Firestone Audio’s Fireye HD | Clean, Clear, Effortless

Firestone Audio is known as a value brand in the world of headphone amplifiers and audio accessories. The Taiwanese company has made a name for itself making the Cute series of small, box-type amplifiers and power supplies. However, over the last couple of years, it has been a little silent with product announcements. Only recently did it come out with an entirely new line of products, beginning with a revamp of the Fireye line. The new line consists of the Fireye HD — Firestone’s flagship portable amplifier as well as the current topic of discussion, the Fireye DA — a combination DAC/amp, and the Fireye HA — a lower-end amplifier based off the output end of the Fireye DA.

‘Fireye‘ has been Firestone’s portable amplifier line for quite a few years — I bought their miniscule Fireye Mini a couple of years ago and thought it was quite nice, but the size was almost too tiny and I didn’t really know what to do with it hanging off my iPod. I’ve always been a minimalist when it comes to using portable sources on the go, and while the Mini was very small and very portable indeed, I still felt it to be a hinderance to my portability. However, the new Fireye line was a completely new endeavor, and seemed to signify that Firestone turning over a new leaf. I was instantly interested.

Even with the Cute series, Firestone never really caught my eye in terms of aesthetics. Its products were always very ‘function first’, and frankly, pretty bland when it came to looks, unlike the colorful offerings from ALO Audio, Schiit, and the like. Perhaps it has just been a late bloomer, but Firestone has definitely caught up in the design department. The Fireye HD looks and feels great.

Its front fascia is fully symmetrical and the circuit board is wrapped in a substantial, seemingly laser-cut block of aluminum. Its fit and finish is as good as any that I’ve seen — the power, bass-boost, and gain switches click positively and the volume knob is buttery smooth. The brushed aluminum case is textured differently on the top face, forming a sandblasted matte look that contrasts well against the traditional brushed finish of the title panel and the other faces of the case. The text printed all over is very high quality and shows no signs of corner cutting.

At ~150g, it is a solid, solid product — substantial, but not exactly heavy, either. The footprint of the Fireye HD is almost exactly the same size as my second generation iPod Touch, and is only 1.5 cm thick, so it is well-classified as a portable headphone amplifier.

The front fascia and back panel are held together securely by four hex screws; at the back are a DC 5V jack and mini-USB port for charging. Why the redundancy in charging ports? Firestone claims that, through the DC 5V input, “fast charging” to 80% capacity for its internal Li-ion battery can be achieved within 2 hours (faster in my experience), while charging from an empty battery takes 6 hours via USB. Operation is fully allowed during operation for both ports, and overcharge protection is built in, switching to external DC power when charging is completed to allow the Fireye HD to run purely off wall power if a user so chooses.

At the front, low gain is 1.5x and high gain mode is at 5x — I would’ve preferred even lower gain for more play on the volume pot for IEMs more sensitive than the RE262 (almost every single one, but honestly, I’m probably way too fastidious about volume control and the only things that would satisfy me would either be stepped attenuators or digital volume control), but low-gain mode gives me more than enough to work with at normal listening volumes, even with the sensitive Heir Audio 4.A.

Bass boost sits opposite the volume knob; it only provides single stage functionality (I think it’s +2.5 dB @100 Hz, but I’m not sure) but in my experience, that’s all anyone really needs, especially when it’s dialed in right. When I heard the prototype board, the bass boost wasn’t the most refined and frankly sounded a bit bloated, but the final version sounds much more polished. The boost has enough tightness that it doesn’t bloat, but is generous enough to give listeners a definite change in sound signature. Most of the time, though, I just keep it switched off, unless I really want some heft to the low end.

When it comes to sound, the Fireye HD doesn’t disappoint. At $399 USD (now slashed to $349 at select retailers, and €199), it is definitely no longer a value-conscious product, and while it’s not priced as a premium item through and through, it certainly has aspirations of being one and definitely delivers on those ambitions in the sound quality department.

I’m no expert on amplifier circuit design, but I do believe the Fireye HD is a very nice implementation of TI’s powerful and capable TPA6120, a very wide-bandwidth purpose-built amplifier IC built on a current-feedback architecture with specifications for low distortion (<0.0004%), high dynamic range (>120 dB), and very fast slew rates (1300 V/µs). The TPA6120 is also used in many amplifiers such as the FiiO E9, ASUS Xonar series, and by DIYers looking for cost-effective reference designs. The Fireye HD does well on this platform by delivering a dynamic range of 113.7 dB(A) and a THD% of only 0.0005%.

As per Firestone’s modus operandi, the overall sound of the Fireye HD is clear and neutral (Firestone states +0.02 dB @40 Hz, and -0.14 dB @15 kHz when tested with a 1 kHz sine wave, at 2 Vrms output). It’s highly impressive, and when you give it a deeper listen, you’ll notice that the Fireye HD is very much as transparent as the source you feed it. Some brands throw “HD” around as a buzzword, but the Fireye does indeed have high definition.

