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Interphase Audio EQ Modules

Interphase Audio EQ Modules

Compatible with both the 500‑series and the 51X modular formats, these classy Allan Bradford‑designed modules offer up analogue magic in spades.

Towards the end of 2017, Bruno Wynants, founder/owner of Belgium‑based Interphase Audio, and gear designer Allan Bradford, visited us at SOS Towers to demonstrate their new Ark modular analogue console, a headline feature of which is its ability to host 500‑series modules in line with its own channels. We found ourselves impressed, and Technical Editor Hugh Robjohns remarked that: “It appears to be a very well‑conceived and beautifully engineered console, designed with considerable flexibility in mind. The set of modules available at launch is equally impressive and should meet the needs of most clients, but there are advanced plans for additional future options.”

Interphase have managed to sell and install several consoles in a range of different configurations since then, despite the logistical difficulties presented by the pandemic, and have recently started to deliver on their promise of new modules. The first trio, reviewed here, are all equalisers, and there are plans in place to follow these up with a mic preamp and an intriguing multi‑type analogue compressor, amongst other things.


As shipped, the module’s rear card connector is compatible with 500‑series racks, but Interphase Audio can also provide a clip‑on adaptor to take advantage of the 51X format’s higher‑voltage power rails.As shipped, the module’s rear card connector is compatible with 500‑series racks, but Interphase Audio can also provide a clip‑on adaptor to take advantage of the 51X format’s higher‑voltage power rails.

As supplied, the three EQs, named the Iridium, the Carbon and the Helium, come ready to plug into any 500‑series chassis but, unlike most such modules, they aren’t limited to use in 500‑series racks: a small adaptor can be fitted onto the rear to provide the additional connectors required by 51X racks. Popular in the pro‑audio DIY community, the 51X format augments the usual 500‑series connections, adding ±24V power rails, which allow module designers to develop products with greater headroom — they can accept and output higher‑level signals. It’s perhaps not a surprise to learn, then, that one of the other products in the Interphase pipeline is a chassis capable of hosting 500‑series and/or 51X modules; to date I’ve only seen these as DIY projects or built on a very small scale.

All three of these modules are fully discrete Class‑A devices, whose beating heart is Allan Bradford’s Top Amp, a low‑noise, low‑distortion discrete op‑amp that he developed specifically to be more compact and better suited to mass‑manufacture techniques than the other boutique options out there. He tells me his design is guided by the ‘wire with gain’ ideal and that, in developing the Top Amp, he conducted extensive listening tests of a range of op‑amps, both IC‑based and boutique discrete; he says that on good speakers it’s possible to discern a significant difference between them. The chief practical benefits of his compact low‑noise design are that it can be manufactured at scale (great for building mixing consoles!), and it’s easy to accommodate several of them in small devices, such as these modules.

The review units were all very nicely constructed, exhibiting a solid feel and a slick finish. The circuit boards are populated with high‑quality components, of course, and are surrounded by a folded metal case with a small slot at the rear to allow the card connector through. Considering the number of knobs on the front plate, I was pleased with the amount of space available, and never felt as though my fingers would accidentally move an adjacent control. The aesthetic is pleasing to the eye too — very ‘hi‑fi’. That said, while the legends are clean and clear, the small black‑on‑grey typeface could make it tricky to read everything in a dimly lit studio, and when the units are placed below eye height much of the information is obscured by the tall knobs and their shadows.

Three Elements

The Iridium is essentially the same EQ as that created for Interphase’s Ark console — a four‑band inductor‑based design. Each band has a knob to switch the centre frequency and another that turns a ±15dB gain pot, so there are eight knobs in total, and the only other control is a button described on the panel with the words Discrete Class‑A. It’s an accurate description of the electronics, but this backlit (green) button is in fact an in/bypass control. When the unit’s in bypass, the signal still flows through the balancing/unbalancing circuitry.

