Questyle CMA600i
Questyle is a newish company on the scene, drawing on high-tech enthusiasm and skills from China. We’re not entirely sure, in fact, whether it is pronounced ‘cue-style’ or ‘quest-aisle’ — prevailing wisdom goes with the latter. What we are sure of is that the Questyle ‘DAC with Headphone Amplifier’ is a well-built, versatile and fine-sounding headphone amplifier and DAC.
The first thing you notice is that this is one well-constructed piece of kit. The case is made of slabs of aluminium. It’s smaller than standard component width, but still weighs a very solid 3.2kg.
It has a certain old-timey/hobbyist feel to its styling, thanks primarily to the three main front-panel switches. They remind us of the switches on the Electronics Australia projects we built 40 years ago. The quality on offer here, however, is undeniable.
Questyle CMA600i
Also on the front panel are two 6.5mm headphone outputs conforming to normal standards, plus a four-pin socket which, if it weren’t for the fourth pin, would look like an XLR socket. The first two are, of course, for regular headphones. The third is for balanced headphones (in which there’s a separate return for each channel, and typically a push-pull configuration for the amplifier, allowing for improved noise rejection). Even with regular headphones, things were quite revealing enough, as we’ll see.
The front panel has four LEDs to indicate status: whether the signal is PCM or Direct Stream Digital (DSD), whether the input is USB or digital. There’s an analogue volume control, a power switch, an input selection switch (USB or digital) and another input selection switch for choosing between the relevant digital input and the analogue stereo input on the rear panel.
‘Digital’ means the optical or coaxial digital audio inputs on the rear panel. USB means the USB Type-B socket which allows the unit to act as a super high quality audio device for computers. It is natively supported by Macs and ought to be with the next version of Windows 10, due out early in 2017. Meanwhile a driver for current and older versions of Windows is supplied on a mini CD-ROM.
In addition to the headphone outputs, there are RCA line-level outputs on the rear panel, 
along with balanced XLR outputs for those with higher end gear.
A small but classy-looking black and gold remote allows volume control and muting (the other buttons on the remote are ineffective). The volume isn’t digital: a servo turns the front-panel volume knob.
From what we’ve been able to glean about the current-mode amplification, it is to do with the amplifier topology. It is not to do with some kind of magical ability to deliver particular currents at the output independent of voltage and impedance; Ohm’s law has yet to be broken. Instead the internal feedback loop draws not on the voltage of the output, but the actual current delivered at the output of that section of the circuit. The advantages of current mode amplification are significantly higher slew rates (the speed at which voltage can change), greatly reduced rates of transient intermodulation distortion, and much higher effective bandwidth. Questyle’s implementation is also pure Class A in the amplification stage, ensuring zero crossover distortion.
The DAC used is the 32-bit, 768kHz AK4490 chip. This also supports DSD files up to Quad speed (aka DSD256).
Each Questyle CMA600i is factory tested to ensure that “over 30 factors of specification” are met. Two graphs are provided on the test card: frequency response and THD plus noise for the amp section. For the review unit, the frequency response showed the output down by around 1.5dB at 200kHz. The THD graph showed distortion reducing at the headphone output from 0.15% at 10mV output to less than 0.0005% at three volts output. Distortion remained ridiculously low until just short of seven volts output.
There’s no cut-out switch on the headphone sockets, so the line-level outputs continue to 
work when headphones are plugged in. On balance, for a device like this we think that’s the better option for versatility. Neither can you use both balanced and 6.5mm outputs at the same time. The warning about this on the insert is so stern that perhaps it would have been better to add another switch to force manual selection.
Even if you knew nothing about this unit, you could guess that it uses Class A amplification. It generates quite a bit of heat even in the rest mode with nothing playing. The top panel is almost as warm in this mode as when it has been working hard for a while, when it can get quite close to becoming uncomfortable if you leave your hand atop it.
