iFi Pro iESL Electrostatic Headphone Energizer Review

Discussion in 'Headphone Amplifiers and Combo (DAC/Amp) Units' started by Marv, Jun 5, 2021.

  1. Armaegis

    Armaegis Friend

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    That's why I want to know if the dynamic output has any gain, and/or if it is affected by the impedance/gain knob.

    edit: in my head it would make sense if the impedance/gain knob adjusts the primaries, and the estat and dynamic outputs are just on separate secondaries... this is a wild guess on my part though.
    edit2: probably more complex than that since there are four impedance settings but only two "gains", so there might be multiple things switching around, I dunno.

    edit 3: I know this is wrong, horribly horribly wrong, because there has to be some sort of lumping in order to keep gain the same for the top two and bottom two impedance settings, but this is sorta how I picture it:
    upload_2021-6-7_13-6-18.png
     
    Last edited: Jun 7, 2021
  2. purr1n

    purr1n Building Magnis part time because it's peaceful.

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    Good enough. I've been convinced on the basis of gain. Measurements pending.
     
  3. Doug_D

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    REALLY interesting review. Thank you for sharing.

    I hadn't really considered the iESL seriously before, and now I'm thinking it's definitely worth a hard look. It takes a big chunk of cash and a leap of faith to jump up to the primo level of custom electrostatic amps, and the top Stax amps are also pricey but mostly not well-loved. Plus I have some vintage normal-bias Stax h/ps and would like to try the Koss. The iFi Audio box offers a lot of flexibility at, compared to the alternatives, a very good price point.

    I'm not enough of a gear expert to recognize your so-called "flea-powered headphone amp" ... powerwise, how does that unit compare to say a Jot-2 ?
     
  4. Marv

    Marv Super Friend

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    My custom amp utterly destroys the Jotunheim 2 from a subjectively point of view with most headphones and certain high-efficiency speaker setups. Well, it better considering the major parts, the interstage, the output transformers, the tube types, were picked out by me to suit exactly what I wanted - much of this being learned over the past 5 to 7 years working with Craig at EC. Craig's the genius, I'm just the weekend hack.

    Tonally both amps are similar, with the Jot 2 being just a tick warmer. With respect to timbre, slam, microdetail, and especially microdynamics and head/soundtage, it's no contest. The Jot 2 will output more power at much lower distortion. When it comes to objective measurements, the Jot 2 utterly destroys the tube amp.

    Either I like the sound of distortion, a specific distortion pattern, or that super low distortion is vastly overrated. FWIW, I also feel that amps with distortion less than Jot 2 / Magni 3 levels, those ultra-low distortion amps, actually start to sound worse. 100db of feedback makes stuff sound perfectly insipid. Think Nicole Kidman. Absolutely gorgeous, but I don't get a boner.
     
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    Last edited by a moderator: Jun 7, 2021
  5. Marv

    Marv Super Friend

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    Will do in a bit.
     
  6. Doug_D

    Doug_D Rando

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    Thanks Marv.
     
  7. Biodegraded

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    Using your measurements with the Hive, sadly it seems it's not as simple as powers of 2. Here's the ratio between the iESL's 64 & 16 Ohm settings; I'm sure somebody can derive a relationship, or better, can come up with one from EE first principles:

    [​IMG]

    Here are predicted linear distortions for the Hive at the iESL's 16 & 64 Ohm settings for amps of four different output Z (4R, 10R, 14R, 26R) relative to a notional 0R amp (horizontal line). The bottom set of curves corresponds to the lower (tan) impedance curve (Hive measured with iESL at 16 Ohms / x32 gain setting); the top set corresponds to the upper (green) curve (iESL at 64 Ohms / x16 gain setting).

    [​IMG]

    The smaller deviations at the higher iESL impedance setting might seem counterintuitive given the shapes of the headphone's impedance curves, but that's the way the voltage-divider formula works (it's the relative amplitudes that matter). So if you have a higher output-impedance amp, you might think it'd be better to use the iESL's higher-impedance setting - but if the amp is a SET thing that's no great powerhouse (like Marv's example of the EC45 in post #14), you'll likely need to use the lower impedance / higher gain setting to avoid clipping at listening volumes.
     
  8. Marv

    Marv Super Friend

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    1. Basically plug DAC into source preamp/amp or integrated amp. In this case, my EC 45.
    2. From the amp speaker outputs
    DSC02320.JPG

    3. To the iESL speaker inputs
    DSC02322.JPG

    Note the speaker outputs on the iESL are for bypass if you want to listen to speakers. The idea is that this is a box that sits between your amp and speakers that can also be used for electrostatic headphones. The balanced input deal on the right side is a connection supported by iFi amps like Pro iCan to go directly to the iESL.

    4. Front panel provides three outputs: Normal (ancient) bias STAX, Pro (selectable higher bias), and regular headphones (that you happen to want to use your power amp for). Plug in headphone here.
    DSC02324.JPG
     
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  9. Degru

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    Interesting, I've been using an SRD-7 with a custom bias supply board paired with an Adcom GFA-545ii as my main Stax driver. Recently, I had the guy responsible for the custom bias board build me a rather cursed upgrade: a pair of Lundahl LL1630-PP's on an external PCB, attached to the main board via a pair of 5-pin XLR's. The original Stax transformers were also put on their own board, for comparison (and having some higher-ratio transformers on hand for lower gain amps)

    https://i.imgur.com/hl8QfUd.png

    The stock Stax transformers are supposed to be about 1:25 ratio. The Lundahls are specced at 7.2:1 ratio in their intended use case, tho obviously this is reversed here. I measured almost exactly 11db decrease in volume at the headphone when switching between the transformers, which seems to correlate to the difference in ratio if my math is right.

