iFi's Pro iESL

This one has been a long time coming. I've spent a lot of time with this product, and now have a much more holistic understanding of both the technology and its sound. This is a very full-featured product, just as the Pro iCan and the Micro iDSD Black Label were. And as an aside, iFi continues to get massive props from me for paying insane attention to microscopic detail in all of their products. But we'll get to all that below.


Electrostatic headphones are a very niche market. With that niche market, comes a lot of relatively complicated technicality that I'll try to get out of the way here.

The fundamental way that electrostatic headphones operate is by taking an absurdly thin sheet of electrically interactive material (think of an incredibly light plastic bag, but about 10 to 20 times thinner) and placing it between two electrically charged plates. The bias voltage on these plates is massive, compared to the voltages on which most dynamic and orthodynamic headphones operate. The voltage, by numbers, is usually in the range of 550V DC. A current is then sent that alters the charge on the membrane in relation to the charge on the plates, allowing electrostatic attraction and repulsion to move the membrane without any mechanical interaction. This is part of why such low THD numbers are seen with electrostatic headphones, and why such great clarity and detail are heard. To get this bias voltage, though, a very large and very competent transformer is necessary. This is where this product, and other energizers come into the picture.

The iESL

There are lots of energizers on the market, many of which are absolutely spectacular, by all accounts. The iESL has a lot that separates it from the rest of the market, as is the common trend with all of iFi's products.


I want to get out of the way at the outset that this energizer does an incredibly competent job. When fed signal in solid-state mode, with all the settings correct for the given headphones, it presents the headphones faithfully, offering no noticeable distortion or coloration, whatsoever, aside from what the headphone imparts. That's the easy part, admittedly, but (from reading around on other products), some models don't even get that far. I want to get that out of the way because this product does a whole lot more, largely on the part of the iCan that I have powering it, but it is nonetheless very impressive. To do what I just described would be more than enough for this product to be a solid contender in the market, but what separates it from the crowd is what else it can do, and that's to what I intend to get.

Back to It

So beyond just powering electrostatic headphones with great fidelity, there's a whole lot else that this energizer can do. But before I get to all that, and sorry for the teasing there, I want to run down a bit of specs. There's a lot that separates this from the rest of the market in terms of acoustic features, but there's also a lot of very well thought-out technology inside that deserves mentioning.

iFi uses what they call a Pinstripe Permalloy Core Transformer. It has a very wide bandwidth of under 5 Hz to over 60 kHz, which is far more than is necessary for the range of human hearing. Additionally, it has very low distortion, below the standards of Siemans (Germany) and Peerless (US). These two capabilities, coupled, allow this energizer to perform its functions exactly as would be desired.

Also employed is a unique capacitive power supply, which iFi appropriately calls the Capacitive Battery Power Supply. Intuitive, right? The intelligent part of this is that the circuit is charged up to the require bias voltage, then shut off, entirely. This allows the bank of capacitors to essentially "float" (in iFi's own words) at the required voltage, leaving a greatly reduced probability of electrical noise interfering with the listener's experience.

Here's the really cool technical benefit: iFi has included a 4-pin XLR connection (that's right) to power dynamic headphones. iFi claims that the unique properties of the transformer circuit can elevate flagship dynamic driver-based headphones, but I regrettably did not have a pair of dynamic headphones with an XLR connection with which to test. That claim is left untested by me, but iFi has yet to fail, in my eyes, pretty much whatsoever, so I have faith that this was executed well.

Final note on spec sheets, the iESL can interestingly be used as essentially a passthrough for speakers, with (very high-quality, I'd note) banana plugs on the backside, four for inputs and four for outputs.

A lot of the above section is me quoting iFi, but they provide the best information on their own product, particularly in terms of technical aspects, so I did my best to rephrase in the way that was most conducive to understanding. I've been a consistently big fan of iFi in the recent past for, beyond just acoustics, going the extra mile (and in many cases, miles) to put the absolute best technology in their products, with little regard paid to the extra price. I highly recommend, to any prospective buyers or interested readers, reading the user manual and tech notes for this product, as both give a more in-depth look at the microscopic level of the technology employed in this product than I can, linked here and here, respectively.


The design mirrors, in many ways, the iCan, which makes sense. It has the same dimensions, and a very similar front panel setup, as shown. The left-most connector is 6-pin, for use with older Stax models. Center is a 4-pin XLR for, as previously mentioned, dynamic headphones. The right-most is a 5-pin Pro Bias connector for use with modern Stax headphones and the vast majority of other modern electrostatic headphones. The left-most dial is used to select the output, and to turn the unit on. Once turned on, the logo in the upper-left glows an attractive green. Moving from left to right from there, the next dial allows the user to set the AC termination. I'm going to directly quote iFi here on what different AC termination makes, as I don't feel entirely comfortable phrasing it differently:

AC termination affects the operation of the headphone, by either making the bias node that is shared between both channels high impedance, and low impedance for audio signals. This affects a complex set of parameters, but audibly mainly affects the presentation of the 3D soundscape. 