We noticed that in the story that Questyle tells of its origins, its founder Jason Wang reports being disappointed that even with fine headphones, the amplifiers he had been using prior to developing Questyle products had produced a sound that “was dry, empty and emotionless”.
We listened to a lot of music, a very large amount of music indeed, using a variety of headphones and employing the CMA600i and we did not find it injecting into the music dampness, filling any emptiness, nor adding any sentimentality, nor any other emotion. What we found was an astonishing accuracy, a just-about-perfect transparency.
It was actually rather disconcerting. Through a combination of control of the headphones and unrelenting accuracy in its delivery, this unit revealed what was there in the recording. Without masking. Without making anything up. Music often sounded substantially different from the expensive near-field speaker system that we often use for critical listening, because the headphones driven by the Questyle CMA600i revealed so much more and utterly refused to hide anything.
Which meant that an emotionless recording remained emotionless. A dry one most definitely remained dry. Any emptiness in the music remained a gap. A headphone amplifier should not create something that isn’t there. (If it did, that would be called ‘distortion’.)
Anything with heavy post-production was mercilessly revealed for what it was. But music that was recorded with little processing shone through. Almost regardless of age. The original film soundtrack of ‘West Side Story’, for example, features relatively primitive recording technology but no dynamic compression (which hadn’t really been invented then) and this sounded wonderful, far less like a 55-year-old recording than we’d previously experienced. To be fair, the ultrawide separation of Jimmy Bryant and Marnie Nixon in Tonight was definitely 1960s stereo show-off stuff. (You didn’t think Richard Beymer and Natalie Wood sang their own parts, did you?)
Moving to a different genre, but a recording with an equivalent lack of compression, the finale of the Shchedrin version of Bizet’s ‘Carmen’ on a Chandos CD delivered every nuance from every member of the audience, in an extraordinary demonstration of the amount of detail able to be delivered by CD standard audio.
With higher resolution material — for example, Rickie Lee Jones’ album ‘It’s Like This’ delivered in Direct Stream Digital — the detail was perhaps even higher. On the opening track, Steely Dan’s Show Biz Kids, there seemed initially to be a slightly reduced sense of body in Jones’ voice, a touch less reverb. But this, we realised, was control. Control by the CMA600i, control by the headphones. There had been plenty of air around other music elements on other tracks. What the system was doing was revealing the relatively dry recording of her voice on this track. Does this detract from the performance? We think not. We like it just the way it was delivered, which is just the way she sang.Don’t like it? Then you don’t like the source.
This is an ongoing debate among music lovers. Do you want a system that somehow sounds ‘good’? Or do you want a system that delivers precisely what’s in the recording? We plump for the latter.
We must add that across the range of music we enjoyed (there was 1970s prog rock in the 
mix, ‘Rage Against the Machine’, fine acoustic numbers recorded direct to DSD, among others) there was no limitation. No limitation to volume level. No limitation to bandwidth.
Clearly there must be limits, but they were well beyond anything any reasonable set of headphones, or set of ears, would find appropriate.
Since the unit performs so wonderfully well with headphones, you won’t be surprised to read that it works just as well at the line outputs, delivering a great performance to a stereo system. The level control also affects the line outputs.
There was one clear improvement possible: eliminating the somewhat startling switching noise when going from PCM to DSD tracks.
If you like listening with headphones, do yourself a favour. Find a shop that sells the Questyle CMA600i DAC and headphone amplifier and have a listen. Seriously. Do it. 
Questyle CMA600i

Questyle CMA600i DAC with headphone amplifier
Price: $1599

+ Superb performance both in DAC and headphone amp sections
+ Excellent facilities
+ Good value for money

- Some switching noise going to DSD
- Could add protection for user error connections

Inputs: 1 x optical digital audio, 1 x coaxial digital audio, USB Type-A, 1 x stereo analogue audio

Outputs: 1 x stereo analogue audio (RCA), 1 x stereo analogue audio (XLR), 1 x coaxial digital audio, 2 x 6.5mm headphone, 1 x 4-pin balanced headphone
Dimensions (whd): 330 x 55 x 200mm
Weight: 3.20kg

Contact: Audio Dynamics Pty Ltd
Telephone: 03 9882 0372