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Fostex HP-A4BL 4-pin XLR Balanced Headphone Amplifier & Dac

  1. Wiljen
    Fostex HP-A4BL - a DAC/Amp with Power to spare
    Written by Wiljen
    Published Oct 12, 2018
    Pros - Good build quality, very good output power, Easy driver install
    Cons - Micro-SD for firmware only, high noise floor with sensitive IEMs.
    Fostex HP-A4BL DAC/AMP

    I recently got to try out the Fostex HP-A4BL dac/amp as part of the Dekoni Blue review tour. Dekoni wisely had decided to send this unit with the Blue as the T50rp drivers are known for being a power sponge and it seemed wise to make sure that reviewers had an amplifier that was up to the task. I’ll admit that since I have a Burson Fun, I didn’t use the little Fostex for most of my testing of the Blue, but my curiosity got the best of me and this review is the result.

    Packaging / Accessories:

    The box is unassuming enough but well padded with a separate compartment for the power supply and enough foam to protect the main unit from bumps and bangs in transit. I particularly liked the use of the bottom of the box as a way to show all the controls although for those who generally pitch the box this may be less than helpful. The accessories are utilitarian enough a simple power adapter and USB cable are provided.


    Build Quality:

    The unit feels light but still feels well made. All the seams are well fitted with no gaps or mis-alignments. Buttons are all well-centered in the cuts for them as were the LEDs. Controls are all very positive and gave tactile feedback to let you know they have been set. Overall, a well made unit.


    The USB and Optical inputs are on the rear of the unit as shown below. The USB supports USB 2.0 and supports 16 or 24 bit quantization with sampling rates from 44.1 to 192kHz for PCM and 2.8(64), 5.6(128), and 11.2mHz(512) DSD when the Fostex drivers are loaded. The Optical input gives the same bit rate choices for PCM but does not have the DSD support. The other input on the back panel is something that seems exciting but actually isn’t. Unfortunately ,the micro-sd slot is only for firmware updates and cannot be used for music playback directly from a card. For those that need to update the firmware, you will need a blank 32gb or smaller micro-sd card formatted with fat32. The firmware has to be copied to the root of the card, then the card installed into the unit before powering it on. I know, I got excited too when I first saw it and thought it was a playback device, sorry to disappoint.



    Outputs are split between the rear (RCA and Optical) shown above, and the front (6.3mm and 4-pin XLR Balanced). I used the device both as a DAC paired to the Burson Fun as the Amp and as a stand alone via both the 6.3 and the XLR connections. I did not test the Optical output as I didn’t have an amp with an optical input handy. The 6.3mm output had no problem driving the Dekoni Blue while the XLR doubles the output power and was more than enough to handle even my 600Ω Beyers. I did find the noise floor to be slightly high for use with IEMs as a perceptible hiss is present when using my Eartech Quints or Magaosi K5s, but when you move to slightly less sensitive or higher impedance models like the Campfire Comet the hiss disappears. this phenomenon is not wholly surprising considering the target audience for this device is quite obviously Fostex planars.



    From left to right, the front panel has the XLR connector, the 6.3mm Jack, the input selection button, the filter button, the Gain Switch, the output selection switch, and finally the volume control. Input selection allows switching between USB and toslink. Filter is either sharp(1) or slow(2) roll-off when using PCM or high cut-off frequency(1) or low cut-off frequency(2) when using DSD. Gain is a 2 position switch with the high gain position offering a 10dB boost over the low. The output selector lets you enable either the headphone jacks or the RCA outputs. The Toslink output seems to be enabled only when RCA is selected as well although the control is not marked as such. Above the controls are a series of LEDs that show the frequency during playback with both PCM and DSD options shown. The Sole control on the rear face is the auto-standby mode switch which allows for dropping the unit into a low power state when not in use. the downside is that some PCs will lose the unit when it drops into this state and require powering the unit off and back on to find it again (Windows 10 particularly). If using USB input, it may be advisable to leave the power saving turned off.

    Sound Quality:

    This is always the toughest part of the review of a DAC/amp for two reasons. First, if the device is doing its job, it shouldn’t be contributing to the overall signature and second, if it is contributing to the signature which component is responsible for that contribution. The HP-A4BL has a near neutral signature with perhaps just a shade of lower treble enhancement. The unit had good detail retrieval and better than average dynamics which is quite a feat when using 600Ω headphones. I found no major areas of coloration of the sound when using the unit either as just a DAC or when using both the DAC and amp sections. Power was more than adequate for all the headphones I tried and more than needed for the vast majority of them. Using the single ended output with the Dekoni Blue, I had the volume knob at roughly 11 oclock on high gain and that was as high as I could go and tolerate it. With the 600Ω Beyers, I was able to use the XLR output and managed to get to about 2pm on the dial before the level was sufficient to make me back it down. In both cases a sustained listening levels would have been a couple hours before the listed positions on the dial.


