Universal Audio Apollo X4 Gen 2

Is this the best interface currently to do it all?

PROS: Excellent build and finish quality, standout design, industry-leading Unison preamps with analog emulation, pristine AD/DA conversion with wide dynamic range, Real-time UAD plugin processing for zero-latency tracking, Sonarworks Apollo Monitor Correction, Auto Gain, compact yet robust build quality, flexible I/O with ADAT expansion for scalability.

CONS: Premium price (expensive), basic power adapter (wish for C13 integration), no thunderbolt cable included, DSP limitations if one would like to use onboard DSP for running all UAD plugins, expensive individual plugins and bundles, additional costs for Sonarworks monitor and headphone calibration after the promotional period.

Links – Official Website ($1999/$2299) | Promusicals (Indian Distributor – ₹206,159)

Introduction.

Universal Audio (UA) has firmly established itself as a leader in the professional audio industry, revered for blending state-of-the-art technology with the warmth and character of analog sound. With the Apollo X4 Gen 2, UA expands its Apollo lineup to cater to creators who demand studio-grade performance in a compact, desktop-friendly form factor. Combining exceptional audio quality, real-time UAD plugin processing, and Unison-enabled preamps, the Apollo X4 Gen 2 is positioned as a versatile powerhouse for musicians, producers, and audio engineers alike. In this review, we’ll take a deep dive into the Apollo X4 Gen 2, examining its features, sound quality, usability, and how it compares to other interfaces in its class.

X4 Gen 2 Inputs and Outputs.

The Apollo x4 Gen 2 is designed to handle various recording situations, from simple vocal and guitar tracking to full-band sessions using external preamps via ADAT expansion.

Microphone/Line Inputs (4x Unison-Enabled Preamps).

Type: 4 x XLR-1/4” Combo Jacks (mic/line)
Preamps: Unison-enabled, digitally controlled analog preamps
Impedance: Dynamically variable with Unison technology
Phantom Power: +48V individually switchable per channel
Gain Range: Up to 65 dB

The four Unison-enabled preamps allow users to record with near-exact emulations of famous preamps (Neve, API, Manley, SSL, etc.), providing an analog-like experience with real-time DSP-powered plug-ins. The preamps have an extended gain range and low noise floor, making them ideal for both high-gain dynamic microphones (e.g., SM7B) and low-output ribbon microphones. The mic/line combo jacks allow flexibility for different input sources without requiring additional adapters.

Hi-Z (Instrument) Inputs (2x Front Panel).

Type: 2 x 1/4” Hi-Z instrument inputs
Location: Front panel

These inputs are designed for direct guitar and bass recording, offering authentic impedance matching with classic amp simulations like Fender, Marshall, and Ampeg. Lower latency compared to DI boxes allows for real-time monitoring with effects applied without performance lag. The inputs automatically disable the corresponding mic/line input on the back panel when used, avoiding signal conflicts.

Optical Input (ADAT/SPDIF).

Type: 1 x Toslink Optical Input
Format: ADAT (8 channels) or S/PDIF (stereo)
Sample Rates: Up to 96 kHz (ADAT), 192 kHz (S/PDIF)

This input expands the Apollo X4 Gen 2’s capabilities, allowing users to connect an external 8-channel preamp (such as the UA 4-710d, Focusrite OctoPre, or Audient ASP880) for additional microphone inputs. When used in S/PDIF mode, it allows high-fidelity digital signal transmission from other studio gear such as external mic preamps, digital synths, or mastering converters.

Outputs: High-Quality Monitoring and Routing Options.

The Apollo X4 Gen 2 offers a variety of outputs designed to accommodate different monitoring setups, external gear routing, and headphone mixes.

Monitor Outputs (2x Balanced 1/4” TRS).

Type: 2 x 1/4” TRS Balanced
Dynamic Range: 129 dB
Maximum Output Level: +20.8 dBu
Impedance: 100Ω

These high-quality monitor outputs offer one of the cleanest signal paths in any desktop interface, with improved D/A conversion ensuring accurate playback. The improved bass management system allows users to integrate a subwoofer seamlessly, making this interface ideal for mixing with full-range monitoring. The Apollo Room Correction system works in tandem with these outputs, applying Sonarworks SoundID room profiles to optimize monitoring accuracy.

Line Outputs (2x Balanced 1/4” TRS).

Type: 2 x 1/4” TRS Balanced

Use Cases: Sending signals to outboard gear, additional monitor speakers, or re-amping These outputs allow flexible routing for external gear such as analog compressors, EQs, or effects processors. They can also be used as an alternative speaker output, allowing for easy switching between different monitoring setups.

Headphone Outputs (2x 1/4” TRS).

