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.
ADJUSTABLE CLASS-A BIASING
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.
SELECTABLE POWER SOURCE
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).
GENERAL SOUND IMPRESSIONS
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.
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.