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iFi Audio Pro iESL

Electrostatic headphone energizer

Item # 246PROIESL

This energizer works together with a headphone amp to provide the high voltage needed for electrostatic headphones

This energizer works together with a headphone amp to provide the high voltage needed for electrostatic headphones

Item # 246PROIESL

Low stock

About the iFi Audio Pro iESL

Give your high-performance electrostatic 'phones the voltage they need

I'm glad we're carrying the iFi Audio Pro iESL electrostatic energizer form many reasons, but one in particular. It gives us a way to power and play the Dan Clark Audio (MrSpeakers) VOCE electrostatic headphones — some of the absolute best headphones I've ever heard. This energizer works together with a headphone amp (like iFi's Pro iCAN) to provide the high voltage needed for electrostatic headphones, like the VOCE.
iFi Audio Pro iESL energizer

The iFi Audio Pro iESL energizer can power electrostatic headphones, like the MrSpeakers VOCE. I was beyond impressed by the combination.

Give your high-performance electrostatic 'phones the voltage they need

I'm glad we're carrying the iFi Audio Pro iESL electrostatic energizer form many reasons, but one in particular. It gives us a way to power and play the Dan Clark Audio (MrSpeakers) VOCE electrostatic headphones — some of the absolute best headphones I've ever heard. This energizer works together with a headphone amp (like iFi's Pro iCAN) to provide the high voltage needed for electrostatic headphones, like the VOCE.

And it drives those particular headphones well. I heard the combination together at CanJam NYC, and it was really special. There's just something about the overall electrostatic listening experience that's greater than the sum of its parts. But the sonic highlights include rich dynamics, revealing high-end detail, and deep, potent bass.

Powers legendary headphones from Stax, Koss, Sennheiser, and others

What separates the iESL energizer from the other energizers out there? Versatility. Many electrostatic energizers are purpose built for a specific set of headphones. The iESL offers both 6-pin and 5-pin sockets for connecting both modern and classic electrostatic headphones.

There are also front-panel controls and bias voltage settings for customizing the amount of power. These settings are tailored to some of the best headphones in the rich history of audio. Along with the MrSpeakers VOCE, you'll find voltage settings for the old-school Stax electrostats, the Koss ESP/950, and even the iconic Sennheiser Orpheus headphones.   

How do electrostatic headphones work?

Electrostatic headphones and more common planar magnetic models share a similar basic design concept: a large ultra-thin diaphragm moves quickly to create sound. But electrostats actually rely on an entirely different principle to generate that motion. Electrostatic diaphragms are suspended between two metal stators.

Here's where the electrostatic energizer comes into play. It delivers a fixed, high voltage to the diaphragms, and each stator receives a varying audio signal to push and pull the diaphragm back and forth. 

iFi keeps the power consistent

All headphones (and speakers) perform their best with clean, consistent power, but the electrostatic process absolutely requires it. iFi goes to great lengths to keep this energizer's power as pure as possible. They use premium capacitors and built-in batteries to bank the power that energizes the electrostatic diaphragms. This keeps the operating parts isolated from line noise and dirty power from the grid, and helps avoid spikes or sudden drops.    

iFi iESL back panel

iFi offers several ways to connect the required separate amp. The back panel includes a proprietary input for their Pro iCAN headphone amp.

How to power the Pro iESL energizer

In order to operate the Pro iESL energizer, you'll need a separate headphone amplifier. You have lots of options, but the easiest and most natural pairing is the iFi Pro iCAN headphone amplifier.

First off, the Pro iCAN is one of the most versatile and powerful headphone amps available. And it also has a proprietary iESL connection to deliver the right amount of amplification to this energizer, using the cable provided with the iESL. These two heavy-duty components are even built to stack up together.  

But the iESL's back panel also has inputs for connecting other types of headphone amps or even a regular power amp. There's a balanced XLR 4-pin input that can accept up to 28V into 64 ohms (10V into 64 ohms is the minimum required). And there are speaker binding posts for connecting a traditional power amp or integrated amp. Those connections are a little tricky, so be sure to consult the manual before you dive in. (And Crutchfield offers lifetime tech support for every product you buy from us.)


