Pro – iESL

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Ultra-wide Bandwidth Hand-Wound Permalloy Core Transformer Capacitive Battery Power Supply Perfect Matching
Impedance, Bias and Termination
Orpheus, Koss
KS compatible


The Pro series iESL is the best way of driving electrostatic headphones.

The iESL Pro driven by either the iCAN Pro or your existing amplifier can go toe-to-toe with the best amplifiers out there for electrostatic headphones. Its defining characteristic is a transparency and an ultra-wide dynamic range that is totally natural.

Imagine that your electrostatic headphones are like the top of the range Porsche. A Porsche engine requires a lot more power than your standard car. So do electrostatic headphones! They need to be ‘charged’ or more accurately, ‘energised’.

Traditional amplifiers do not deliver the either the EHT (extra-high tension) voltage or the required audio voltage that electrostatic headphones need. A special ‘energiser’ is required to ‘energise’ the electrostatic plates in the headphones, typically a maximum output of 300V!

The Pro iESL makes use of 3 core components, all developed using the very best classic approaches combined with the latest in cutting-edge technology, to deliver the ‘perfectly matched’ headphone experience.

One-off Pinstripe Permalloy Core Transformer
Bias Voltage Generator
Capacitive Battery Power Supply



The Pinstripe Permalloy Core Transformer (PPCT) – Making a transformer with the wide bandwidth and low distortion needed in the iESL is very challenging. To attain this, the custom transformers use complicated hand-wound, multi-section windings with both vertical and horizontal sectioning. They exceed the standards set by Siemans (Germany) and Peerless (USA). With a bandwidth of < 5Hz to > 60kHz (-3dB), transformers in the iESL far exceed the requirements of even the JAS High-Res Logo.
The Capacitive Battery Power Supply. They are charged up to the required voltage and then the charger circuit is turned off completely, leaving the capacitor bank to ‘float’ at the required bias voltage. An essentially perfect bias voltage source with no electronic noise whatsoever at any frequency as the headphones are in essence, powered by the battery bank and not the mains!
Wima Capacitors – Another quality component and perfectly suited for the capacitive power supply. These 1,000Volt rated capacitors are from Germany. The key is the very high insulation resistance and minimal current leakage, as any such leakage causes self-discharge. The whole capacitor pack has around 35,000,000 kiloOhm insulation resistance, which means that self-discharge time (to 30%) is around 100,000 seconds or 27 hours. So one is always listening on battery (not mains) power.
Vishay MELF resistors – MELFs offer superior SMD resistor performance in terms of accuracy, stability, reliability, and pulse load capability. The cylindrical construction of MELF devices provides the optimal power rating and pulse load capability need for the iESL. MELF are more complex to make and so cost more. When assembling PCBs, MELF resistors require special machines and treatment but at iFi we do not shy away from such complexities if it means higher component quality and ultimately ne plus ultra sonics.
Elevate flagship dynamic headphones – The unique transformer properties gives the iESL an ear tingling transformer coupled-stage that is unsurpassed. So each and every flagship dynamic headphone when used through the iESL enjoys a whole new level of smoothness and airiness.


Maximum Output voltage: 640V RMS (16Ω/20V in)
320V RMS (64Ω/20V in)
Frequency Response: 5Hz – 50kHz (-3dB)
Input Voltage (Pro iESL): 5V – 9V/1A max
Input Voltage (iPower): AC 85 – 265V, 50/60Hz
Power Consumption: < 1W
Dimensions: 213(l) x 206(w) x 63.3(h) mm
Weight: 2.5kg (5.5 lbs)
User Manual Tech Note
The iESL has a lot that separates it from the rest of the market!

“I was very impressed with the iESL. It benefitted dramatically from a lowering of noise which opens up the upper midrange frequencies to new and complex detail…the iESL is packed with value too. Any electrostatic headphone fan needs to grab one!”

Many thanks Paul Rigby (The Audiophile Man) for your excellent review


The iESL has a lot that separates it from the rest of the market!

“…so much extra beauty breathed in the Pro iESL. Thanks to the Pro iESL, you can now enjoy equally superb sound quality through electrostatic headphones.”

Many thanks HiFi Philosophy and iFi Poland (Moje Audio) for this excellent review

Translated with Google


“Pro iESL – We can reach the new attraction of headphones that we have not noticed before!

“…common trend with all of iFi’s products. They really hit the nail on the head with this product.”

Many thanks hearfidelity for your wicked review.


“Pro iESL – We can reach the new attraction of headphones that we have not noticed before!

“Pro iESL boasts contents that can be said to be the definite version of such a capacitor type headphone adapter. It is an item where you can enjoy the depths unique to condenser-type headphones.”

Many thanks Phileweb and iFi Japan (Top Wing Corporation) for your excellent review.

Translated with Google


““With the recommended settings, everything is fine. There is a wonderful detail and distinct dynamics and most surprisingly, a comfortable musical balance.”

What on earth is Tarasov from Salon AV on about?

(Drum roll)

The Pro iESL of course which also got the Expert’s Choice award!

Thank you Sohu for your review.


Pro iESL Main Accessories
• HDMI cable
• ipower 9V
• Adapter for iPower
• Warranty Card
• User Manual
• STS Card