Questyle created one of our favorite all time portable DACs in the M15, and has continued to provide a very high level of quality in their ultra-portable devices – as well as in their desktop DACs – but they haven’t quite nailed the space in the middle yet. The CMA18 Portable was solid, but had a few shortcomings that kept it from landing at the top of the portable DAC pile. The recently released Sigma ($599) made some improvements over CMA18 Portable, but it was the Sigma Pro ($899) that really caught our attention. As a more advanced version of their best portable DAC yet, it elevates every aspect of the device, but is it enough to conquer a very competitive space?

Build and Design

Questyle has a unique design language for their portable products, typically featuring a sleek black (or silver) chassis and a window showing the circuit board and inner workings of the device. Sigma Pro is the biggest version of this setup that we’ve seen yet, adding a nice volume wheel and controls for something that looks like a hybrid of their flagship desktop DACs and their portable units. Sigma Pro is a fairly large device, about the same width and length as an iPhone Max series smartphone and more than twice as thick.

For inputs and outputs, Sigma Pro features pretty much every digital input you could ask for: USB-C, Digital Coax, Optical/TOSLink, and Bluetooth 5.4 (with LDAC, AAX, and aptX support). Sigma Pro also features analog inputs, with 3.5mm and 4.4mm line in. For output you have a full sized 6.3mm headphone jack and 3.5mm for single-ended output, and 4.4mm for balanced.

In terms of the form factor and general usability, Sigma Pro mostly fits into the transportable category, but it’s just small and light enough to have some potential for real portability. It’s too big to fit in most pants pockets, but isn’t entirely uncomfortable in something like a sweatshirt pocket. It’s best for situations where you need a simple device to bring from place to place: whether around the house, from the home to the office, or on vacation.

Features

Sigma Pro has three gain modes: low, high, and super. Low gain is best for IEMs, High is good for most headphones, and Super should be reserved for the hardest to drive over-ear headphones. You can expect a notable reduction in battery life with Super gain, as in our testing, we got up to 12 hours with Low gain and IEMs, but only 6 hours with Super gain and the HIFIMAN HE1000.

Bluetooth performance is solid, and basic operations like entering pairing mode are intuitively designed. The range is as expected, and the sound quality is excellent. With streaming services and casual listening Bluetooth sound quality was hard to distinguish from USB, though you’ll notice some quality loss with high-res files and more careful listening.

To control the various inputs and outputs on Sigma Pro, you use the Output and Source buttons on the front of the device to cycle through. Your current status is displayed on the front screen, along with the bitrate of the source audio if you’re using digital input. There are also indicators for the gain level on the screen, and adjusting the volume, will show the current volume level. Altogether, Sigma Pro feels premium and easy to use, accessing various features should be intuitive for users.

Sound

Sigma Pro offers a clean, reference sound, with strong dynamics that provide a strong sense of engagement and musicality. The most notable elements of Sigma Pro’s sonic delivery are the clean, crisp detail and strong separation. Sigma Pro also offers the unique combo of sounding detailed and articulate while also offering natural timbre, and a genuine lifelike quality to the music. This combination of technical performance and musicality rivals devices that are significantly more expensive than Sigma Pro. 

With IEMs, Sigma Pro delivers a clean, low-noise background with all but the most sensitive of IEMs. Even with that 1% of most sensitive IEMs, the background noise is minimal and not noticeable while music is playing. Using Super gain, it can handle a pretty wide range of IEMs, up to and including high impedance dynamic headphones like the Audio-Technica ATH-ADX7000 and lower sensitivity planars like the HIFIMAN HE1000.

Questyle released Sigma Pro alongside of the standard Sigma, which itself was an update to the CMA18 Portable. Sigma is closer in operation to the CMA18, sharing the same AK4493 DAC chipset and form factor. Sigma Pro uses dual ESS ES2093 DACs, resulting in a significantly different sound. Where Sigma is warmer and smoother, Sigma Pro is cleaner and more articulate with a more neutral signature. In spite of the standard Sigma’s dedicated IEM mode, I found Sigma Pro to be preferable with most IEMs. Sigma’s IEM mode reduces the noise when no music is playing, but there’s more noise noticeable in highly dynamic tracks, while Sigma maintains a lower noise level throughout.

Comparison: iBasso D17 Atheris

Sigma Pro is a great sounding DAC/Amp, no doubt, but is it really on the same level as significantly more expensive products? To test that out, we did some extensive comparisons with the $1999 iBasso D17 Atheris (which we found to be a solid competitor at its price,) and found that, yes, Sigma Pro really is in the same class as devices at more than twice the price.

In terms of the build and design, the two devices are different takes on a similar aesthetic. Atheris has a little more of a vintage inspired look, while Sigma Pro is a bit sleeker looking – especially in black. Atheris is slightly larger, so if Sigma Pro could barely fit in your pocket, Atheris is definitely not going to fit. Sigma Pro offers a few more input and output options though, as Atheris simply has USB-C input, and 3.5mm and 4.4mm output.

In terms of sound, Atheris provides tube and solid state options for two different takes on the sound. In tube mode Atheris offers a thicker, weightier sound, but loses a small amount of resolution, while solid state mode sounds cleaner, but lacks the weight and punch. I preferred the balance of detail and musicality of Sigma Pro over Atheris’s solid state mode, and the tube mode is largely on par in overall sound quality: just different. If you want the cleaner, more articulate detail, Sigma Pro is better. If you want that detail mixed with a more euphonic warmth, Atheris is what you’re looking for. At less than half the price though, Sigma Pro sounds like a steal.

The Bottom Line

Sigma Pro steps up the features, performance, and sound quality of Questyle’s larger portable items, providing a clear upgrade over past models like CMA18 Portable and its non-”Pro” counterpart. Sigma Pro isn’t just an upgrade for Questyle’s line, it’s also a genuine price-bracket busting device, that offers build, design, and performance that rivals flagship DACs and DAPs that cost nearly twice as much.