• Standard 500 series modular size 1.5″ x 5.25″
  • 3-bands with 9 selectable frequencies per band
  • ±16 dB of boost and cut using continuous variable potentiometer
  • Push button shelving option for low and high frequencies
  • All discrete design
  • High headroom
  • Low noise
  • Jensen transformers couple the input & output
  • Custom machined aluminum knobs
  • Gold plated rotary switches and 15-way connector for long life
  • Utilizes 1122 opamps

E27 Modular Equalizer


The E27 was created for the need of a high quality modular equalizer. High performance, versatility, ease of use, and simplicity is the initial core of the E27’s design. Some of the best analog gear from the past was the inspiration for its character in sound. Outstanding quality and most suitable parts are used throughout – gold-plated switches, high performance capacitors, high-quality transformers, and custom machined aluminum knobs.

Today’s engineers deal with many different types of recording equipment from older high-end analog gear to the newest digitally controlled recording equipment. With this in mind, the E27 can interface easily with many types of gear including balanced or unbalanced sources, high or low impedance loads, even unbalanced loads.

The level knob controls up to 16 db’s of cut and boost but for those who only need a few db’s, the travel of the pot starts slow and speeds up at the extremes giving the user more fine control at levels of 6 db’s or less. Knob positions are not only easy to see from a distance but are also felt – the level pot knob has a small point so you know where you are without looking at the eq. The outer frequency control knob has a knurl texture for ease of grip.

Frequencies (Hertz)
Low Freq Mid Freq High Freq
33 680 6K8
63* 800 8K2
82* 1K2 10K
120* 1K8 12K
200* 2K5 14K
300* 3K 17K5
370 3K8 20K
430 4K6 24K
550 5K6 28K

* These low frequencies are shelving capable.
All high frequencies have an optional shelving switch.

Input Impedance 13KΩ transformer balanced
Output Impedance Less than 45Ω
Nominal Operating Levels −15 to +15dbu
Frequency Response −0.2dbu@10hz, −3dbu@58khz
Power Requirement ±16 VDC @ 85mA Total
THD+N (full bandwidth unfiltered) 0.004% @ 0dbu unequalized
Intermodulation Distortion (IMD) 0.004%, unequalized − 1.5% max equalized
Filter BW (Q) ≈0.85−1.1
Max Output Level 600Ω load +27.2dbu @ ±16V
Max Output Level 20kΩ load +28.5dbu @ ±16V
Connector 15 pin 0.156″ spacing
Recall Sheet



Why frequencies up to 28kHz?

When you boost or cut a chosen frequency, it is shaped like a bell unless you push in the shelving button – then it’s half a bell where the top of it extends beyond the selected frequency. The top of the bell would be 28kHz, for example, but the curve leading up to the top starts at frequencies within the fundamentals, or within 20khz. So you will easily hear the effect at the very tail end of the audible spectrum without accentuating siblance or already aggressive high frequency in the source. After you hear it, it will be part of your eq vocabulary.

Is it a clone or a reproduction of anything?

The E27 is not a clone or a reproduction of anything else. Certainly some things about it have been influenced by gear of the past and present, but the overall design is unlike any other.

Is there a characteristic sound to this EQ?

One way to tell is to have some music playing on your favorite monitors, and suddenly patch the eq in the chain, but without any boost or cut and even with the IN button not engaged. Listen carefully and what you will hear is the characteristic sound. Though the frequency response is flat when no eq is used, an increase in depth is often noticed with this simple test.

Nothing is transparent; everything has a sound.

Will your E27 equaliser plug into my rack?

The E27 should fit and work in all 500 series major brand racks, lunchboxes, consoles, and rackmounts like Brent Averill Enterprises 6 & 11 space rackmounts, and API Audio racks as part of the VPR Alliance. If there are any questions, please email us.

How narrow is the bandwidth?

The bandwidth (also called Q) varies depending on how much boost or cut there is. The more you boost, the narrower the Q, going from a fairly a broad bandwidth to medium.

Here is a graph of this Relative Q feature.

Why don’t the frequencies overlap?

Overlapping frequencies are popular with parametric eq’s which typically have a bandwidth control, but overlapping is not necessary for stepped frequency type of eq. When you overlap with a stepped eq, you force each band to have a wider range of frequencies than you really want. What if you want to adjust a siblance at 5K6 but still want to boost a little bit at 20Khz- if both of these frequencies are on the same band then you cannot use both. It’s much better for this type of eq to keep the bands separate but have the frequencies side by side.

It did not seem reasonable for this unit to putting 200 or 300hz in the midband and 1k2 in the low frequencies which may work for some things but limit more in other areas making it a heavy trade-off. The E27 does a great job of covering lots of areas between 33Hz – 28kHz

Can I also use it as an impedance converter? Or to interface unbalanced to balanced gear?

Yes. Because the input impedance of the E27 is 13k ohms and the output impedance is less than 45 ohms, this means that the E27 could interface two unlikely pieces of gear.

For example, say you had a CD/MD/DAT player that has unbalanced outputs and you wanted it to drive an old compressor or equalizer you have. The problem here is that the output of the CD player cannot handle a heavy load of 600 ohms and when you do connect them, you will likely find that you lose low end and the high frequencies don’t sound very good. You can connect an E27 in between them, and even with the eq button not engaged you still go through the transformers and the 1122 opamp, which now has enough output current to drive 600 ohms easily.

Another example is, say you have a console or outboard gear that has electronically balanced outputs and you need to interface it with something that is unbalanced, and let’s say that it’s that old compressor or equalizer again. Some electronically balanced outputs can handle being unbalanced if it has been implemented in the design, but some cannot deal with being unbalanced in any way. Even unequal impedances to ground in a balanced system, like passive attenuators using potentiometers can cause problems. If you try pluggin the two together, what you will hear is a loss of level as well as a loss of low frequencies because what you are doing is driving one of the push-pull opamps into ground – and it does not like that. Put an E27 in between the two, and due to the transformers on the input and output, everything is happily working together.

So what you are saying is that transformers are a good thing? What about those ads about "transformerless" circuits?

Transformers are invaluable for audio. They solve many real world problems. Well made transformers are inherently quiet (they are passive) and have a very high common-mode rejection ratio (CMRR- this is how well it rejects noise in a balanced line), they can be run unbalanced without needing any extra help, can handle high levels without severe clipping and they have a great range of characteristic sounds and applications. However, good quality transformer cost a lot more, they are heavy and take up lots of space in circuit boards, and not as readily available. These are not things you could easily clone overseas. Although there are very well made professional transformerless designs, some companies have decided to try and make “transformerless” a thing to desire – this is not a move for better sound but to cut cost in parts.

Much of the older or “vintage” quality gear which contain tubes are sought after because of their great sound but the key factor making those units sound so good isn’t much the tube as it is a fact that older designs have transformers.

But transformers are not made equal, and like many things, you can have good quality, poor quality and everything in between. A well-made transformer, designed and applied specifically for the job it will do, is itself the most valuable component even after many years of use.

Bell vs Shelf using the two push-buttons at the bottom... Explain.

The Low and High frequencies have a Shelf option which can be used to affect the band before the chosen frequency when using the Low band making it a great adjustable High-Pass Filter, or it could affect the band after the chosen frequency in the High band for larger scale tone shaping. See Graphs.

On the Low band, why does the Shelf start at 63Hz and not at 33Hz?

If you are at 33Hz, the -3db point with the bell is near 15Hz so it’s not really practical to have shelving at 33Hz with this eq. As the graph can also show, having a Shelf at 33Hz would not do any good.