    While not an inexpensive unit, the HP-A4BL certainly checks in at a reasonable price point but one with pretty stiff competition. I had on the bench at the time of writing this, an iFi Micro BL, a Schiit Jotenhiem, and the Burson Play all of which fit the same bill. Some (ifi) offer portability, others (Schitt) offer modularity, still others (Burson) offer your choice of op-amps to tailor the sound to your liking, and yet with all these other options, the HP-A4BL manages to carve out a niche for itself. For those who appreciate a simple device that does what they need without the need to fiddle with it or fight with drivers incessantly, the little Fostex has a lot to like. If you are in the market for a DAC/Amp combo, this little unit should be on your list to audition. Matter of fact, if you put it first on your list, you may be able to stop after #1.
  2. Takeanidea
    Fostex HP-A4BL perfectly capable Dac/Amp just lacking the versatility of the Chord Mojo
    Written by Takeanidea
    Published Apr 8, 2017
    Pros - Output power increased threefold from previous design
    Cons - Mains powered - non transportable. Firmware update is too fiddly


    With many thanks to Matt Esau of SCV Distribution and @glassmonkey for introducing us in the first place, I had the opportunity to spend almost a month with the Fostex HP-A4BL Dac/Amp. This was a loan review model offered to me in return for a write up of my opinion on it. The opinions are mine and mine alone, I welcome other's views of course. So much so that it just so happens I have actually recorded a sound sample of the Dac/Amp itself and volume matched it to an Aune S6 and a Chord Mojo. If you wish I shall provide the link and you can listen over and over again to your heart's content. As I had to put myself through..... You  may find that your ears and your tastes put the Fostex on such a pedestal that you will feel compelled to part with your hard earned for it. Or you may not. That , my dear reader, is entirely up to you.

    The Sound Sample

    I will tell you what I found about using the DacAmp on a day to day basis for a little while and you can use this as a guide against some real world listening to it for yourself. The recording was made using a semi professional Art USB Phono + Analogue to Digital Converter which was fed by the headphone outs of each DacAmp. This was then fed by USB2 back into my MacBook and recorded using Audacity recording Software. The bitrate used was 16 bit 44.1 - standard CD quality. It was recorded as a WAV file for simplicity's sake. The track was a Vinyl Rip of Crucify by Tori Amos. It is track 1 from her debut album Little Earthquakes. The recordings , by their very nature, are lossy. They aren't designed to showcase the very best in Digital Reproduction and won't be of the quality of the original. What they will do is (arguably) show you the differences between the sound signatures of each of the DacAmps. Have a listen here.

    About Fostex

    The Fostex TH610 was part of the package and was a beautiful sounding headphone, more details of which can be found here. Clearly the phones and Dacamp complement each other.
    My experience with Fostex products go back a way. I have been listening to Fostex TH900 flagship closed cans for some time now at meets and CanJams. The TH900s aren't quite my personal flavour due to the portrayal of the low end and lacking the airiness and crispness of open cans like the HD800 and HE-6 . But there's no doubting these flagships have gained a following. In fact, I ended up with a pair of Mr Speakers Alpha Dogs. These are modded Fostex TH50 MKI headphones. And quite a facelift they had too. The cups were bespoke 3D printed. The drivers were tweaked and the cables were replaced. The MK II models are considered not commercially viable to mod and Mr Speakers of course have moved on to bigger and better things. 
    TH900_003.jpg 500x1000px-LL-440cbd84_f2.jpg

    The Design

    The Fostex build quality is good. Here we have a number options at the front of the device. They can change between the inputs gain and the Dac filter modes by pressing the buttons corresponding to the options. For your info all of my sound files have used sharp roll off filtering. This is how @Joe Bloggs explains my preference :
    Sharp rolloff: your high frequency hearing doesn't go up to the Nyquist frequency (22050Hz in the case of 44.1kHz sample rate) so you choose to use a sharp rolloff filter. This way you get flat response up to the limits of your hearing and no audible ringing (only inaudible ringing at 20000-22050Hz)
    So there you have it. There are other filters , as per pretty much every Desktop Dac/Amp I've come across recently. More information can be found here

    On the back there are the standard RCA outputs , Optical In and Out and USB in. There is also a micro sd card slot.  Fostex have also decided to put 12V through this unit to push some of the more power hungry headphones to greater outputs than their predecessor. The theoretical output is now 300mW at 32 Ohms on the balanced input Ouch! A more conservative 150 mW can be had through the 6.3 socket. Still more than enough to cause a headache.