This Iridium’s upper band can be set to 8, 9, 10, 12, 14 or 16 kHz; the higher mid band to 1.5, 3, 5, 6, 7 or 8 kHz; the lower mid band to 125, 250 or 500 Hz or 1, 2 or 4 kHz; and the low band to 50, 80, 100, 150, 300 or 600 Hz. Each band is a peak/bell type with a fairly broad curve (a shade over an octave, the peak becoming narrower the more you boost/cut), and it exhibits a pleasingly vintage sort of vibe, that I’ll describe subjectively below. Despite the coloration harking back to days gone by, audible noise is notable only for its absence.

Unlike the other two, the Carbon has no inductor EQ bands (it is based on resistor‑capacitor networks) but it features Allan Bradford’s Top Amps, and has very generous frequency overlap between bands.Unlike the other two, the Carbon has no inductor EQ bands (it is based on resistor‑capacitor networks) but it features Allan Bradford’s Top Amps, and has very generous frequency overlap between bands.The Carbon is another four‑band EQ, but this time all four bands are based on resistor‑capacitor (R‑C) networks. The two middle bands are bell/peak filters, while the two outer bands are switchable between bell/peak and high (top band) and low (bottom band) shelving filters. There’s the same bypass button, and two more such buttons (orange‑backlit this time) are used to select the outer band types. There’s much greater overlap between the bands than on the Iridium and the frequency controls are continuously variable rather than switches. So while the Carbon may lack the other’s silky charm, it is rather more versatile — and it sounds gratifyingly clean too. The top band can be set anywhere from 1.2 to 22 kHz, the upper mid from 0.5 to 8 kHz, the lower mid from 100Hz to 1.8kHz and the low band reaches from 440Hz all the way down to 25Hz. Again, each band’s frequency control is accompanied by a ±15dB gain pot.

Finally, we come to the Helium, which is a sort of halfway house, with the middle two bands being borrowed from the Iridium, and the outer two being R‑C types. The latter offer bell filters, identical to those of the Carbon, but this time they can be switched to become 12dB/octave high‑/low‑pass filters (rather than shelves). Given their generous frequency range, you can use these filters for all sorts of useful things, from rolling off low‑end rumbles, through ‘bracketing’ a source to sit it in a mix, to taming the side‑effects of generous boosts using those wide mid‑range inductor bands.


I tested all three EQs on a range of different sources in active projects over the course of several days. These included lead vocals, acoustic guitars, kick, snare and a drum bus, as well as some commercial mixes with which I’m familiar. I should note that, since I had only one of each module for review, most of these tests were performed on mono signals (although I did partner the Helium with the Iridium on occasion, to give a sense of the inductor EQ on stereo sources, albeit limited to two bands). There’s currently no stereo version of any of these modules (a double‑wide version with a single set of controls and more panel space could be interesting!) but Interphase do offer a discount for Iridium pairs.

My first impressions of all of them were great. They didn’t seem to add anything noticeable in the way of noise, and the control layout is about as intuitive as possible. By this I mean that although I was interested to note what settings I’d settled on, with just two knobs per band I was more than happy adjusting things and making decisions by ear and intuition alone.

After a short time, I generally found myself reaching for the Iridium first. Why? Well, there’s such a lovely fullness and smoothness to the high and low bands...

If you’re looking for the sort of richness and sheen that most digital EQs can’t supply, the inductor bands on the Iridium and the Helium will definitely appeal. While the Helium offers that bit more control, courtesy of its continuously variable R‑C bands and high/low filters, I soon found myself reaching instinctively for the Iridium. Why? Well, there’s such a lovely fullness and smoothness to the high and low bands in particular, and this not only made gentle rebalancing of sounds a joy, but it almost always seemed to get me where I wanted to go rather more quickly. This difference was perhaps best revealed to me when shaping a slightly dull‑sounding, fairly slowly strummed acoustic guitar. I was able to bring out all the ‘sparkle’ I wanted with no hint of grit or brittleness, and to tame some unhelpful boom quickly without any unpleasant side‑effects. It’s not that the Carbon or Helium sounded at all bad on the same source (they’re really nice EQs in their own right, the R‑C bands sounding very clean, yet somehow also ‘present’ and ‘lively’) but I found I needed to work that bit harder to achieve a similar sort of result, particularly at the top end.

