Portable Class-A: Just Audio AHA-120


British amplifier manufacturer Just Audio came up with something that I haven’t seen before in the portable amplifier market: a class-A portable amplifier. The reason is this: class-A amplifiers are really inefficient designs, as with a theoretical maximum efficiency of 50%, you’re wasting one watt of power for every watt delivered. Not only does Class-A design demands large power supplies and heat dissipation on desktop amplifiers, on portable amplifiers that would mean a very, very large battery otherwise you can’t get a decent running time out of the amp. And yet, similar to big block engines on muscle cars, class-A amplifiers simply give you the best sonic performance there is.

Recognizing the power hungry nature of class-A amplifiers, he installed a massive 4.2 Amp hours Lithium Polymer battery that can last for a full week without a recharge. In comparison, a typical Nokia cellphone battery is 0.8 Amp hours, and a standard Canon DSLR comes with a standard 1.1 Amp hours battery (BP511). Looking over the PCB of the AHA-120, one of the reason that the enclosure was made to be that big is so that it can fit gigantic-sized batteries inside. Using Lithium Polymer batteries helps keep the weight low, and while the AHA-120 can’t be considered a lightweight portable, it’s not quite a box of brick either.

According to Justin, theoretically it would take 60 hours when running the amplifier at the 300 Ohm setting to fully drain the batteries, though real world usage will be lower. I’ve tried testing the battery life with the amplifier set at 300 Ohms with the Sennheiser HD800, and I’ve clocked over 32 hours and it’s still running strong with no flashing LED (Power indicator LED is supposed to flash when the battery life is at around 50%). With that kind of a battery life, this may be the longest battery life portable amplifier I’ve ever tested. And since you can charge it from the USB port (using the Ipad’s charger is a good idea to shorten charging time), I can’t imagine any average use scenario where the AHA-120 would run out of power.


In addition to the extra large battery, the user selectable class-A bias makes sure that the amplifier is not wasting extra power when not needed. The amount of current needed for high impedance headphones are noticeably lesser than what’s needed for low impedance headphones, and by selecting the appropriate impedance setting of your headphone, the amplifier will be able to run more efficiently, resulting in a longer running time. For further explanation of how the selectable class-A biasing works, please refer to the diagram below:

Although the impedance setting only go to as low as 32 Ohms, that doesn’t mean you can’t use the amplifier with lower impedance IEMs such as the Westone 4 (31 Ohms) or the JH16Pro (18 Ohms). As most of you know, though 32 Ohm headphones would be more current hungry than 600 Ohm headphones, low impedance IEMs like the JH16Pro are far less demanding in current levels, and I can run the 18 Ohms JH16Pro on the 300 Ohm setting without having any current shortage at all, hence conserving battery. Likewise, if you’re driving relatively light weight, easy to drive headphones (normally headphones that run fine out of an Ipod, or mid-level closed headphones like the Sennheiser HD-25 or the Audio Technica M-50, then you’ll be fine leaving the bias setting at 300 Ohms.

Remember, the switch has nothing to do with impedance matching, but rather it tells the amplifier of how much current reserve is needed to drive a particular headphone. Another factor to consider is also the type of music you’re listening to. Bass heavy electronic, house, or trance music is bound to be more current hungry than a slow paced simple vocal-guitar song. When the headphone/IEM demands more current than what the bias setting allows, it will simply revert to class AB operation, rather than denying the headphones the current they demands.


Another feature that is designed to prolong the battery life of the AHA-120 is the capability for the amplifier to run from USB power, while also giving a boost on the voltage swing (3.1V on batteries versus 4.1V on USB).

USB port for charging, LED indicator, APSS Switch, and Fast/Normal Charging switch.



The Just Audio AHA-120 is quite different than anything I’ve heard on the portable amp realm. The RSA amps have a dark coloration, the ALO Rx puts an emphasis in the treble and bass for a lively sound with good bass impact. Between them you have warm and mid-centric amps like the TTVJ and the original Pico, or something energetic like the Pico Slim or the Ibasso PB1-PB2. Being different than any of the previous amplifiers I’ve listened to, the AHA-120 is very neutral with absolutely no treble or bass emphasis. Recently we’ve had a number of very neutral, flat-line IEMs like the UM3X, the SM3, and the Westone 4, and in a way, the AHA-120 sort of takes the same philosophy of frequency tuning.

The flat-line sound of the AHA-120 happens to be very full sounding at the same time. And with a good amount of solid bass punch, you never feel like the amplifier is trying to mimic the AKG K701. The sound is thick and very analog, with a little grain in the sound. It reminds me of the signature of the Graham Slee line of amps, but with much better punch and PRaT. Despite the flat-line voicing, the sound of the AHA-120 works because of its thick mids, punchy bass, and very good depth and resolution in the sound. The thick sound doesn’t give a feeling of air, space, or clarity, but the detail level and the resolution of the amplifier is among the best I’ve heard in a portable, and clearly can take on a lot of the sub $500 desktop amps. Remember that this thing is running in class-A, and with a fully discrete output stage (BCP56, BCP51 amplifiers) and a high quality Vishay potentiometer.

