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iFi Audio Pro iESL - The Official Thread

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  1. The Pro iESL
    First among energizers

    The Pro series iESL is an unsurpassed energiser for electrostatic headphones (and transformer-coupled stage for flagship dynamic headphones too).

    The Pro iESL driven by either the iFi Pro iCAN or a powerful amplifier takes electrostatic headphones to an unprecedented level of performance.

    Its defining characteristic is a transparency and an ultra-wide dynamic range that is totally natural. The Pro iESL works with Stax, Sennheiser Orpheus and many more.

    Classic & Cutting Edge Technology

    The Pro iESL’s three key elements combine the very best in classical approaches with the latest in cutting-edge technology AND superb component quality to deliver the ‘perfectly matched’ headphone experience.

    • Atypical Pinstripe Permalloy Core transformer
    • Bias voltage generator
    • Capacitive Battery Power Supply

    Outstanding Component Quality
    • One-off, hand-wound, Pinstripe Permalloy Core Transformer
    • Wima Capacitors
    • Vishay MELF resistors
    • Gold-plated silver and sealed-silver alloy contact relays


    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)

    The retail price of the Pro iESL is US$1,399 (ex-tax) or €1,575/£1,395 (incl VAT).

    Stay tuned, tech stuff coming up shortly!
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    Fastnbulbous, XSAMURAI and Tom Blake like this.
  2. Tom Blake
    @iFi audio - great to see the details on the iESL at last. It is a very ambitious design, much more so than expected. I look forward to hearing it soon! I do have one question - when will the Pro series iRack be available? With 2 components in the line now available it will be useful. Please don't wait until the iDSD Pro is available to release. Until iRack is available is it okay to stack Pro iCAN and iESL? I assume Pro iCAN should be on top with how warm it runs?

  3. Whitigir
    I can try and compare this against Kevin Gilmore Grounded Grid :D . Good to see new manufacturers getting into ES
    Last edited: Jun 10, 2017
    FLguy likes this.
  4. Tom Blake
    I have a friend with a KGSS (I think). We're getting together to do some listening soon and I will bring the Pro iCAN+iESL combo to compare. If the Pro pair can get close to some of the Kevin Gilmore estat designs that would be outstanding!
    Whitigir likes this.
  5. Matez
    That HDMI interface looks interesting from a cable maker's perspective :ksc75smile:
  6. iFi audio
    Pro iESL and Micro Seiki
    Modern Marvel meets Vintage Classic

    Check out our Pro iESL rubbing noses with a pair of vintage Micro Seiki headphones. We estimate these are between 30 – 40 years old…

    Micro Seiki is best known for their turntables. When they decided to make a set of electrostatic headphones they went to the market leader, Stax.

    The headset seems based on the SR-X while the power supply seems a variant of the SRD-7 "Energiser".


    Unlike the Stax branded products, the MS-2 sports high-compliance polyurethane drivers are set with micro-photo etching for excellent uniformity and improved power-handling linearity. The MS-2 weights in at only 240g and boasts a frequency response up to 25kHz

    The MP-1 works with a 9V battery lasts ~2000 hours. It is intended to plug into a speaker amplifier and has a switch to alternate between speakers and headphones.
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    Tom Blake likes this.
  7. Tom Blake
    @iFi audio - Got my iESL today and have a few questions. I am using it with my Pro iCAN via the provided ESL link cable and Stax SR-L500 earspeakers...

    1. Can you please address the stacking question I posted above? There is basically no choice but to stack the 2 units (with iCAN on top) due to the short ESL link cable, so I assume it is okay. Hopefully the iRack for Pro sized components is coming out soon :)

    2. Manual says iESL LED is orange but mine is green. Is that expected?

    3. I really have to turn up the volume level on the Pro iCAN to get decent volume on my Stax with Pro bias settings. I don't notice much difference between the 96 ohm and 64 ohm Impedance settings on the iESL so am currently running at 64 ohm (the output is noticeably louder at 24 ohm). At 64 ohm impedance and the Pro iCAN at Med gain I nearly have to max out the volume control. At High gain I am running between 12-2 o'clock. With impedance set to 24 ohm Med gain is more than adequate. Is this expected performance for the Pro iCAN + iESL combo via iESL link? Any issues with running at 24 ohm impedance and current gen Stax Lambda series? I rarely have XBass set higher than 20 Hz and 3D usually off or at the lowest setting.

    4. Any burn-in time recommendations? I ask only because the Pro iCAN had such long burn-in time suggestions for each circuit.

    So far it sounds like a big improvement over my Stax SRM-252S energizer but I will wait to make major conclusions until after it has had some burn-in time.

    XSAMURAI and Mshenay like this.
  8. iFi audio
    Tech Notes - The Pro iESL part 1
    A little background

    The Pro series iESL is the best way of driving electrostatic headphones. We are confident that 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.


    What is an electrostatic headphone amplifier and why do I want one?

    We have quoted from Wikipedia:

    Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes). The electrical sound signal is applied to the electrodes creating an electrical field; depending on the polarity of this field, the diaphragm is drawn towards one of the plates. Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated. Electrostatic headphones are usually more expensive than moving-coil ones, and are comparatively uncommon. In addition, a special amplifier is required to amplify the signal to deflect the membrane, which often requires electrical potentials in the range of 100 to 1000 volts.

