Questyle Sigma's Review - Through The Looking Glass

 

General/Packaging/Build

Opening the Questyle Sigma feels less like unboxing a gadget and more like handling a piece of modern art. The packaging is minimalist but effective, sufficient protection to prevent the Sigma from damage during transportation with the device resting in a perfectly shaped indentation. Inside, you get the essentials: a short USB-C to C cable, a longer charging cable, and a USB-A adapter, being an audiophile centric company, i do hope the cable can be off better quality

The device itself is striking. Questyle sticks to their "transparent" industrial design language, sandwiching a CNC-machined aluminum frame between two panes of optical-grade glass, It looks incredible—you can see the entire PCB layout, the chips, and the red LED indicators that are glowing from within. At 599, I would expect to have a certain protective film or at least some silicon case included, sadly there’s none and it will be an optional purchase.

In terms of size, it is definitely bigger than most dongles for sure, but still it is fairly transportable and small, not really an issue for me. It is roughly the size of an old iPod touch, but thicker and slightly shorter in terms of length. Pocketable? Yes but not recommended due to the surface finishing which might be prone to micro scratches

Gears used for this review

• xDuoo XD05T

• Questyle Sigma

• Fiio JT7, Dunu 242, Letshuoer Ember, Dita Mecha

• Foobar 2k/Tidal Streaming exclusive mode

• Google Pixel 9 Pro XL

Specifications

• DAC Chip: Single AKM AK4493

• Amp Tech: Patented Current Mode Amplification (CMA), fully discrete, Class A operation

• Outputs: 3.5mm Single-Ended and 4.4mm Balanced

• Inputs: Bluetooth 5.4, USB-C Data, USB-C Charge

• Battery: ~4300mAh to 4800mAh, rated for ~12 hours (Real world closer to 10-12 hours)

• Bluetooth Codecs: LDAC, aptX HD, aptX Adaptive, aptX Lossless, AAC, SBC

• Peak output power : >5w (5000mw)

Foreword

My review is solely based on what I hear via my equipment and I never consider my reviews to be objective in any way rather a subjective approach. Do take into consideration that everyone’s ear anatomy is not the same, so the psychoacoustics perception might be different as well, but i believe it will not stray too far

Driving Power

• From efficient IEMs to demanding planars like the HiFiMAN Sundara or HE1000, the Sigma keeps the bass tight and the dynamics uncompressed.

• It utilizes Questyle’s proprietary "Current Mode Amplification" (CMA), this gives it a level of authority and grip that you rarely find in standard voltage-based dongles

• You get toggle switches for Gain (Standard/High) and a dedicated "Headphone/IEM" mode. Headphone mode bumps the voltage and current for high-impedance gear, while IEM mode is designed to optimize the noise floor for sensitive monitors, which to me, it works somehow like an impedance adapter but without the actual adapter plugged in, but personally i have always let it at headphone mode as i personally find that it sounded so much better compared to IEM mode, IEM mode somehow is darker and not as open and clean sounding compared to headphone mode

Bluetooth Capability

• Bluetooth connectivity in terms of range is quite stable, with my phone placed on tabletop in my living room, while bringing the Sigma with me into the room which is approximately 5-6m away with some obstructions, i did not experience any connection issue

• Listening test with LDAC codec, and the LDAC quality set to the highest via developer mode in my Pixel phone, i am not sure if the issue is my phone or Sigma itself, but it does experience some hiccup and stuttering, but with the LDAC quality set to balanced, there is no stuttering and the sound quality is clean

• As with the rest of the codecs, AAC and SBC, they work without any issues, but of course if you are using Apple’s device, you are only limited to AAC

Features

• The Interface (or lack thereof): There is no screen. You have to rely on a series of LED lights (dots) to figure out your sample rate, volume, and battery life. Honestly, deciphering blinking orange dots to guess if you have 25% or 50% battery left feels a bit outdated and annoying compared to competitors with OLED displays

• Dual USB-C Ports: This is a massive win. One port is for data, the other for power. You can charge the Sigma while listening without introducing noise or draining your phone’s battery

• Connectivity: Bluetooth 5.4 is rock solid. I found the connection stable even through walls, and having LDAC and aptX Lossless support means wireless listening doesn't feel like a huge sonic compromise

• IEM Mode: While designed to kill background hiss, results are mixed. Some find it pitch black, while others note that the noise gate seems to cut out only when the music stops, with some hiss audible during quiet passages in dynamic tracks

• Missing Modern Comforts: There is no companion app. No parametric EQ. No digital filters. You get what the hardware gives you.