The idea here is to come up with a high quality audio amplifier that strikes all the right points in the technicalities, while remaining to be very musical sounding. Some amps may give you a good technicalities, but they don’t let you feel the emotion of the music. Some others give a good musical presentation, but fall short in the resolution aspect. After all, everyone can come up with a warm sounding amplifier, but good resolution and musicality are two things that are rarely found in the portable realm. It is also recommended to try the AHA-120 with a good resolution headphone as lesser headphones may not be able to reveal the level of depth in the sound produced by the amplifier. Although most people will probably use the AHA-120 with a portable source, you can try hooking up the AHA-120 to a high quality desktop source (I used the CEC TL51XZ) just to see how much the AHA-120 improves with a good source.

The neutral voicing makes the AHA-120 quite a good all rounder amp. It’s not extremely snappy or fast like the Pico Slim, but the tight and punchy bass is a lot of fun for Linkin Park or Muse, while the full sounding midrange is great for Jazz Vocals like Diana Krall or even for playing Indie bands like Mumford & Sons or She & Him. The amp is so good with so many different genres, and as long as the recording is good, you will hear the superior resolution from the class-A topology.

The AHA-120 does make the Ipod Classic look rather small.



I’ve tried using the AHA-120 with the JH16Pro and the Westone 4, and although the Vishay potentiometer is still having balance issues at low volume, it was gone at my normal listening volume at around 9 o’clock with the JH16Pro. The AHA-120 volume control won’t give you the level of control that you get with digital volume controls, but it’s definitely usable with IEMs.

Perhaps one factor that may get in the way of a solid recommendation is its size. It’s not Triad Audio LISA 3 big, but it’s still bigger than the i-Qube or the RSA SR-71A, both being the bigger amp I had on the portable amp shootout. The footprint probably makes it a good stack for the Hifiman HM-801, though I haven’t tried it with the HM-801. But it’s definitely too big to be pocketable. I certainly am not going to carry this amplifier around when I want to be mobile, but I think it would make a very nice companion for travelling, or for those times at home when you don’t want to be tied down to a particular AC outlet.

The sound quality is so amazing, I think it just fall a little bit behind in terms of resolution than the Burson HA-160D amplifier!

Gears used for review:

Headphones: Sennheiser HD800, HD650, German Maestro GMP 8.35, JHAudio JH16Pro, Fostex TH-7B, among others.
Source: CEC TL-51XZ, Hifiman HM-602, Ipod Classic 120GB, Audinst HUD-MX1.

Rate this review

  • Donunus

    How much do these cost? This looks like an awesome portable. Looks like power and sound quality combined with great battery life and a charging option is finally here!

    So how does the power of this one compare with that of the fiio e9? I know both of them don’t have massive power specs but since the e9 can really pump out the volume, I thought it could be a good baseline for asking the question about power and gain.

    • Anonymous

      Hi Donunus,
      It’s £348.49 from Hifi Headphones

      The voltage swing is probably not as high as the E9, but I will check
      and get back to you.

    • Anonymous

      Just checked,
      Voltage swing is much higher on the Fiio E9, not even close. But I don’t
      think it is in any way an indication of sound quality or power output.

      • Donunus

        I think Voltage swing matters more for high impedance cans. What i was wondering more about was if we were to compare these against the e9 driving something like an LCD2

        • Anonymous

          Yes high voltage swing is required for low efficiency headphones and
          high impedance headphones, but the majority of amplifiers we have these
          days should have no issues for even 600 ohm beyers.

          I don’t think the E9 is adequate to drive big orthos like the LCD-2. I
          don’t know though it has never crossed my mind so I’ve never tried it.
          Likewise the AHA-120, I don’t think it has enough voltage swing for
          either the LCD-2 or HE-500.

  • Anaxilus

    Hey Mike, care to elaborate on the treble quality, clarity and especially source transparency as well? I would not be pleased with any short comings here at this price and size. Thx.

    • Anonymous

      The treble is smooth but fairly relaxed (especially upper trebles). It is not the sparkly treble kind of sound, and treble fans would probably find treble presence to be lacking. Hence I think this also affects the perception of clarity, similar to how people say the HD650 is veiled.

      Source transparency is very very good. It scales up very well to my CEC player.

      Quality wise, I think it’s a strong candidate for a portable amp with the most high-end sound, if there is a category like that. Too bad I didn’t have it early enough for the portable amp shootout. It displays depth and layering in the music that I don’t hear with any other portable amps.

      I’ve yet to find a shortcoming of the amplifier. I realize the sound signature may not be for everyone though.

      • Jpperez11

        I wonder how this Amp compares to the SR71A. I’m trying to decide on an Amp with the best overall sound (not too concern abou the size of either amp).