Type: 2 x 1/4” TRS Stereo
Independent Volume Control: Yes
Impedance: Low-impedance output for better compatibility with a variety of headphones

Unlike many interfaces that only offer one headphone output, the Apollo x4 Gen 2 has two, each with independent volume control, making it ideal for collaborative sessions. The newly improved headphone amp provides better low-end response and overall clarity, ensuring a more detailed listening experience for critical mixing or tracking.

Optical Output (ADAT/SPDIF).

Type: 1 x Toslink Optical Output
Format: ADAT (8 channels) or S/PDIF (stereo)

This allows the interface to send multi-channel digital audio to another device, such as a digital mixer, external DAC, or another ADAT-equipped interface. The S/PDIF mode is useful for sending high-quality stereo signals to external gear like a mastering converter or an external reverb unit.

Thunderbolt 3 Port.

Type: 1 x Thunderbolt 3 (USB-C)
Daisy-Chaining: Supports multiple Apollo interfaces and UAD devices

This high-speed connection ensures ultra-low latency and high-bandwidth data transfer between the Apollo X4 Gen 2 and a computer. Thunderbolt also allows expansion with additional Apollo interfaces, making it possible to grow from a desktop setup to a full professional studio.

Table Inputs & Outputs.

Category	Type & Count	Key Features
Mic/Line Inputs	4 x XLR-1/4” combo	Unison-enabled, high-gain range
Hi-Z Inputs	2 x 1/4” (Front Panel)	Realistic amp modelling with Unison
Optical Input	1 x ADAT/S/PDIF	Expandable up to 8 additional inputs
Monitor Outputs	2 x 1/4” TRS	High-fidelity D/A conversion (129 dB)
Line Outputs	2 x 1/4” TRS	Routing for outboard gear or re-amping
Headphone Outputs	2 x 1/4” TRS	Independent volume controls, low impedance
Optical Output	1 x ADAT/S/PDIF	Multi-channel digital signal routing
Thunderbolt 3	1 x USB-C	High-speed connectivity, expansion support

Unboxing and Build Quality.

The Apollo X4 Gen 2 arrives in premium packaging that reflects its professional-grade pedigree. Inside the box, you’ll find:

The Apollo X4 Gen 2 unit.
A robust external power supply with regional adapters.
Quick-start guide and documentation.

The unit itself exudes elegance with its sleek, low-profile aluminium chassis finished in matte silver. The craftsmanship is top-notch, with solid construction and thoughtful design elements that prioritise both aesthetics and durability. The large rotary encoder knob on the front panel is smooth and responsive, complemented by tactile buttons with LED backlighting for quick visual feedback. The primary difference from the earlier Apollo interfaces is that it’s no more a click rotary encoder that was used as a mute button. Instead the mute button has now been shifted to one of the buttons under the rotary knob. This is a welcome step as the click type rotary encoder was known to get a little erratic over time and I’m happy to see UA make this change (mine conked off thrice). The overall meticulous attention to detail ensures the X4 Gen 2 not only performs well but also looks and feels premium on any desktop setup.

At 2.37 lbs (1.08 kg), the Apollo X4 Gen 2 is compact and lightweight yet feels substantial. Its compact footprint is perfect for home studios, yet it’s durable enough to accompany professionals on the road.

Key Features of the Apollo series.

UA Console – The UA Console is Universal Audio’s powerful real-time mixing and routing application designed for Apollo audio interfaces. It allows users to manage input levels, apply UAD plug-ins with near-zero latency, and control monitoring settings effortlessly. With features like Unison preamp integration, cue mixes, and flexible signal routing, UA Console provides a streamlined workflow for tracking, mixing, and recording. Its intuitive interface ensures seamless control over Apollo’s DSP-powered processing, making it an essential tool for both studio and live recording environments.

Unison Preamps – The Apollo X4 Gen 2 boasts four Unison-enabled mic preamps, a signature feature of UA’s Apollo series. These preamps emulate the tone and behavior of iconic analog gear, such as the Neve 1073, API Vision, and Manley VoxBox. Unison technology dynamically adjusts input impedance, gain staging, and circuit behavior to authentically replicate the analog counterparts, giving users access to a virtual rack of high-end preamps.

Real-Time UAD Processing – The X4 Gen 2 comes equipped with a quad-core DSP processor, enabling real-time tracking and mixing with UAD plugins. This processing power eliminates latency issues and provides access to UA’s extensive plugin library, including classics like the Teletronix LA-2A, 1176LN, and Pultec EQP-1A. Tracking vocals or instruments with zero latency and studio-grade effects is a transformative experience for any creator.

Connectivity – Inputs: 4 XLR/TRS combo inputs for mic/line sources, 2 Hi-Z instrument inputs on the front panel for guitars and basses. Outputs: 6 balanced line outputs and 2 independent headphone outputs with dedicated volume controls. Expansion: Optical ADAT/SPDIF input, supporting up to 8 additional channels of digital audio. Thunderbolt 3: Ensures ultra-low latency and high bandwidth for seamless integration with macOS and Windows.