Something the Iridium lacks is high‑/low‑pass filters, but in many ways it doesn’t really need them. Interphase’s philosophy is to design gear that performs one task exceptionally well and, these being bell bands, it’s not like you need to tame the far reaches of a generous shelving boost. Also, there’s not a huge amount of space left to accommodate more features. But with up to 15dB of gain available, it does limit how radical you can be at the extremes, unless you pair it with another device that does have filters.

It also has switched frequencies for all its bands, which the Carbon and the Helium’s top/bottom bands don’t. This has pros and cons, but for the most part I much prefer the switched settings. It makes recall and A/B comparison far easier and, with broader curves like these, I usually enjoy working with limited options anyway; I spend less time tweaking things that make very little difference so just find the nicest position to cut/boost, commit and move on. Naturally, though, the greater precision of the continuous pots is sometimes helpful, and not least when trying to massage acoustic drums into shape. On the one hand, I could often get a result I preferred using the Iridium — that silky richness when boosting the low end of kick drums in particular is a joy — but on the other, I sometimes found it tricky to pull out a boxy resonance that poked its head above the parapet when boosting. For example, a generous 60Hz boost sometimes meant a little 200Hz needed pulling back, and the Iridium can be too broad‑brush a tool for that job; in those circumstances, the Carbon and Helium were preferable, because I could centre that cut right where it needed to be.

The Iridium inductor EQ is derived from the four‑band EQ created for Interphase Audio’s modular Ark console.The Iridium inductor EQ is derived from the four‑band EQ created for Interphase Audio’s modular Ark console.

It’s easy to see why the Helium is the ‘factory choice’ for the EQ on the Ark console: recording or mixing on a console laden with these on every channel would be a delight. But the Ark can accommodate two 500‑series modules in place of that design (which is twice the heigth and uses faders for the gain), and I can see the attraction of putting an Iridium and one of the other modules on a handful of channels: they’d make a very powerful pairing, just as they could in your 500‑series rack. To some extent, of course, that’s precisely what the Helium aims to deliver: the best of both worlds. It largely succeeds in that too. On many sources, not least lead vocals, the two inductor bands on the mids deliver a silky touch of midrange magic, while the outer bands can be used to sculpt the top and bottom ends. The generous frequency ranges of its high‑/low‑pass filters make it really versatile if you enjoy doing all your mixing in the analogue domain. Still, there were definitely scenarios in which I preferred the Iridium’s high and low bands. Of course, bank manager permitting, the two modules could make powerful partners, combining to form a six‑band inductor EQ with high‑ and low‑pass filters.


So, there we have it. This trio of high‑quality discrete Class‑A EQ modules delivers exactly what their manufacturers claim: three different takes on the ‘console EQ’ concept which work well on their own, but might also be useful in combination. For me, the Iridium steals the show, with the beautiful, effortless elegance to its sound. Occasionally, I wished it had either shelving or cut filters. I had the luxury of achieving that by routing its output to the Carbon or Helium, but it’s also worth me mentioning that the same inductor EQ circuitry is also available in three different 19‑inch rackmount products — which happen to include high‑ and low‑pass filters, as well as a Swing (in other words ‘tilt’) EQ.

If you’re in the market for a 500‑series EQ, and particularly if the idea of a 51X‑compatible EQ appeals, then I’d heartily recommend checking out these modules. I look forward to seeing what Interphase release next!


  • All sound lovely, but the inductor bands particularly so.
  • Stylish and beautifully constructed.
  • Great to have different options.
  • Compatible with 51X and 500 series.


  • Legends could be easier to read.


This classy trio of EQ modules all sound great, but the ‘analogue magic’ lies mostly in the inductors of the Iridium and Helium.


Carbon €690 each. Iridium €785 each or €1446 per pair. Helium €835. Prices include VAT.

Interphase Audio +44 (0)1626 650 017.

Carbon €570 (about $550) each. Iridium €650 ($620) each or €1195 ($1140) per pair. Helium €690 ($660).

Interphase Audio +44 (0)1626 650 017.