    Using my 404LE, I could definitely notice the improvement in sound. While I would strongly disagree with the impression of stock transformers sounding like "syrupy, non-resolving goo", I did notice a not insubstantial improvement in resolution and distortion/coloration with this upgrade. The Stax transformers had a substantial amount of bass distortion which disappeared on the Lundahls (I am thinking this is partially down to the easier load on the amp as well). There was a coloration to the mids that went away. The treble also got quite noticeably more airy and well defined, which is interesting since the FR measurement shows the Lundahls having gentle rolloff of about -1db at 20khz compared to the stock. Now I get a sense that I am listening to the amp directly with nothing in between; not bad for a total of ~$600 combined.

    Also, I am thinking there is some calculation missing between the measured 350 or 1500 ohm impedance curve in this thread and the actual load that the amp is driving. The stock Stax transformers I have were very clearly running into current limitations of my amp in the bass at very loud volumes, and I get about 6.4 ohms across the input when measured with a multimeter with Stax plugged in and powered. Seeing how iFi specifies the input impedances as 16 ohm, 64 ohm, etc. I'd suspect those graphs need a closer look.
     
  10. Marv

    Marv Super Friend

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    I get a DCR of 11.8-ohms across the input with the iESL set to (x32 gain) "16-ohms". For another perspective on the impedance curve, I've set the range from 2Hz (instead of 20Hz) to 2kHz. 2Hz is the lowest I can go. Extrapolating to DC, the 11.7-ohms measured with the ohm-meter seems very reasonable. These findings seem to be consistent, the same range, with your DCR reading of 6.4 with the Lundahls. With transformers, it's usually best to think of any resistance spec rating from the manufacturer with a grain of salt, a usage guideline for consumers, and not literally. As we can see, the real answer is much more complex. And regardless such impedance curves don't have a huge effect with frequency response per @Biodegraded's calculations.

    Koss ESP950
    Impedance at iESL speaker inputs 16-ohm setting (x32 gain per spec)
    11.7 ohms DCR with DMM
    upload_2021-6-7_20-22-3.png

    Koss ESP950
    Impedance at iESL speaker inputs 64-ohm setting (x16 gain per spec)
    25.7 ohms DCR with DMM
    upload_2021-6-7_20-25-46.png

    Now whether this behavior is more attributable to the electrostatic driver or the transformer, I am not sure. I suspect the latter, at least with < 20Hz - primary inductance?
     
    Last edited: Jun 7, 2021
  11. DrForBin

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    hello,
    @Marv, thanks ever so for the pix!
    cheers!
     
  12. Tachikoma

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    @Degru it sounds like you're running your stax transformers into saturation in the bass region. Whats the dcr measurement without anything plugged in? If its on the lines of 0-1 ohm you may want to add a couple more ohms of dcr, to lessen saturation fron dc offset.

    I think the Lundahls have reasonably high dcr by themselves, if I remember correctly.

    fwiw the sowters i run are spec'ed at 1:50 and all of the amps i've tried have no bass distortion (including a 10w class d topping) once i add a little dcr.
     
    Last edited: Jun 7, 2021
  13. Degru

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    If I measure the input of just the Stax transformer itself full disconnected from the SRD-7, I get about 0.7 ohms. However, once it's connected and I measure the SRD's input terminals, I get 6.6 ohms with or without the Stax connected. It seems Stax already thought of this.

    By comparison the Lundahls measure 5.5 ohm on their own and 11.4 ohm in the SRD.
     
    Last edited: Jun 8, 2021 at 12:50 AM
  14. Tachikoma

    Tachikoma Facebook Friend

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    I guess the Stax transformers are simply too small to handle bass.

    The Lundahls may sound even better without the SRD's input resistors - the extra 6 ohms on the primary might explain the rolloff at 20 khz.
     
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  15. Marv

    Marv Super Friend

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    Part of this could be because of the SR009, which I believe is less sensitive, harder to drive, than the Lambda 404LE. I've heard the 404LE through the SRD-7 driven from a Leben CS600, and it was pretty good. The owner of the system understood what he wanted and had a "goal" in mind.

    I didn't realize the Lundahl LL1630s wired in PP could be used in reverse as an electrostatic step-up. Cool idea.
     
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    Last edited by a moderator: Jun 8, 2021 at 7:56 PM
  16. iFi audio

    iFi audio MOT iFi Audio

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    @Marv I'm digging through that content and am super happy with your findings! Thanks a ton!
     
  17. Armaegis

    Armaegis Friend

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  18. iFi audio

    iFi audio MOT iFi Audio

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    Yep, that's our official shot I believe, we never were secretive about Pro iESL's inside ;)
     
  19. Armaegis

    Armaegis Friend

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    Can you tell us how the dynamic output is tapped off this transformer as well?
     
  20. Degru

    Degru Facebook Friend

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    Bit off topic, but curious, why do you blur out the labelling of chips on some of your other products in the official shots?
     

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