Reviewing the Manley Pultec EQP-1A - by Dr. Frederick J. Bashour
Taking the classic Pultec equaliser circuitry of Eugene Shenk, Manley has produced the updated EQP1-A EQ. George Shilling checks the price of progress.
Back in audio's dark ages (which for me was in the early seventies), I decided I needed an out-board equalizer to help flatten out the drooping low-frequency response of the hypercardioid Schoeps CMT mics I used to make my recordings. Vacuum tube equipment had not yet made its comeback, so I followed the conventional wisdom and purchased the first of the I.TI. parametric units, the MEP-230. It was a bit noisy, but it did what I wanted it to do. I used it happily until the mid-8Os, when I considered replacing it with the latest in its evolutionary line, the George Massenberg parametric.
Unfortunately, I found I didn't like the "smallness" of the GML unit's sound, nor did I like the later Sontec versions of my original I.T.I. box. Also, unfortunately, I had sold my original I.T.I. before I started to shop for its replacement. Bad move. I ended up never buying another analog EQ.
But the digital era was upon us and, over the next 10 years, I went through every digital equalizer I could get my hands on (since, of course, it was considered preferable to do program EQ in the digital domain). For the past several years, I've been using the five-band parametric EQ built into my Studer Dyaxis II workstation, although I occasionally supplement it with the mild 6 dB/octave circuit in the Lexicon 300, as well as my Roland E-660 (which, by the way, sounds much better if you patch its digital output directly into a de-jittering box, like the Digital Domain VSP Pro or one of the Audio Alchemy units, before sending it elsewhere).
I had frankly not even considered analog equalization to be a viable option for me, Mr. Dufay Digital. However, since I use only vacuum tube equipment wherever possible in my signal chain, and I do own a complete Manley recording front end (six tube mics, mic preamps and mixers), I asked David and EveAnna Manley to send me a pair of their EQP1-A "Enhanced Pultecs" for evaluation. Boy, am I glad I did! It had been so long since I had heard analog EQ that I had forgotten the happy feeling my original I.T.l. parametric gave me, when I first dialed in +1 dB of low-frequency shelf at 50 Hz, and heard such a big difference in the sound.
In my opinion, there seems to be a very different quality of sound with digital EQ, compared with the best current analog circuits. Before I describe what I hear a little later in this article, the reader should understand that I do my listening through a rather sophisticated audiophile-type system in my control room (custom Eclair Engineering vacuum tube monitor board, McIntosh and VTL tube amplifiers, and Manley/Tannoy dynamic loudspeakers with IMF transmission line subwoofers). I consider myself quite sensitive to the tiny 'just noticeable differences" in audio quality audible with such a high-resolution monitor system.
The Manley Enhanced Pultecs are, at $2,150 per unit, single-channel, single rack-space vacuum tube units. David Manley has licensed the original Western Electric passive circuit, as designed originally by Eugene Shank, formerly of New Jersey's PULse TEChnology Labs. But Manley has built it using modern components and the standard Manley Labs line amp, which is his own "totem pole" version of the classic "White cathode follower" circuit. The two tubes used in the equalizer's gain makeup amplifier are the 1 2AU7WA (input) and the military 6'114 (output). Typical passive parts include conductive plastic potentiometers, sealed gold-contact switches, polystyrene and rolled film and foil capacitors, and custom in-house manufactured mu-metal encased input transformers. The control facilities themselves are "enhanced" by the addition of one more LF and one more HF equalization point than the original Pultec unit, and the HF quasi-parametric boost control has more range than the original.
Speaking of boost, I should explain that if you've never used a "real" Pultec before, the boost and cut controls are a bit...well, unique. That is to say, thc low-frequency section has both boost and cut pots, not a single control with a center '"flat"" position, such as the l.T.l., Sontec, and Massenberg parametrics.