With it, I can hear layering and a depth to the stereo image that I can’t in my D7 and its well-regarded OPA1611-based output, and much less from offerings from FiiO, Styleaudio, or Audinst. Driving headphones with it felt pretty much effortless, on low or high gain. Transients feel ultra quick (this was apparent to me even before I knew the Fireye HD was based on the super-fast TPA6120). It casts a tall, deep, and wide stereo image, and I couldn’t notice any problems playing soft or loud, whether it was through the RE262, the HD598, or the Fidelio X1.

There is no hiss at all with the Fireye HD; even with the most sensitive IEMs, I could not pick anything up, with the exception of a high frequency hum when the volume knob is turned way up on high gain. At those SPLs, though, even on a full-sized headphone, the output would sooner destroy your ears than it would take for you to complain about the hum.

The one real issue that I can think of with the HD has to do with the output impedance; for the TPA6120, TI recommends a 10Ω series output resistor to isolate reactive loads, and thus, the Fireye has 10 ohms of output impedance built-in. While not a problem for the 150 ohm RE-262, most in-ear monitors are going to run into problems with the output impedance. Compared to the output of the D7 (which has <1 ohm of output impedance), I could hear a small, but significant difference in the tonal balance of my 4.A and FI-BA-SS when I plugged them into the Fireye HD.

It’s a shame, really, that the TPA6120’s wide bandwidth and speed requires current overload protection in such a way; otherwise, the Fireye HD would’ve been a near-perfect portable amplifier. With high impedance loads, however, the Fireye HD is simply sublime. It’s too bad that most balanced armature-based IEMs range between 8 and 50 ohms in impedance (and sometimes drop even lower in the lowest bass regions) and are simply unsuitable loads for the HD to drive. Only something like the ER4P/PT (which actually benefits in tonality from electrical under-damping) may possibly sound better with the Fireye HD, as well as high-impedance IEMs like the Heir Audio Tzar 350. Thus, the Fireye HD as an amplifier is best suited for high-impedance cans and portables, as well as dynamic insert earphones less affected by electrical damping (very linear impedance response).

The second, less crucial issue is the battery life; Firestone lists the battery life as 5 hours under continuous usage, which is not great if you take it outside for long periods of time, but is perfectly fine if you need to use it for audio meets and such. In my tests, continuous usage lasts over 5 hours, and the LED in the back will begin to flash red when it’s time to give it another charge. With the DC wall wart (or any compatible DC 5V power supply), you can leave the Fireye on wall power at the desk and not have to worry about battery life. Another interesting feature (or perhaps drawback) of the Fireye HD is that it never really fully turns off. Even when the power switch is in the “OFF” position, if you turn the volume pot way up, you’ll get a significant bit of volume coming out of the amplifier. I believe this feature is part of the “pop” protection circuit for cycling power implemented by Firestone (the protection circuit is built into the TPA6120, but requires the manufacturer to take full advantage). Anyways, be prepared to charge the Fireye weekly just to make sure you have enough juice inside.

Overall, the Fireye HD is a very, very capable portable amplifier worthy of mention amongst the others. Firestone has created an amp that provides a powerful, effortless output stage, wrapped it in a very attractive exterior and combined it with useful features.

It’s about to get even better, too — word is that Firestone is readying a balanced version of the Fireye HD; it features a proprietary balanced plug but includes multiple adapters for Kobiconn or XLR users. I believe this version is probably going to be even more appealing to folks that use portable amplifiers for full-sized headphones. I also hope that FSA figures out a way to lower the output impedance without sacrificing current protection, such as with an inductive output, rather than resistive. Nevertheless, be on the lookout for when the balanced version gets released.


  • Beautiful Fit and Finish
  • Effortless Driving Power
  • Very Neutral Sound, Wideband Presentation, Ultra-Fast Sense of Speed
  • Selectable Gain (1.5/5) and Bass Boost
  • Very Soft ON/OFF “Pop”


  • Output Impedance of 10Ω Not Suitable for Low-Impedance Headphones/IEMs
  • Below Average Battery Life (5 hrs. Continuous Operation), Never Completely “OFF”
  • Cheap Included Interconnect

For more information and in-depth specifications, please visit the Fireye HD information page on Firestone Audio’s official website:

Firestone Audio also has an official Facebook Page, one in English and another in Chinese. The Chinese language page is updated more often.

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Redux of my original review, posted on, on 2013/02/02

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Mr. T

Mr. T is an in-ear fanatic by day, and writes SOAP notes by night. He pities the fool who actually has the patience to read through his stuff.

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