FiiO K9 Pro ESS


Reviewer at hxosplus
FiiO goes desktop
Pros: + Musical and organic sound
+ Outstanding technical performance and transparency
+ Deeply detailed yet not analytical
+ Excellent performance as a standalone DAC
+ Multitude of inputs and outputs
+ High resolution Bluetooth reception
+ Built-in low energy Bluetooth technology allows to use FiiO Control application
+ Internal Linear Power Supply
+ Side mounted USB type C port
+ Excellent build quality and luxurious appearance
+Great accessories pack including a stand for vertical positioning
+ Compact dimensions
Cons: - Competition can be a little more dynamic and holographic but that comes for a price
- Somewhat dry sounding
- Might not pair well with brighter headphones
- Sampling rate limited to 32-bit/384kHz PCM, DSD256 and 8x MQA
- Very warm to the touch
- No dedicated isolation feet
- No remote control
- Function markings are impossible to read
The review sample was kindly provided free of charge in exchange for my honest and subjective evaluation.
I only covered import fees and tax.
The current price is €849 and you can order it from



When you read about FiiO then your mind instantly goes on earphones and portable sources.
Truly enough the only larger sized FiiO device was the K5 PRO, well until relatively recently when they released their first ever true desktop DAC/amp with an internal linear power supply, the K9 PRO.
The first batch was a limited one featuring the last pieces of the AKM AK4499 DAC chip that got out of production because of the AKM factory fire.
Life goes on and the AK4499 was eventually replaced by the ESS ES9038 PRO, thus the K9 PRO ESS version was born.
However the ESS version is not just a copy - paste of the original with a different DAC chip but various adjustments have also been made in the circuit, the power supply and the physical appearance.
The key differences are illustrated in the following picture.


Technical highlights

Two ES9038 PRO DAC chips are used, one for each channel, utilizing all of its 8 parallel outputs in a fully differential and balanced congratulation.
A total of 8 TI OPA1612 op-amps are used for I/V conversion, low pass filtering and to drive two pieces of the THX AAA 788+ amplifiers, one for each channel.
The core component of the specially developed THX AAA 788+ amplifier is the TPA6120A2 and thanks for this new, innovative, design the K9 PRO ESS provides 1.1W/300Ω and 2.1W/32Ω of undistorted power with a voltage swing as high as 26Vp-p.
The PCB of the K9 PRO ESS uses a partitioned layout where not only the power supply and signals are separated but the analog signal loop is also put in a different section in order not to interfere with the digital loop.
The K9 PRO ESS is the first device from FiiO to feature an internal linear power supply.
A custom made, low frequency, transformer and 4 massive 4700μF capacitors makes sure that the rest of the system gets abundant clean power.
The analog and digital power supplies are isolated while each stage of the audio circuit has its own LDO for voltage regulation.
Dual asynchronous femtosecond clocks are used to decode USB and Bluetooth sources, while automatic PLL clocks are used to decode optical and coaxial sources.
The XMOS XU-208 digital receiver supports resolution up to 32-bit/384kHz, DSD256 and 8x MQA unfolding.
Special care has been given to the volume control, FiiO is using an ADC curve reconstruction method of volume adjustment that effectively minimizes any left-right channel volume imbalance and noise.
It also results in a smooth change of volume across all levels.
A hardware gain circuit with three settings (low, medium,high) is used to accommodate for various impedance loads, from high sensitivity earphones to full sized, difficult to drive headphones.


Wireless connectivity

The K9 PRO ESS, employs the latest Qualcomm QC5124 chip to handle Bluetooth duties, with support for formats such as LDAC, aptX HD and aptX Adaptive.
This chip ensures that Bluetooth connections are both reliable and quick, satisfying both the low latency needs for games/videos and the high bitrate necessary for high resolution audio.
An external antenna is used for better signal reception.
Sound quality is excellent, of course not on the same level with the wired but still very good considering the ease of use and the convenience of the wireless connection.

FiiO Control application

A special feature is the built in Bluetooth low energy technology that is always active and allows you to use the FiiO Control App at any time even when not in Bluetooth mode.
With the FiiO Control application you can select between the various low pass filters, enable the equalizer, select the input source and customize various other functions.


Inputs/Outputs and operation

The K9 PRO ESS has three digital inputs (coaxial, optical and USB) that are located at the back of the chassis.
A handy USB type C input is also included at the right side of the unit for connecting with USB OTG devices.
Except for the obvious use as a combo headphone DAC/amp the K9 PRO ESS can also be used as a DAC only with a fixed or variable output, both balanced and unbalanced.
You can even use it as a standalone headphone amplifier bypassing the internal DAC by the means of the two analog signal inputs that are located at the back of the unit, one single ended RCA and one 4.4mm balanced.
You can select the desired operation mode (headphone, pre or fixed DAC) by the means of a switch at the right side of the front face.
Another switch is used for gain selection (H,M,L) and there are also two buttons, one to cycle through the available inputs and another
to enter standby mode.
At standby mode the power consumption is as low as 1W and if you wish to completely power off the unit then you should use the master switch at the back.
There are five small LEDs to indicate the chosen input while the luxurious volume control knob is surrounded by an RGB illuminated ring that will change color according to the current playing sample rate.
There is the option to disable this function through the FiiO Control application.


