve covered it in depth before, but the short version is that compensating for an additional band pushes the EQP line amp beyond what it was designed for, causing bandwidth loss when that stage is inserted.)
Designing the AudioScape EQP-A
When we decided to tackle the MEQ’s older and more famous sibling, we made an interesting discovery along the way. The original EQP-1A schematics that circulate online actually contain an error that, if implemented to a T, creates some odd behavior in the high frequency bands at certain settings - which can’t be replicated in an original unit. Many cheap clones recreate this error anyway. It’s something to watch out for when shopping for an EQP design!
Inexpensive clones also often omit the interstage transformer, which is a crucial part of the circuit, and the rectifier tube. These may lead to cost savings, but you will notice a difference! The best EQP recreations use these elements like the originals, and in our case we stick to NOS tubes whenever possible, to capture that extra bit of authenticity in the sound. They don’t make tubes like they did back then!
We’re proud to carry on the legacy of this iconic unit in our own way, and look forward to building the next 4,000!
You can learn more about our EQP-A on its product page here
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