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The Fostex HP-A4 DAC / headphone amplifier -- "The Little Black Box That Could"

post #1 of 4
Thread Starter 

Preliminary impressions of the Fostex HP-A4 DAC/headphone amplifier




"The Little Black Box That Could"




Fostex? Who dat?


Fostex has been around for a while. In addition to manufacturing their own products, they OEM for other brands (think Denon headphones, for example). They are a subsidiary of the Japanese company Foster Electric (they merged with Foster in 2003). Foster has been around since the 1940s and is probably the biggest audio company you have never heard of.


History of the HPA4, or "The days have gone down in the West behind the hills into shadow. How did it come to this?"


Earlier this year, Fostex upgraded their little HPA3 DAC/amp with an new design, the HPA4. There hasn't been much buzz or hype about this model, and I didn't know much about it until recently. That's when I got interested, initially because the HPA3 is a USB-powered amp/DAC that utilizes a DAC chip made by a company you don't commonly see -- Asahi Kasei's AK4390 chip, and I couldn't figure out what DAC the HPA4 was using.


But as I did my research, the HPA4 stood out for a bunch of reasons, and soon I realized I was obsessing about it all the time. When a product's webpage ends up as a Favorite on my browser, that's when I know I am screwed, and will probably end up buying said product. So sure enough, both logical arguments ("I need another DAC like I need another testicle") and emotional pleas ("USB powered amps don't have enough juice to even drive a Grado") failed miserably, and I ended up buying the HPA4.


What is it?


The HPA4 is a USB powered amp/DAC. It has USB and optical in, and can support up to 24 bit/192 kHz frequencies on PCM and SPDIF, and 2.8 and 5.6 MHz DSD.


It has a single 1/4 inch headphone jack that delivers 100 mw at 32 ohms, dropping to 20 mw at 300 ohms. It also has RCA out, so can act as a preamp for active speakers.


As usual, within a few hours of powering it up I had to open it up and take a look inside. It does not feature an Asahi Kasei DAC chip, instead settling for a more run-of-the-mill PCM1792A. There's a custom crystal oscillator to run asynchronous mode, the PCB design is efficient, and the build quality is excellent.


Most of the reviews I have read about the HPA4 claim that is the "little brother" of the acclaimed Fostex HPA8 DAC/amp, which is considered by some to be an endgame setup. From my look inside the HPA4, that description is, well, untrue. Different DAC, different architecture, different components -- IMO these are two entirely different DAC/amps that merely share the Fostex brand and a couple of common features. The hype train strikes again.




I have four interesting features to report to you.


First, the HPA4 has optical out, so it can convert USB or optical in to a SPDIF optical out signal, which is useful if you want to daisy-chain another DAC with optical in.


Second, it has a gain switch that adds an extra 10 dB. This is very useful given that the HPA4 is USB powered.


Third, it has a filter switch. With a PCM source you can select between a slow and sharp roll-off digital filter, and with a DSD source you can choose a high cutoff (185 kHz with gain of -6.6 dB) or low cutoff (85 kHz with gain of -1.5 db).


And fourth, it has a microSD slot at the back. Unfortunately, this can't be used to play music, it is designed to allow easy firmware updates using a microSD card.


For those looking for button and connection pictures, here is the front end, and the back end.





It's small, but heavy. It comes with 4 rubber feet that you have to stick, and once they are on the unit is solid and does not slide. It has a sleek industrial black look similar to the HPA8 (but no display). It has a row of LEDs which are green and red, like Christmas all over again. The metal enclosure is a fingerprint magnet -- it attracts more fingerprints than Lindsay Lohan attracts DUIs.





How can you have a device with a knob and not describe knobfeel in a review? Alas, the potentiometer does have a little friction -- it's not buttery smooth. No low end imbalance though, so I'm happy.



The sound


I read a a review a while ago (on Headphonia?) where the HPA4 was pitted against the Benchmark DAC1, and won.Ha ha, I said to myself, since the DAC1 is my gold standard. It takes challengers and breaks them like Gregor Clegane. So of course my first set of tests were against the DAC1.