The FiiO K9 PRO ESS has three headphone outputs to cover most plugs without the need of an adapter.
These sockets are located at the left side of the front face and they include a 6.35mm single ended, a 4.4mm pentacon and a 4-pin XLR that is covered by plastic plug when not in use.
The unit generates quite a lot of heat even when in standby mode so it should be placed in a well vented area.
The markings at the front face that indicate the various functions are so faintly imprinted that it is nearly impossible to read them even under direct light.
FiiO has acknowledged the mistake and they have printed some adhesive labels that look awful and I do hope that the issue will get resolved within the next batches.

Build quality and appearance

The K9 PRO ESS might be advertised as the first desktop unit by FiiO but the truth is that it still is relatively compact and lightweight.
The dimensions are 200x224x73mm so you can either place it in a dedicated hifi rack or at your desk without occupying too much space.
The best part is that you can use the included custom stand for a vertical positioning to reduce even more the amount of space.
The rigid chassis is made from solid, CNC-milled aluminium with a sandblasted matte black finish of the highest quality.
The unit is definitely beautiful looking and luxurious, certainly worth the price of admission.



The K9 PRO ESS comes in a relatively big box which except for the main unit and the stand, it also includes the power cable, a replacement fuse, a USB A to B cable, a Bluetooth antenna, six pieces of adhesive rubber feet and a 6.35mm to 3.5mm headphone adapter.
The accessories package is pretty comprehensive but at this price point some dedicated high quality isolation feet directly screwed into the chassis would be far better than the adhesive rubber feet.


Associated gear

The K9 PRO ESS was tested both as a DAC/headphone amplifier and as a dedicated DAC connected to a 2-channel system consisting of the Kinki EX-M1 integrated amplifier and Audiophysic Step 35 speakers.
Headphone listening was done with the Focal Clear Mg, Sennheiser HD8XX, Sennheiser HD660S, Meze Audio Liric and HiFiMan Arya V3 plus a couple of IEMs to check the noise floor.
The main input source was the Silent Angel Munich M1T steamer together with the M11 Plus ESS that was used as a transport through the USB type C port.
All subsystems were powered from an iFi power station and DC blocker.


Power output and noise floor

The K9 PRO ESS has a pretty powerful headphone amplifier and was able to drive all my headphones with ease and headroom to spare.
At the same time the noise floor was totally inaudible making it a perfect companion for more sensitive earphones.
Of course with 2.3W at 32Ω, the K9 PRO ESS is not the most powerful amplifier, there are other models with greater power output but you are not going to find it lacking unless you own some of the famously inefficient headphones like the HiFiMan Susvara.
Interestingly enough the K9 PRO ESS single ended output has the same power rating, under 16Ω and 32Ω loads, with the balanced one so you can drive most of your headphones without the need of a balanced cable.
Thus said, the balanced output has larger voltage swing for 300Ω loads and since the K9 PRO ESS is a fully differential and balanced circuit it is advised to use the balanced output whenever possible.
The K9 PRO ESS was a great match with the Sennheiser HD8XX and the HiFiMan Arya V3.


Listening impressions

Starting with the obviously expected for a dual ES9038PRO and THX AAA machine, yes the K9 PRO ESS is one of the most transparent and linear sounding DAC/amps you could possibly imagine.
Colorless, flat and perfectly balanced it stays true to the source material without adding any character of its own while it is highly exposing of poor quality so do expect crap to sound like crap and remember that this is not the best machine to manipulate your headphones.
But on the other hand, is not as punishing or ruthlessly revealing as you have probably imagined, the K9 PRO ESS is not strident or the cold blooded professional tool that was made for analyzing and finding out the deficiencies of a recording.
On the contrary, the K9 PRO ESS can get surprisingly musical and organic, when the source material allows for, by highlighting all of the glory that the sound engineer has captured in a good quality recording.
As when listening to the following album where choruses sounded brilliantly expanded, instruments were full blooded and harmoniously intense while singers were doing their part with the most natural timbre and large sized images.


Listening track after track and album after an album, it became clearly evident that FiiO has made a truly remarkable job into implementing the flagship ES9038PRO.
This is a DAC chip that came my way dozens of times but it must be admitted that with the K9 PRO ESS it was one of the least digitally sounding experiences.
The sound was almost absent of digital glare with a timbre that I wouldn't hesitate to call analogue and naturally textured.
The K9 PRO ESS is highly resolving and deeply detailed but it never crosses the line to become analytical and mechanical sounding.
During the evaluation it became clearly evident that the final tuning was definitely done by the ear, the engineers must have done a lot of listening and not designed just sitting behind a PC monitor.

