ASTELL&KERN ACRO L1000 headphone amplifier
Led by its frankly enormous knob, Astell&Kern’s headphone amp is unusual in more ways than merely its looks.
Astell&Kern is the brand name used by long-time portable audio player iriver for its high-end stuff. We’ve covered a few of those over the years, including the company’s multi-thousand-dollar flagship portable music player. This time, however, we have something a little different. It’s the Astell&Kern ACRO L1000 Desktop Headphone Amp.
Equipment
It is a slightly unusual device in terms of functionality, so it’ll be best if we start by making that clear. It is a headphone amplifier — but only for computer-like devices. It has only one input. That’s a Micro-B USB socket. You plug it into a computer and it becomes an audio playback device for the computer. An external sound card, if you will.
But there’s more. The box is labelled ‘Made for AK Portable Player’. I guess it’s taking the lead from Apple’s ‘Made for iPod’ logo. Anyway, instead of a computer you can plug in any of eight A&K portable music players, and we’d better list them (since some models aren’t supported) — you can use the SP1000, AK380, AK320, AK300, KANN, AK240, AK70 MKII and the AK70.
I haven’t tried it yet, but I’m guessing you could also use most Android phones and iPhones.
So that’s input. What about output? Headphones and, surprisingly, loudspeakers. On the back there are four proper gold-plated loudspeaker binding posts, along with a slide switch. You use this switch to choose whether you want headphones or speakers.
As for headphones, there are four connection options. On the left side is a 6.35mm stereo headphone socket, plus a 3.5mm electrical duplicate. Those are both for unbalanced headphones. Also on the side is a 2.5mm socket for balanced headphones. On the back is a four-pin XLR socket, also for balanced headphones.
When the switch is in the speaker position, the headphone sockets produce no sound. When in the headphone position, the sound goes to the most recently connected headphone socket. You can’t use multiple outputs at the same time.
There isn’t a line output as such. If you wish to use it as a DAC for a different audio system, you’ll just have to use one of the headphone outputs. The output level will be subject to the L1000’s volume control.
Speaking of which, it has one hell of a volume control knob! It’s 97mm in diameter, 18mm tall, and features a gently glowing red ring around its base when it’s switched on. Eleven LEDs around the edge indicate the volume level. Approximately. There seems to be about 10 barely perceptible volume level clicks between each LED illuminating.
Another LED is at the bottom of the volume control indicators and this glows blue, green or red according to the Filter mode. (The explanation of the three settings made little sense to me. I’ll sort out what they mean in the panel overleaf.) Also on the side is a power/standby button.
That’s about it. I won’t spin words to describe the styling; you can see it for yourself in the photos. I’ll just note that while you may think having the speaker/headphone selection switch on the back would be inconvenient, the way the unit leans makes it easy to reach over and find the switch surely by touch alone.
The DAC supports high resolution audio... and low resolution audio according to Astell&Kern. Apparently it will play audio with sample rates as low as 8kHz all the way up to 384kHz, and with resolutions of 8, 16, 24 or 32 bits. Plus DSD in regular form (DSD64), double-rate DSD128 and quad-rate DSD256. The unit packs two AK4490 decoder chips. According to Ashai Kasei’s spec sheet, the chips themselves can accept up to 768kHz PCM. It performs 256× oversampling and offers a choice of antialiasing filters. It also has a 255-level 0.5dBstep digital attenuator.
The loudspeaker outputs are rated at 15 watts per channel into four ohms. The only specification given for the headphone outputs is 6V RMS for the unbalanced sockets, and 8.5V RMS for the balanced ones, in both cases unloaded.
Performance
I used the unit primarily with a Windows computer. Current versions of Windows 10 support the USB Audio Class 2.0 standard required by this DAC, but since this was only recently introduced results can sometimes be iffy. In particular, DSD can often require the use of an ASIO driver, and the native Windows support doesn’t include that. So I downloaded and installed the specified Astell&Kern driver on the computer. (No driver is required for Macs.) But before doing that, I did run the unit briefly with the native Windows drivers and it worked perfectly well, except that there was no DSD.
All went smoothly. I installed the software, then plugged in the DAC and after a few seconds it was installed. Windows made it the default audio device and gave output options of 16 or 24 bits (but not 32, see screengrab), for 44.1kHz to 384kHz sampling.
Of course, I didn’t use Windows’ regular audio facilities for critical listening. For that I used Foobar 2000 which can bypass all that stuff (once you install the necessary components and configure them properly). But the Windows audio facility remains necessary for such things as Spotify, YouTube and assorted other regular music services.
Configuring Foobar2000 was a little different because for the first time I was ready to give DSD256 a whirl. Direct Stream Digital is not a format supported by USB Audio Class 2.0. So it’s usually handled by disguising it as PCM. It’s not converted to PCM, it’s merely given headers and such, so that it looks to USB as though it is PCM. The DAC recognises it for what it is, and decodes it as DSD. This is called DoP, for ‘DSD over PCM’.
But DoP for DSD256 requires it to be disguised as 705.6kHz PCM, which most stuff doesn’t support. So DSD256 has to be delivered in ‘native’ format. I don’t know how USB is fooled by this, but it works.
I spent a fair bit of time enjoying the limited selection of DSD256 and DSD128 material I have managed to gather in recent years. Playback was flawless. I mostly used a pair of Oppo PM-3 headphones, and my old (and much loved) pair of Sennheiser HD 535 headphones. The sound was lovely throughout. The Sennheiser headphones are a bit more demanding, being higher impedance and lower sensitivity than is common these days. Indeed, they can sound a bit thin unless the headphone amplifier has the bass thoroughly mastered. Clearly the Astell&Kern ACRO L1000 has the bass mastered.
The unit handled everything I threw at it, including PCM music sampled at 352.8kHz and 384kHz. All worked perfectly well. And it all sounded excellent. There was plenty of gain available. The large volume control tended to make it slower to adjust the volume. The wheel couldn’t really be spun, to do it faster. But it was possible to adjust the level very precisely.
Using the unit with a pair of loudspeakers, it did a reasonable job. With only 15 watts available you’ll want fairly sensitive speakers. I used my desktop models and the sound was very good. They don’t go deep into the bass (I normally use a subwoofer for that, and there’s no subwoofer output with this DAC), but the upper bass, mids and treble were all well handled. However the gain was a little on the low side with this function. To get good volumes it was necessary to have the volume indicator close to the top LED.
Conclusion
Unless you have a particular preference for a slow and early filter with your CD music (see panel above), select the red LED filter mode. It stays selected even if you turn off the unit and turn it on again. As for the unit as a whole, the Astell & Kern ACRO L1000 is a highly competent headphone amplifier for those who want first-class sound from their computers. And it has the added convenience of running loudspeakers if you need them.
Stephen Dawson