    The Micro SD Card Slot

    This certainly piqued my interest when I noticed the slot. My imagination ran riot. I had visions of a DSD capable playback system using a card with tons of music on it. I was excited to think what Fostex could produce, albeit without an interface. Of course, I was barking up the wrong tree. The micro sd card slot allows for firmware updates. I have a MacBook running OS Sierra. It didn't recognise the HP-A4BL. I've had my laptop over 3 years and this is a rarity for hardware. After a bit of head scratching and some close perusal of the Fostex website this is how you get the 2 devices talking to each other.
    You need a Fat 32 formatted micro sd card with nothing on it. It cannot be any larger than 32 Gb. Download the firmware update onto the card. Insert the card into the slot. Switch on the power plug into the laptop. Switch off the power take out the sd card. Something like that anyway.

    The HP-A4BL in use

    The HPA finally recognised by my MacBook I got to grips with the various types of files I had stored offline. The simplest way I found to get these recognised properly was through the freeware player Fostex supply from their website. It is a decent enough sounding player to me and may be worth a look. The link is here to the Fostex Audio Player. There are lights above the push buttons on the front of the Dacamp which light up reassuringly to let you know things are nicely under control. No glitches or hiccups were in evidence with the offline playback. Online playback was another story....
    At first I could not get the HPA to playback online audio at anything more than 1/4 speed. Very puzzling. Eventually I had a look at my midi settings in the applications folder of the Mac. The Midi settings had been put at 32 384. I put them back to 16 44.1 and voila! Everything working fine online. I go through this so you don't have to........
    OTG worked beautifully. No updates were needed. I unpacked the Fostex and my Android Marshmallow using UAPP worked straight out of the box. My phone is a Moto G4.
    Optical in worked fine. My MacBook's headphone input also doubles as optical out. With the usual limitations of the maximum output as 24 192 optical worked just fine. Some argue that Optical can produce less interference, so for you this may well be of greater benefit than using the standard USB connection. I couldn't discern any obvious glaring smack you in the face night and day audible differences between the connections.

    Sound Quality

    I hope there is no particular controversy here (for once!) I have supplied you with the means to listen to what the HP-A4BL actually sounds like. It sounds very much like this. If you are interested in purchasing this to further your listening pleasure and you can't get to a show or a showroom to listen to it then at least this is another way to get an insight. It takes several hours to get this sampling done right. It provides a souvenir for me  as a constant reference. Whenever I get a spare moment I can critically AB. It is much quicker than plugging and unplugging the various dacs.
    These are my thoughts on the differences between the 3 DacAmps. If you wish to hear for yourself without being influenced, look away now.

    Vs Aune S6 & Chord Mojo

    The S6 is a smooth sounding dac that was a very enjoyable listening experience. It was edged out in punch and liveliness by the Fostex. The Fostex sounded rawer and less refined than the Chord Mojo. The Chord Mojo had a linearity and detail that surpassed the other 2 in my opinion. I own the Chord Mojo. I have owned it pretty much since the day it came out. I reviewed a Mojo here. I have had mine since October 2015. As you can imagine, I am comfortable with the sound signature and it will take an awful lot to convince me to part with mine in favour of something better out there. There are better decamps available - the Hugo II was amazing and the Dave - well, what can I say about the Dave? That is a marvel, plain and simple.


    The Fostex was perfectly fine as a DacAmp. The balanced connection is becoming more prevalent on stuff at this price range and may be of interest especially if you find the sound signature is one you particularly enjoy. I still prefer my Chord Mojo. Although the Mojo needs all sorts of connectors and adapters to get things like balanced headphones working, for me I prefer the sound signature. Bear in mind also that, for the same price, the Mojo will run on battery whereas the Fostex is tied to a desk. The HP-A4BL is a great Dacamp but ultimately it didn't persuade me to put my Chord Mojo in the for sale threads.



      obsidyen likes this.
    1. obsidyen
      How does it pair with TH610?
      obsidyen, Apr 8, 2017
    2. Takeanidea
      Single ended but you can buy a balanced cable
      Takeanidea, Apr 9, 2017