Product highlights:

  • high-voltage energizer for powering electrostatic headphones
  • compact all-metal chassis with rubberized no-slip base to minimize resonance
  • adjustable voltage settings for properly powering legendary and popular electrostats
  • requires power from a separate amplifier. We recommend the iFi PRO iCAN headphone amplifier
Audio performance features:
  • custom, hand-wound transformers generate wide bandwidth with low distortion
  • built-in batteries and capacitors for banking power to drive headphones
    • helps to isolate signals from AC power supply to avoid dirty power and line noise
  • premium capacitors for stable, efficient power supply with no spikes or sudden drops
  • outboard "iPower Plus" power supply for clean, substantial power
    • separated and shielded from audio circuitry to avoid interference
Back-panel inputs/outputs:
  • proprietary "ESL-Link" connector for use with iFi Pro iCAN headphone amp (cable included)
  • one balanced XLR 4-pin input
  • dual speaker binding-post inputs for connecting an external amplifier
    • for safe operation, amp rating should not exceed 100 watts/8 ohms
  • dual speaker outputs for connecting speakers
  • DC power input
Front-panel headphones outputs:
  • 5-pin "pro bias" socket for connecting modern electrostatic headphones (MrSpeakers VOCE, newer Stax models, etc.)
    • 500V to 640V adjustable bias voltage
  • 6-pin "normal bias" socket for connecting classic electrostatic headphones (vintage Stax models, etc.)
    • 230V bias voltage
  • one balanced XLR 4-pin output for dynamic headphones
Front panel controls:
  • input selector dial
  • AC termination adjustment dial
    • helps maintain three-dimensional soundfield with different types of connected gear
  • impedance adjustment dial (for incoming load from driving amplifier)
    • 16 ohms to 96 ohms
  • bias voltage setting control dial
  • recommended bias voltage usage for popular electrostatic headphones:
    • 230V — vintage Stax models
    • 500V — Sennheiser Orpheus HE-90 (original)
    • 540V — MrSpeakers VOCE, Sennheiser HE-60 (latest version), King Sound KS-H4
    • 580V — Stax Pro Bias ESHP
    • 600V — Koss ESP/950, HiFiMAN Jade
    • 620V — no current headphones, but ready for future options
Other info & specs:
  • frequency response: 5-500,000 Hz (-3 dB)
  • maximum output voltage:
    • 640V RMS (16 ohms in)
    • 320V RMS (64 ohms in)
  • power consumption: <1W
  • 8-1/8"W x 2-1/2"H x 8-1/2"D
  • weight: 5.5 lbs.
  • warranty: 1 year
  • Our 60-day money-back guarantee
  • MFR # 301003

What's in the box:

  • Electrostatic headphone energizer
  • AC adapter with 5' DC output cord (DC output: 9V 2A)
  • 9" ESL-Link cable (male HDMI plugs on each end)
  • User Manual
  • Warranty card
  • 3 Easy Steps card
  • Audio Band Noise Floor card

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More details on the iFi Audio Pro iESL

Mark G.

Product Research


Overview: The iFi Audio Pro iESL is an energizer and matching device for electrostatic headphones. It is NOT a headphone amplifier. It allows you to use electrostatic headphones with normal headphone or loudspeaker amplifiers. It is matched with iFi's Pro iCAN, or you can use with other headphone amplifiers or loudspeaker amplifers to drive electrostatic, dynamic, or planar headphones.

Electrostatic Headphones: Electrostatic headphones are quite different from dynamic headphones. They require very high voltages for operation, plus they need a bias voltage. The Pro iESL normal bias of 230 volts, plus custom and pro bias levels between 500 and 640 volts. You can experiment with the bias settings for best results.

Pro iCAN Match: The Pro iESL is matched to the Pro iCAN headphone amplifier, allowing the Pro iCAN to drive electrostatic headphones. There is a dedicated input selection for the iCAN for optimum results. You can connect the two components together with a single cable, the included ESL-Link. The ESL-Link provides power to the Pro iESL, so you don't have to use the included AC adapter. The Pro iESL turns on and off as you power the Pro iCAN.

