Why Apollo Sounds Better

In the crowded field of audio interfaces, Universal Audio’s Apollo has risen above all the rest. Used on hundreds of Grammy-winning albums since its introduction in 2012 — from Kendrick Lamar and the Black Keys to Ariane Grande, Green Day, and Tyler, the Creator — Apollo interfaces are commonplace in pro studios around the world.

But what exactly makes them sound better than the competition?

In this article, we'll explore the level of detail we've put into the Apollo line of desktop and rackmount interfaces, specifically its elite-class AD/DA conversion, and talk to the people behind its impeccable sound quality.

Apollo: A Simple Design Philosophy

Although the birth of Apollo was a massive team effort at Universal Audio, one of its main drivers is UA Technical Fellow, Dave Rossum.

A pioneer of the synthesizer revolution, inventor of the polyphonic keyboard, and co-founder of E-mu Systems, Rossum changed the course of modern music with the E-mu SP-1200 sampling drum machine — a groundbreaking device at the center of hundreds of classic pop and hip-hop records over the past four decades.

Through all of his legendary designs, Rossum uses a single core principle as his guiding light, and Apollo interfaces were certainly no different.

"I strive for a simple design," says Rossum. "And for me that means getting a circuit topology that works cleanly, rather than tweaking things to zero in on optimum specs. Plus, we're not chasing specs, we're looking for sound. Sure, specs are vital, but real listening by golden ears matters too. There are nuances too subtle to measure scientifically, but to an experienced ear, they can be distinctly audible.

"The listening tests vary. Some are simple, informal consensus. Others are more sophisticated and scientifically designed for true 'double blind' testing.

"We enhance this by also testing — by including multiple identical copies in the preference list — whether the listeners can actually distinguish when sounds are identical. This allows us to really know what is clearly audible, and what is preferred, versus simple test condition artifacts."

Apollo: Class-Leading Audio Conversion

At the heart of any audio interface's sound is its AD/DA conversion. AD/DA stands for analog-to-digital (AD) and digital-to-analog (DA) conversion. These processes are critical to recording and playing back sound in the digital world. High-quality AD/DA conversion is essential for accurately capturing and delivering every nuance of sound, similar to how a high-resolution camera captures every visual detail.

"With Apollo, transparency is the goal, but there’s always going to be some perceived, subjective alteration to the sound," says Rossum.

"We chose the ADC (audio-to-digital converter) for Apollo particularly because we like the sound of it," he continues. "While the function and specs of it seem comparable to the ones in other interfaces, each modern converter has its unique quirks related to the “chaotic” math algorithms. As for the DAC (digital-to-analog converter), they generally have better specs than ADCs, so with careful component selection it’s easier to get a transparent design."

Over the years, Apollo interfaces have had various improvements, but the latest Apollo X Gen 2 interfaces offer the best AD/DA conversion yet, with unprecedented low noise and distortion (up to 0.000037% THD+N).

"See, modern audio converters all use a technology called 'delta-sigma' modulation," continues Rossum. "This avoids the requirement for matched analog components that were needed in the old-style 'R-2R' converters of the 1980s.

"They use some sophisticated DSP and very high sample rates — 6MHz or even much higher — to move all the artifacts of sampling — the stuff typically called quantization noise or distortion — out of the audio band.

"Unfortunately, these mathematical algorithms can have objectionable resonant modes, so they are deliberately designed to be somewhat unstable or chaotic to break up these modes, which could color the sound. Every interface manufacturer has their own 'secret' implementations, so each sounds a little different, even though they are all designed to test well using the standard Dynamic Range and THD+N tests.

"Thankfully, we discovered numerous circuit enhancements that reduced distortion and noise in Apollo X Gen 2," Rossum continues. "There were a number of small tweaks based on lessons learned, all of which are part of our dedication to continuously improving our products."

Apollo X Gen 2 interfaces offer class-leading jitter performance. In an audio interface, jitter affects the conversion of digital audio signals to analog (or vice versa) and introduces unwanted artifacts like noise, subtle distortions, or a loss of clarity. It’s especially crucial in high-quality recording and playback.

With Apollo X Gen 1 we introduced the concept of Dual-Crystal clocking for Apollo rackmount interfaces. Dual-Crystal clocking is a premium circuit topology that relies upon optimized crystal-oscillators as the clock source to maintain consistency and precise timing.

Apollo X rackmount interfaces use one crystal oscillator for 44.1 kHz related sample rates, and another for 48 kHz and its related multiples (96 or 192 kHz for example). By doing this jitter is minimized across the audio band without compromise, ensuring Apollo X rackmounts sound consistent, no matter what sample rate your session is at.

"Maintaining precise timing for digital audio signals, both internal and external, is crucial." says Rossum. "It really comes down to the fine details of the components, “but also the layout of the PC board and precise, extensive early-stage QA."

Another big improvement of Apollo Gen 2 interfaces is the headphone amp — something very noticeable the minute you plug in a pair of cans, with a jaw-dropping 0.0013% to 0.00025% difference in THD+N compared to earlier Apollos.

"For previous versions of Apollo, we used an off-the-shelf IC (integrated circuit)," explains Rossum. "But with Apollo Gen 2 we felt we could do better by 'rolling our own' out of discrete components. Delightfully, the result was an improvement in specs, but more importantly, sound."

"With Apollo interfaces, the devil is in the details," says Rossum. "It's not something competitors can easily copy. There's way more to it than just the schematic."