Sound Impression

The sound is undeniably "Questyle"—which is a compliment. It leans towards a mature, reference-grade presentation that prioritizes technical ability over artificial coloring.

Tonality The Sigma uses an AKM chip, which gives it a hint of "Velvet Sound" warmth compared to clinically sharp ESS implementations. It is widely described as neutral and transparent but avoids sounding sterile,. There’s a smoothness to the mids and a bit of organic weight that makes it less fatiguing than a purely analytical source.

Bass It’s all about control. The CMA tech tightens up flabby bass lines, adding texture and speed,. It trims the fat, meaning you might miss some of the loose, boomy slam found in "fun" tuned amps, but the extension is deep and the definition is top-tier.

Mids & Treble The midrange is clean with excellent separation. Vocals have a natural body—female vocals sound airy while male vocals have appropriate weight. The treble is a highlight; it is extended and airy without getting sharp. It manages to take the edge off harsh tracks while keeping the detail intact, making it great for treble-sensitive listeners.

Technicalities The soundstage is wide and "holographic," with instruments having plenty of air around them,. Separation and detail retrieval are excellent, allowing you to easily pick out micro-details like reverb tails.

Comparison

Questyle Sigma (599)vs.xDuooXD05T(549)

These two devices sit in the same price bracket and "transportable" size category, but they appeal to different audiences:

• Form Factor & Build: The Sigma is a sleek, glass-and-aluminum slab that’s very nice to look at (then again, this is subjective and more like a personal preference) . The xDuoo XD05T is a utilitarian tank—it's thicker and heavier, not as pocketable, the xDuoo features vents for heat dissipation due to it being a hybrid of tube. The Sigma is much easier to slide into a bag, whereas the xDuoo feels more like a desktop unit you can move if you have to.

• Features: The Sigma is a solid-state device using Current Mode Amplification. The XD05T is a hybrid tube/solid-state amp, allowing for a different sonic flavor. The xDuoo wins on features: it has a proper OLED screen, a volume wheel (versus Sigma's buttons), a Bass Boost switch, and a massive array of inputs including Optical, Coaxial which the Sigma lacks.

• Power: Both put out a more or less similar amount of power, and will drive almost anything with the exception of Susvara i believe? 

• Sound: The Sigma is cleaner, faster, and more neutral-reference. It focuses on transparency and tight transient response, the xDuoo on the other hand offers a warmer tonality  thanks to the tubes, plus a bass boost that can be fun but colors the mids, but for certain IEMs it actually worsen it, making the bass muddy, so i generally avoid using bass boost unless the IEM/Headphone is really very bass light.

There isn’t any good or bad between the two of them to be honest, it will be more like a personal preference kind of thing, if you prefer an uncolored sound signature, go for Sigma, if you like your sound warm and rich, go for the xDuoo’s.

Final Thoughts

The Questyle Sigma is a great device overall, the excellent Current Mode Amplification does wonders and every track played via the Sigma sounds clean and it has very good extension on both ends. There are of course some personal nitpicks such as the exclusion of a case or any sort of film protector applied for both the front and back glass, then again, not really a major concern.

For the most part, it works. The sound is effortless, powerful, and beautifully transparent with just the right touch of warmth. It drives full-size headphones with authority that smaller dongles just can't match. All in all, Sigma definitely deserves a recommendation!

*The Sigma is sent over by Questyle for the purpose of this review. I thank them for the support and opportunity as always

Questyle Sigma Product Page - Non Affiliated

Questyle Sigma Store - Non Affiliated