Inputs: The Pro iESL provides three ways to connect your input source to the device. You can select the desired input on the front panel. If paired with the Pro iCAN, you can plug the included cable via the ESL-Link (HDMI connector). You can also connect the Pro iESL to your headphone amplifier (via balanced cable) or your loudspeaker amplifier (via speaker wire binding posts). There are also binding post speaker outputs for connecting your loudspeakers. When the input selector is switched to "loudspeaker," the Pro iESL turns OFF the loudspeakers so you can listen with headphones. When the Pro iESL input selector is Off, or set to any input except "Loudspeaker," the speaker input is linked to the speaker output, and disconnects the amplifier from the Pro iESL circuitry. The inputs are as follows:

  • ESL-Link: Connect the Pro iCAN via supplied ESL-Link cable
  • Balanced: Balanced 4-pin XLR (male) for connecting compatible headphone amplifier. The amplifier should be able to deliver at least 10V into 64 ohms with a maximum of 28V at 64 ohms.
  • Loudspeaker: Binding posts for connecting loudspeaker cables from your amplifier. The posts will accept bare wire, pin connectors, spade lugs, and binding posts (single or dual). Amplifier rating for safe operation should be between 10 watts and 100 watts at 8 ohms.

Headphone Connections: The iFi Audio Pro iESL offers connections for electrostatic and dynamic headphones. On the front panel, there are the following connections:

  • Electrostatic Normal: a 6-pin socket with normal bias (280V); compatible with older Stax headphones
  • Dynamic: a 4-pin XLR socket for dynamic headphones; compatible with AKG K-1000, Audeze, or other compatible wired headphones
  • Electrostatic Pro/Custom: a 5-pin socket with adjustable bias voltage (via separate control); for modern Stax headphones

Bias Voltage Settings: Before turning on the Pro iESL, you should set the bias voltage to the recommended level given by the electrostatic headphone manufacturer. Connecting to the "Normal" output presents a 230-volt bias voltage. If you connect to the Custom/Pro output, you can use the voltage selector to choose from the following:

  • 230V (Normal): Older Stax headphones
  • 500V (Custom): Sennheiser Orpheus HE-90
  • 540V (Custom): Sennheiser HE-60, King Sound KS-H2/3/4
  • 580V (Pro): Stax Pro Bias headphones
  • 600V (Custom): Koss ESP/950, Jade
  • 620V (Custom): Future option
  • 640V (Custom): Future option

Impedance Adjustment: Using the front panel knob, you can adjust the load impedance for the driving amplifier from 16 to 96 ohms. The lower the impedance setting, the louder the output at the same headphone amp volume, but presents a more difficult load for the amplifier to drive. You can adjust the impedance settings to suit your listening preference to 16 ohms, 24 ohms, 64 ohms, or 96 ohms. Typically the higher settings are recommended.

Capacitive Battery Power Supply: The Pro iESL utilizes a battery of multiple parallel film capacitors to supply the bias voltage. When the battery is charged to the required voltage, the charger circuit is turned off. This leaves the capacitor bank to "float" at the required bias voltage. To keep the battery charged, the charger must be turned on only for a few microseconds every 30 seconds or so, thus leaving the charging circuit off over 99.999% of the time. This results in virtually zero noise on the bias circuit, and an extremely clean audio signal.

iPower AC Adapter: The Pro iESL comes with an iPower AC adapter for powering the device as a standalone device. The AC adapter or other power supply is not required if the Pro iESL is connected to the Pro iCAN via the supplied ESL-Link cable.


  • Frequency Response: 5Hz to 50kHz (-3dB)
  • Maximum Output Voltage: 640V RMS (16 ohms/20V in); 320V RMS (64 ohms/20V in)
  • Input Voltage:
    • ESL-Link: 5V-9V / 1A max
    • iPower: AC 85-265V, 50/60Hz
  • Power Consumption: < 1W
  • Dimensions: Width 8.381", Height 2.506", Depth 8.177"
  • Weight: 5.5 pounds

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