A Pultec is not really fully parametric, since the LF section (with boost and cut) is only a shelf EQ circuit and the HF section (also a shelf equalizer) has only a cut control. It is only with the "High Frequency Boost" section, that we have a quasi-parametric-type circuit, with switch-selectable frequency points and continuously variable Q~
The frequencies available in the three sections are 20, 30, 60, 90, 120 Hz (LF shelf, +A 11 dB boost/cut), 1.5,2,3,4,5, 8, 10, 12, 14, 16 kHz (HF quasi-parametric, + 17 dB boost), and 4, 8, 12, 16, 20 kHz (HF shelf, -11 dB cut only). One might question the lack of a HF shelf boost, or an HF quasi-parametric cut facility (missing in both the original Pultec, and in Manley's version) but, in real life situations, the use of both sections together does, in fact, provide just about any curve one might possibly need.
The input and output circuitry deserves special mention. In the original Pultec, there are three transformers in the signal path: at the input, before the tubes and on the output. Manley's version has only a single transformer -- on the input -- and it is bypassable. On the rear panel one finds both 1/4" unbalanced and XLR balanced input connectors, and a toggle switch to choose between them. I did most of my listening using the unbalanced inputs. The output features both 1/4" and XLR jacks, but they are wired in parallel and are fed from the unbalanced output of the Manley totem pole circuit.
I should point out that the "in/bypass" switch on the front panel bypasses only the passive EQ circuitry; it is not a hardwired bypass of the entire unit. Thus, if one wanted to use the Manley line amp by itself, one could, but the reader should be aware that the line stage is an extremely transparent modern triode circuit, not a highly colored pentode "tubey" one with gobs of negative feedback (such as in, say, an old Ampex MX-l0 mixer), so this unit would not be a good choice if one wanted to "tubeify" a cold digital program.
Having just begun to make value judgments on the sound of the EQP1-A units, let me continue in more detail. During the review period, I patched the two channels into an insert point in my control room monitor board-as mentioned earlier, a vacuum tube "audiophile preamp on steroids" designed and built by Bruce Seifried of Eclair Engineering, the manufacturer of the highly-regarded "Evil Twin" line-level tube direct boxes.
Not once during that time did I notice that the insertion of the Manley line amp stage would compromised my sound in the least degree-whether or not I had the passive Pultec circuitry bypassed!
The units are that transparent. The line amp sound can be described as liquid, relaxed, clear, and effortless -- basically the same adjectives I would use to describe the sound of my reference "monitor preamp" without the EQP1-As in the signal path. In fact, on several occasions, I deliberately tried to fake myself out, and had my wife patch them out of the path, or back in again. I could never reliably tell whether or not they were in the monitor circuit.
Of course, upon flipping the bypass switch up to the in position, and just touching any of the knobs, I immediately knew that not only did I have an equalizer on-line but, also, this was a very special equalizer. The smallest boost increment selected did just that-it supplied a warm and powerful sounding (in the low end) and airy and extended sounding (in the highs) boost completely in keeping with what my "mind's ear" expected.
The cut controls worked similarly, fixing subtle spectrum balance errors without making the program sound as if something were missing. Over the several weeks' evaluation period, I was able to make most of the test material sound "better" to my ears by boosting or cutting no more than 1.5 dB somewhere or other. These units truly possess an immense palette of adjustment capabilities. I mentioned earlier the psychoacoustic "difference" I noticed between the sound of my various digital equalizers, and these units. Bear in mind that all my digital EQs feature conventional F.I.R. filter algorithms, designed to emulate the phase shift of analog EQ. Nevertheless, for some reason, I seem to need much more EQ (as measured with my signal generator and VTVM) with the digital equalizers than with these Manley Pultecs to achieve the same subjective result.
This is a similar finding to that which I noticed long ago when I compared my old I.T.I. equalizer with the Sontec and Massenberg versions. I have no explanation for this phenomenon; I just call it like I hear it.
My conclusion is simple. I can't imagine living for long without these equalizers. Unfortunately, I can't afford them right
now, so back they must go. However, when I am able to purchase a pair, I'm going to have to spring an extra $1,000 for the "mastering" version, with all those precision resistors soldered to those fancy rotary switches. Using them as "program equalizers," I cannot imagine not being able to precisely match the stereo channels.
Dr. Fred Bashour holds a Yale Ph.D. in music theory, and currently perforns as a jazz pianist and church organist, in addition to working as a classical music producer/engineer He is also a contributor to Pro Audio Review.