    Due to the extremely thin and light diaphragm membrane, often only a few micrometers thick, and the complete absence of moving metalwork, the frequency response of electrostatic headphones usually extends well above the audible limit of approximately 20 kHz. The high frequency response means that the low mid-band distortion level is maintained to the top of the audible frequency band, which is generally not the case with moving coil drivers. Also, the frequency response ‘peak’ regularly seen in the high frequency region with moving coil drivers is absent. Well-designed electrostatic headphones can produce significantly better sound quality than other types.

    Electrostatic headphones require a voltage source generating 100 V to over 1 kV, and are on the user's head. They do not need to deliver significant electrical current, which limits the electrical hazard to the wearer in case of fault.


    Let's put this theory into perspective with some figures,
    • Sennheiser HD800 dynamic headphones = 102dB/1V
    • Typical pair of electrostatic headphones = 102dB/100V (!!!)
    Traditional headphone amplifiers do not deliver either the EHT voltage or the required audio voltage. So a special ‘energiser’ is required to match the electrostatic headphones. Typically a maximum output of 300V is needed.This is equivalent to 300 Watt into a 300 ohm headphone load!

    Hence, electrostatic headphones require:
    • VERY high voltages – need 300V in an instant
    • BIAS voltage – adjustable from 230V to 640V

    There are two options to make such a product to ‘energise’ electrostatic headphones:
    • Product A: a dedicated high-voltage amplifier.
    • Product B: an energiser, a solution which is almost opposite to a tube amp as it converts low-voltage to high-voltage.
    In the real world, few amplifiers can deliver these levels of voltage output which is why an ‘energiser’ is required to ‘charge’ electrostatic headphones. Some existing tube amplifiers (like the iFi RETRO Stereo 50) could be modified to drive electrostatic headphones directly and some solid-state high voltage amplifiers also exist, however the need for such extraordinarily high-voltages compromise the active ‘direct high-voltage’ of most amplifiers.

    By comparison, an energiser can use a standard speaker amplifier or a suitably powerful headphone amplifier (> 10V into 16 ohms or > 20V into 64 ohms are needed) and use transformers to produce the high-voltage required.

    The iESL is the second type of a product: an energiser. Yet the iESL is no ordinary energiser. Each one of these three-core components were developed using the best classic and latest approaches with scant regard for cost, complexity or difficulty:

    • The transformer
    • The bias voltage generator
    • The component quality

    ... and let's stop here for now :)
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    XSAMURAI likes this.
    @iFi audio
    how much is the output voltage through 4pin xlr ?
  10. Roen
  11. ptolemy2k6
    1 imagine stacking question is a simple answer. it seems iesl stays cool and pro ican warm, so put warm on top for better air flow :)

    2. not sure.

    3. i imagine impedance will vary between headphones, especially stax and other electrostatics, so find one you like?

    4. good question. i want to know as well.

    I dont have kse1500 but based pictures, it looks like a lemo connector, so for one you'd need an adapter first. per kse1500 pdf, they use 200v bias. i believe iesl starts at 230v for stax eshp.. i am not an engineer, so don't know what it means, but from layman, it seems voltage won't match?
  12. rumina
    @iFi audio

    may i ask how the balanced dynamic headphone out is realised. is it a simple voltage divider or is ist also transformater coupled?
  13. Roen
    That's exactly what I'm getting at and would like ifi to answer.
  14. iFi audio
    1. Stacking is fine as long as iCAN is on top as it gets hot.

    2. To clarify: it should be green. Green is good.

    3. A few things need to be accounted for.
    • In 16 ohm mode, the iESL will turn around 23V max from the iCAN Pro into 736V signal, if the iCAN Pro receives sufficiently high signal levels and/or gain is set sufficiently high.
    • This is way more than the 300V maximum rating from Stax. Using 64 ohm mode will instead produce 368V, a little higher than Stax Spec., but safe.
    • Al the lowest gain setting the iCAN Pro has no gain at all with volume to max, so 2V in from a normal DAC would be equal to 2V out (4V balanced), giving only 64V max in 64 Ohm. Naturally this would be on the low side.
    • Stax headphones are generally rated at 100dB/100V, so with 300V around 110dB are possible.
    • Only at the highest gain setting is it possible to drive the iCAN Pro to maximum output using standard 2V sources.
    • Depending on the source you can either increase iCAN Gain leaving the iESL at 96/64 Ohm, or you can leave the iCAN Pro gain low and crank it up at the iESL Pro end.
    • Just combining high gain and 24/16 ohm Settings are not recommended to avoid overloading these rather expensive electrostatic headphones.
    4. The most significant changes happen in early hours. 100 hours is the right spot to start listening.

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  15. Tom Blake
    @iFi audio - I continue to enjoy my iESL partnered with my Pro iCAN. It sounds fantastic on my Stax L700's and drives them effortlessly at the +9 dB gain setting on the iCAN. I am curious, however, why the Stax output jacks are flipped upside down. The kink it introduces into the flat ribbon cable is highly annoying. Seems like a strange design choice but maybe it was driven by the internal layout of the iESL?
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