When I bought my API 1608 console [Tape Op #81] almost ten years ago, one of the reasons I was attracted to it was the option to put different EQ modules in it. I had two vintage API 550A EQ modules, and I could only afford to buy six new 550A and two new 560 modules at the time. Over the years, as budget allowed, I added more EQs to the 1608’s empty slots. The first of these were two Avedis E27 equalizers, and they haven’t left the console since. If I had to sum up the E27 in one sentence, I’d say it sounds amazing and is built to a standard from a bygone era. Its build quality is beyond that of any other 500-series module I own or have seen, except the API 550A, which is on par. The E27 just looks and feels terrific — and very “pro.” Red and silver knobs, that are custom-machined from aluminum, contrast beautifully with the black faceplate; and the rotary switches and pushbuttons are rock solid. But the best indication of the E27’s build quality is that my two modules have never had a problem in ten years of daily service, and they still look and sound brand new, with no noise or crackling issues of any kind.
The E27 has 27 fixed frequencies spread across its three bands (without overlap) and ±16 dB boost/cut with proportional Q for each band. Each band can be bypassed separately, and the top and bottom bands can be switched between shelving and peaking. A master in/out switch allows you to bypass all three bands while still passing signal through the module’s discrete op amps and Jensen input and output transformers.
The E27 is a creation of company owner/designer Avedis Kifedjian, who is also responsible for the proprietary op amp inside. Avedis spent over a decade working with Geoff Tanner (who was a Neve employee from 1971–1985) and at Brent Averill Enterprises (before it changed ownership and became BAE), repairing API and Neve consoles and modules, as well as designing his own gear — so Avedis knows the “classic” circuits extremely well. He explained to me that older API 2520 op amps were becoming difficult to find, so after a lot of study of the 2520, he developed his own revisions and called his discrete op amp the 1122. One of the design goals of the E27 was to lower the distortion at the center frequency, effectively moving the more musical harmonic distortion to the “rise” and “fall” regions left and right of the center frequency. Another objective was to minimize phase shift, something that all analog equalizers impart to some degree, which translates to the E27 making your music sound “tight” rather than “EQ’d.” Avedis went on to describe some of the differences in equalizer topologies that he analyzed while he was designing the E27.
“Older Neve modules have an active Baxandall and an active LCR-type equalizer. The Baxandall just makes use of capacitors for tuning the LF and HF, both of which share the same card inside — the B205 (or B180 in older 1066 modules). An LCR circuit is used in the MF with the B211 card (or B181 in 1066 modules). Inside of most electronics textbooks, you’ll see mention of LCR. L = inductance, C = capacitance, and R = resistance. If you put the three together, with some feedback around either a discrete or IC-based amplifier, then you can tune to a frequency.
“The API 550A uses a twin-T filter. It’s a way of super-tuning, using just resistors, capacitors, and feedback around a discrete op amp. It can sound very nice and clear, and I would think it’s great for mastering, as long as you had all the frequencies and boost/cut steps you need. Not as colored as you’d think — but the coloration will come from the other components, such as the op amp interacting with the output transformer, each with their own limitations.
“The API 560 graphic EQ uses a type of active gyrator filter, using IC op amps for each frequency, and another IC for boost/cut. The 2520 in the oldest 560s drives the output transformer, but in the 1980s, API removed the 2520 and even the output transformer (optionally) and put in a couple of push-pull transistors instead. I like it, and it does have a certain sound that is more colored than the 550A, due to a little more distortion and a kind of ripple effect around the frequencies you change. The E27 is somewhat related to this topology, except that I use small Class A amps instead of ICs, and I use the 1122 op amps for boost/cut as well as for the output-stage driver through a Jensen transformer. Plus, there’s a Jensen input transformer and another Class A amp, so the transformer sees the load it likes, and the EQ circuit sees the source impedance it likes. It’s a happy EQ, and this allows more freedom for the EQ circuit to add its characteristic sound.
“The API 554 is a state-variable EQ, using three IC op amps for each band, with capacitors, to change your tuning. It sweeps around instead of locking into a specific frequency. It’s versatile, but it causes more phase shift, and, coupled with the series of IC op-amps your signal is passing through, results in that very EQ’d sound. I like it sometimes, and on certain problematic frequencies, it can squeeze into tight spaces in a nice way and solve issues. Many parametric EQs are also the same topology, with some variation — lots of control with the frequency sweeping around, but also lots of phase shift.
“Another rule to keep in mind — when we see an EQ shelving curve, we don’t call it by the frequency point where it peaks, but where it’s down −3 dB. So, with a Neve 1073 high frequency at 12 kHz, that’s where it’s supposed to be, theoretically speaking, down −3 dB, which means its effect will be greater at higher frequencies.
“With a bell curve, two −3 dB points are taken at the sides of the peak, and this determines the Q. If you divide the peak frequency by the bandwidth of the −3 dB points, the result will be the Q number. For example, if you’re EQ’ng at 2 kHz peak, and the −3 dB points are at 1000 Hz and 3000 Hz, then bandwidth is 2 kHz. Divide peak frequency by bandwidth, and you get Q = 1. If bandwidth was narrower, you’d have a Q higher than 1; with a wider bandwidth, Q would be smaller than 1.”
After a lot of experimentation and design tweaks, Avedis was happy with the E27, and he likes to say that it has the best qualities of the popular “British” and “American” EQ topologies. The published frequency response of the E27 is 10 Hz to 58 kHz, but I’m told it’s even wider, and this is reflected in its extremely open, high-fidelity sound. One of the unique features of the E27 is that its high band has points for 20, 24, and 28 kHz — higher frequencies where most EQs don’t dare to go! Like a Mäag or a Pultec, the E27 can add a beautiful “air” to a vocal or to drum overheads when used judiciously.
Because of its non-sweepable frequency points, the E27 is not going to solve every EQ problem you come across, like dialing out troublesome tom rings; but if you’re looking for an EQ to enhance well-recorded tracks, the E27 is in the same league as classic designs like the API 550A and Neve 1073. And there are very few EQs in that league — ones that you reserve for the most important tracks in your mix, like lead vocal, main guitars, drum overheads, kick and snare, etc. My all-time favorite active EQ is the Great River EQ-2NV [Tape Op #45], but Dan Kennedy [#50] says he could never make the EQ-2NV in a 500-series format because of power supply constraints. While not quite as versatile as the EQ-2NV, the E27 comes very close to the same sonic mojo, and that’s high praise. I can safely say that this is the best-sounding 500-series EQ I’ve heard.