So there's that. Next over is the dial that controls the applied bias voltage. This is adjusted, very intuitively, to the bias voltage required by any specific headphone. Different electrostats require different voltages, and this can accommodate any that exist on the market, with 620 and 640 V options for future headphones that might require higher bias voltages.

As a note on the customizability of this energizer, you can actually play with the applied bias voltage to achieve a different sound. Although it is never advisable to exceed the recommended bias voltage for a headphone, moving lower will quiet the midrange, leaving the bass at around the same level. This can give a warmer sound, where moving closer to the recommended bias voltage will yield a brighter, possibly more balanced sound signature. Although this does play with what the headphone manufacturer may have intended, I take it very similarly to equalizing dynamic headphones, as it simply allows the user to more closely match the headphones to their personal taste.

iFi offers a valuable note on playing with bias voltage settings: dropping from a high voltage setting to a lower one will take considerable time, so it is best to start at the lowest setting and move up to the recommended voltage.

The last dial, right-most oriented, selects the impedance load of the headphones. Lower impedances will allow the headphones a higher volume, but are also an inherently harder load on the whatever headphone amp drives the iESL.

Speaking more specifically to visual design, the venting is exactly the same, semi-circular pattern that is seen on the iCan. There is also a looking glass, just like the iCan, that displays the capacitor bank, with a nice LED light.

Here's the attention to detail that I was talking about at the outset of this review.

When I first got this box, a worry popped into my head: there's a looking glass on the iCan, what if they don't stack properly? A canted iESL would not look too great, I don't think. Although I expected iFi to have accounted for this with a divot in the rubber platform on the bottom of the device, I didn't expect them to have branded it with the tubes from the iCan that would be seen through the looking glass that the iESL sits on. I've included an image below to clarify what I mean, since I understand that that sentence is a bit of a nightmare to parse. I know this may come across as me being over-impressed or over-eager to praise, but I was genuinely satisfied that iFi took the time to make a custom rubber press just to show exactly what model of tubes this device is sitting on top of. It's just cool. It's what I would expect, in an ideal universe, from a truly flagship product, but something that I haven't come to expect in our universe. Props to iFi, yet again.


Here's where we get to what I alluded to earlier. Although many of these benefits come sheerly through the iCan that I have powering it, it was iFi that made these benefits possible, so I will still credit them. The iCan is connected to the iESL via an included, branded HDMI interconnect cable. I've been asked by one of the iFi engineers to note that his HDMI cable is wired differently than standard HDMi cables, and that any prospective buyer should absolutely not use a standard HDMI cable, and that using any others could damage the device. This takes the signal that the iCan drives, and transfers it. The benefit here is that all of the incredible sonic features of the iCan can be enjoyed through the iESL. The XBass+ (with a whole lot of adjustment capabilities in this implementation), the 3D Holographic Sound (again, with varying levels of intensity and simulated loudspeaker angles) both come straight from the iCan into your electrostatic headphones. Additionally, and most importantly, when paired with the iCan, the Tube and Tube+ modes can be used. Although tame, the refined GE 5670s can be enjoyed by more than just dynamic and planar magnetic headphones. The more liquid, warmer Tube+ mode carries through just as well.

Although one might expect that, I really appreciate it. When paired this way, it makes what could be a very vanilla product much more diversely useable.

As a complete aside, sheerly for information's sake, I would like to point out that the volume of the energizer is controlled by the amp that feeds it signal, as there is no separate volume control on the iESL, and it does not take a line-level output from the amplifier.


This review is a little bit scatterbrained, and I apologize for that. This product, just like the Pro iCan, is a monster in terms of technology, performance, and design. It hit every mark, dead on, and that's the best praise that I can give it. It can power every electrostatic headphone on the market, and it can do the same for dynamics, and even some speakers. They took into account every potential design flaw or lapse, and accounted for every one that I could think of. They included an immense amount of flagship technology in their flagship device, and that's respect-worthy. They really hit the nail on the head with this product, in every regard. I was excited to see what iFi had in the works when I reviewed the Pro iCan, and I'm now even more excited to see what comes next.


This product was provided to me for review by iFi Audio. I am not being paid by anyone to write this review, to endorse the product reviewed, or for the content that I put in the review.

The signal for this energizer was provided by an iFi Pro iCan, fed signal by a JDS Labs OL DAC, fed signal by USB from my computer.

I have had this product for about a month, and have used it for considerably more than 100 hours, but I stopped keeping track after that metric.

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