        • Anonymous

          It would rank very well, compared to the SR71a.

          I think it would also depend on the sound signature that you’re going
          for. The SR71A is darker, more low end body, but also more airy. The
          AHA120 is more neutral, smoother mids, more upfront, more refined, but
          also less airy.

          • Jpperez11

             Thanks Mike, nice review as always. I own DT88O’s, Sansa Clip, Xune (older version) and nationite S:Flo2. I like Bass but also like some clarity to go with it and as such, I’m tryint to put something togheter emphasizing these features. I don’t mind getting  a differnt pair of headphones to further this cause if the DT88O’s aren’t up to par.
            The AHA120 seems to be a good candidate but I also like the RSA amps as they have been proven to be good amps.

  • Andy Smith

    Guys, I have the Just Audio uHA-120 which is the smaller amp in the range and it is wonderfully dynamic, punchy and has tremendous bass wallop. Comparing it to my Stepdance and SR71-A it doesn’t have quite the same level of transparency, but ironically the amps musicality negates this and the uHA-120 is now my portable amp of choice.

  • denis

    Hi Mike,

    Have you tested this amp with the HD-800’s?

    • Anonymous

      Yes, good performance with the HD800.

  • なんか気になるなー

  • Denis

    Mike, is this amp a good match with Hifiman HE-5LE’s?

    Thanks, Denis.

    • Anonymous

      Hi Denis,
      I wouldn’t recommend it.

      Get the Matrix M-Stage instead.

      • Denis

        What about the ibasso P4?

        • Anonymous

          Likewise, Denis.
          The M-Stage will still be the better amp for the HE-5LE.

          • Denis

            OK Mike, thanks a lot. Will go for that M-Stage. 

  • Muncrats

    Mike, do you think this amp would be able to changes JH’s sound signature into more neutral and monitor like

    • Anonymous

      Nope. The Fostex HP-P1 is more like it. Come over and I’ll let you try it.
      But it looks like you need to change IEMs.

  • Dookie182

    hi Mike,

    I just found this review and have to say I’m pretty interested in it.
    any chance you could make a quick comparasion with dacmini or burson amp?
    kindest regards

    • Anonymous

      Well the Just Audio is a very smooth amp, a little slow in pace, thick and full sounding. It’s smoother and thicker than both the Burson or the DACmini. The Burson and the DACmini amp both have a faster pace and has more bass impact.

      • Dookie182

        thank you very much mike
        it seems like it’s hard to find an amp with both smoth and thick sound but with faster pace and good bass impact in the same time.
        Any advise from you experience?
        kindest regards

        • Dookie182

          ” It’s not extremely snappy or fast like the Pico Slim, but the tight and punchy bass is a lot of fun for Linkin Park or Muse”

          I guess that is what you where talking about, but this one looks like a great all rounder isn’t it? any advise of other good ones?

        • Anonymous

          The Burson is the closest combination I’ve heard that is both thick and fast and with good bass. However it is not as smooth as say the Justaudio or the Graham Slees. Perhaps I can generalize and say that thick amps tend to be slow. The thicker sounding, the slower the sound.

  • Quentin

    Hi Mike,

    How does this amp pair with the HD600/650 and how does it compare to the Schiit Asgard in overall performance?


    • The Just Audio is more refined than the Asgard. It should work nicely with the HD650.

  • Hi there, I just bought a LCD-3, and I’m wondering which amps to get of course. I want a portable one and possibly a stationary one. I’m now wondering if I should chose this for portable use, or a SR71b? It would also be used with a T1 a lot on the go.
    Im considering a Musical Fidelity M1HPA for home use since i read SS class A amps are good with the LCD-3, which is also why this came across my mind…

    Or is there a third amp you’d recommend? I prefer ones with good deep punchy bass response, full mids and some lively treble/mids, if that brings up any amp ideas… 

    • Hi Victor,
      I actually haven’t had the chance to try the LCD-3 so I can’t help you there. The LCD-2 is not so hard to drive and amps like the ALO Continental pairs very well with it, but I can’t say about the LCD-3.

  • Eagle1776

    What sounds closest to the AHA-120 that is a desktop DAC/AMP? Budget is between 300-400…

  • Ryan

    Would this amp pair well with the LCD 2’s in regards to sound? Also, is it powerful enough to drive them with authority?

    • I haven’t tried it to be honest. I think the Portaphile is a safer bet since I used it to drive the Hifimans

      • Ryan

        What about the sound though?

        • I think I wrote some comparison above

  • stan zorin

    Did you try it with T1 ? Beyerdynamics generally need tube amps to sound the best but in this case the thicker mids of AHA-120 might do the trick like tube colorations of sound do and make T1 sound more full in tone. Any thoughts and experience on pairing T1 with AHA-120 ?

    • Its been so long this review so I am not sure I remember, but I think I did try it with the t1 though I can’t remember if it was good or not