The results? To paraphrase Lloyd Bentsen in his famous debate against Dan Quayle: "Senator, I served with the DAC1. I knew the DAC1. The DAC1 was a friend of mine. The HPA4 is no DAC1."


It comes close though, which I thought was surprising. Shocking, even, given the difference sin design and price points. The sound is detailed, if a little smooth. Treble is modestly accentuated, but not harsh at all. Bass is well represented, tight, and without artifacts or muddiness. There is zero noise floor, both in low as well as high gain.

I couldn't make out any difference between the slow and sharp PCM filter. Sorry. I convinced myself I heard a difference with the DSD filter, but that was probably a combination of expectation bias and the dB attenuation more than a real, tangible difference.


I used Fostex's own Audio Player to listen to DSD files. It is very bare-bones, but does the job. It utilizes custom ASIO drivers by default, and can decode DSD over PCM or native DSD. It also has a "load to RAM" feature for handling audio data, for which of course you need a decent amount of RAM. My gaming rig has 32GB of RAM, and although 16GB is taken up by a RAMdrive, the remaining 16GB does not struggle with running the Audio player in its Expanded RAM setting. DSD files sound great, of course, but that's more a function of the file type than the DAC.


I have been rotating headphones only over the last few days, so have not had time to go through my stable. Most of my testing was done with my modded T50RP, which I thought was only fitting since I was flying the Fostex flag. The HPA4 is IMO synergistic with the HD650 and LCD3, but struggles to power the high impedance 600 ohm Beyer DT990. The high gain mode helps, but is merely passable. Ah well, what did you expect from a USB powered amp?



In sumThe good


It's a compact, solidly built, convenient DAC/amp that does not require its own AC power line. It sounds remarkably good, with no sonic flaws. If you are looking to get into a futureproof DSD capable DAC, then this a a great deal from a price-performance perspective. Of course the Aune X1 is the bargain basement option in this category, but Aune QC is hit-or-miss, and Fostex build quality is impeccable.


It also has RCA and optical out, so you can connect it to another optical-in device and to active speakers. The pot on the HPA4 ramps the volume of the RCA-out signal, so it can be used to control active speaker volume.


For the price, it sounds really good. It doesn't quite beat the DAC1, but it comes close. The DAC1 retails for $1000+. The MSRP on the HAA4 is $400. So from the price-performance perspective, I recommend this product.


The bad


It struggles with high-impedance headphones. The filter switch might not be hooked up to the amp at all, for all I can tell. The LED lights are bright and the green-and-red combo is a tad cheesy.RCA and headphone gain levels are not independently controlled, so when you switch from RCA out to headphone jack, you have to make sure the potentiometer is dialed down or you may blast your headphones.


The enclosure isn't a fingerprint magnet in the normal sense, it attracts smudges with the magnetic field strength of a dozen MRI machines chained together. And oh yes, the potentiometer isn't buttery smooth. I know, that's nitpicking, but it is a big deal for me.


The ugly


There really isn't anything ugly about this little black box. Even the USB cable it comes with is well built.


The bottom line


What it is: This is a great portable DAC/amp. For folks who carry their gear from home to work and back again, this is a great choice. For folks looking to get into DSD on the cheap, this is a great choice. For folks looking for a compact bedside amp/dac unit without the usual spaghetti of wires sticking out the back, this is a great choice. For Fostex fanboys, this is a great choice.


What it isn't: It's not the "little brother of the HPA8". So if you are looking for HPA8 performance on a HPA4 budget, that aint going to happen. If you have high impedance headphones, pass on the HPA4. If you want to play music off your microSD card, this isn't the unit for you. If you are paranoid about fingerprints and smudges, stock up on non-abrasive neutral electronics-safe cleaner. By the gallon.

post #2 of 4
How does HP-A4 compare to Audioengine D1 and iFi Nano iDSD power DAC in term of sound?
post #3 of 4
What an interesting post. Very informative too! I can tell you took your time.
post #4 of 4

I love the review, I had the A3 and thought the Dac sounded excellent! This has made me wanting to revisit Fostex!

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