People who listen to classical music know that piano can sound very artificial, fake and lifeless when reproduced through a DAC that is designed for excelling in measurements rather than playing real music but thankfully this was not the case with the K9 PRO ESS.
Here, there was a great balance between the skilful technical performance and musical expression of the highest quality.
Albéniz's Iberia is a masterpiece that with the aid of K9 PRO ESS sounded grand, lifelike, harmoniously saturated with a multitude of tonal colors.


The K9 PRO ESS is absolutely fine sounding, engaging and enjoyable with all kinds of music while it has an end game technical performance and the review could have easily ended here.

Just in case that you are interested in the detailed but usually boring tonal analysis, then the bass is full bodied but tight, clear and controlled, fast with great timing even under a heavily populated bass line.
Layering is perfect, so is the dynamic antithesis and if there is something to be missed when compared to some of the competition, that it is the relative lack of ultimate impact and the less visceral texture which is somewhat lean and on the dry side.
But it is not fair to compare because in order to get this little extra you have to pay a lot for some discrete designs.
After all the K9 PRO was very convincing into presenting the large dynamic swings and the sound contrast.

Mid range is also somewhat dry and not that lucid but then it is certainly not lacking in final engagement and musicality factor.
The region is well expressed with some fine articulation, realistic timbre and an organic sound signature while clarity always stays on the highest level.

If you have been worried about the treble realization then you should stop because here again FiiO has done some excellent job into balancing the uncompromised extension with the lack of brightness, edginess or sharpness.
The K9 PRO ESS is vivid and agile, sound is flowing like sparkling water but there is no roughness or harshness.
The overall treble response is luminous and crystalline so some bright sounding headphones might not be the best match for certain users.
As said earlier the K9 PRO ESS might be a little dry but it is not sounding skinny.
There is an excellent cohesion throughout the whole frequency range and he notes decay in a natural way without losing in body weight as to become thin.

Soundstage is very good, the K9 PRO ESS will not be the bottleneck and is going to do justice to all of your headphones thanks for its spaciousness, the accurate imaging and the layering but it must be said that it is not as grand and holographic as some other discrete topologies.
Thus said, the K9 PRO ESS together with one of my favorites staging headphones, the Arya Stealth, were very effective in communicating the ambience and the scale of large works, like Bach's Mass in B minor.

CKD 354 Sleeve.png

Not only for headphones

All the previous listening impressions apply both for headphone and dedicated 2-channel use, the K9 PRO ESS is just outstanding as a standalone DAC without performance compromises.
From the balanced output is a highly competitive DAC that can become the main source of any mid-tier stereo system or pair with active monitors and higher end headphone amplifiers.
So the value to performance ratio is very high as the K9 PRO ESS can substitute for multiple roles and form the center component for both headphone and speaker listening.


Desktop vs portable

Some of you reading the review might be wondering how the K9 PRO ESS compares to the similarly specced but much more expensive FiiO M17.
Well, although numbers might suggest an equal level of performance between the K9 PRO ESS and M17 (which is really outstanding under the enhanced over the ear mode), the truth is that the former has the slightest edge when it comes to actual music listening.
Except for better driving higher Z loads it also sounds fuller, more effortless and impactful, somewhat grander and more holographic.
Timbre is more or less the same but on the K9 PRO ESS the texture is slightly more organic while it also has a smoother treble and more liquidity to the mids.
I guess that there is no way for a portable device, despite being as good as the M17, to surpass an equally well designed desktop unit with an integrated linear power supply.
Of course the comparison is not fair, it is like comparing apples to oranges because there are fundamental design differences and the two devices are called to fulfill different needs.
Thus said if you are not interested in transportability and you don't need a DAP that can also substitute for a desktop use then the K9 PRO ESS should be your choice.


In the end

The K9 PRO ESS is the first full sized desktop unit by FiiO also featuring an internal linear power supply and oh boy, what a hell of a start it is!
With excellent sound performance, immaculate technicalities and a multitude of inputs/outputs the K9 PRO ESS is one of the best all-in-one DAC/amps available right now with a price to performance ratio that is very hard to beat.
This is an outstanding premier for FiiO that really left this reviewer particularly satisfied while wondering what the future holds for next.

Test playlist

Copyright - Petros Laskis 2022.
Last edited:
Not just THX any other nuetral amps as well and this includes delta sigma dacs. Have to do honest a/b test. Most of the time, I unable to discern the difference. And what do people mean about musicality ? Coloration ?
Thanks for posting this review. Do oyu know whether the USB input is galvanically isolated?
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Thank you for reading.
I don't know, maybe you should ask FiiO directly.