Monitoring and Control (Hardware) – The Apollo X4 Gen 2’s monitor control system includes dedicated buttons for mono, mute and alternate speaker switching. The large encoder knob doubles as a monitor level control, making it easy to adjust volume on the fly. The unit also supports up to two headphone mixes, allowing collaborative work without compromise.

LUNA Integration – For users of UA’s LUNA Recording System, the X4 Gen 2 offers deep integration, including features like tape emulation and integrated summing. LUNA transforms the X4 into a comprehensive music production environment, combining the best of analog and digital workflows.

User Experience.

Setting up the Apollo X4 Gen 2 is straightforward, thanks to UA’s intuitive software ecosystem. The UA Console application serves as the control centre for routing, monitoring, and plugin management. Its user-friendly interface makes configuring even complex setups a breeze. The dual headphone outputs with independent volume controls are a standout feature, enabling seamless collaboration between artists or producers. Additionally, the optical ADAT input allows users to expand their setup with external preamps or converters, making the X4 Gen 2 a scalable solution for growing studios. Thunderbolt 3 connectivity ensures ultra-low latency performance, and compatibility with both macOS and Windows systems makes the X4 Gen 2 versatile across platforms. However, Windows users should note the importance of ensuring Thunderbolt compatibility before purchase.

In-Built Sonarworks Room Correction (Add on).

The Universal Audio Apollo x4 Gen 2 brings a host of refinements over its predecessor, and one of the standout additions is its built-in Sonarworks SoundID Reference integration via Apollo Monitor Correction (AMC). This feature marks a significant step toward more accurate and consistent monitoring, making the Apollo X4 Gen 2 an even more compelling choice. Apollo Monitor Correction (AMC) allows users to apply Sonarworks SoundID Reference calibration profiles directly onto the Apollo X4 Gen 2’s DSP, ensuring that speakers and headphones deliver a neutral, flat response without requiring external plugins or software. With this feature, users no longer need to insert the SoundID Reference plugin in their DAW master bus or system audio, streamlining the workflow and freeing up CPU resources.

Sonarworks AMC for my home studio.

Key Benefits of AMC:

Hardware-based DSP calibration (no CPU load)
Zero-latency correction (ideal for tracking and mixing)
Seamless integration with UAD Console
Support for both speaker and headphone correction
Customizable target curves for personal taste

Sound Quality and Accuracy – The core appeal of Sonarworks SoundID Reference is its ability to neutralise room inconsistencies and ensure a flat frequency response. When applied through the Apollo x4 Gen 2’s DSP, the correction sounds transparent and natural, with noticeable improvements in clarity and balance.

Speaker Calibration Performance – When using Sonarworks’ speaker calibration, the difference is immediately noticeable:

Flat response ‘reference grade’ monitoring
More accurate stereo imaging
Tighter low-end response (reduction in room-induced boominess)
Smoother midrange and high frequencies (removes harshness from problematic reflections)

Headphone Calibration Performance – The headphone calibration mode is equally impressive. With over 500 headphone models supported, users can apply factory-tuned correction profiles to achieve a flatter response. This is especially useful for producers who rely on headphones for mixing and mastering.

More balanced frequency response across different headphones
Reduction of overhyped bass or scooped mids in consumer headphones
Improved stereo imaging and transient detail

For example, calibration testing the Sennheiser HD6XX revealed a less pronounced mid-bass boost, a clearer midrange and much better upper-end extension – making them far more usable for critical listening.

Workflow and Usability – Setting up Apollo Monitor Correction is straightforward:

Measure your room using SoundID Reference software and a measurement mic.
Export the calibration profile to a compatible format.
Load the profile into the UAD Console via the Apollo Monitor Correction tab.
Apply the correction to your monitors or headphones and adjust target curves as needed.

Once loaded, the correction is applied independently from the DAW, meaning it works with any audio source—from DAW playback to streaming services and system audio.

Seamless DAW Integration – One of the biggest advantages of having DSP-based correction is that it eliminates the need to insert SoundID Reference as a plugin on the master bus. This not only reduces CPU usage but also prevents accidental mix exports with correction enabled.

No need for extra plugins
Works across all audio sources (DAW, streaming, system audio)
Frees up CPU for other tasks

Performance and Latency – One of the biggest benefits of hardware-based correction is zero latency. Many software-based calibration solutions introduce a few milliseconds of latency, which can be problematic for tracking and live monitoring.

Near-instant response with no delay
Ideal for real-time recording and monitoring

For comparison:

Software-based SoundID Reference introduces around 10-15ms latency depending on buffer size.
Apollo Monitor Correction runs on the DSP, eliminating latency entirely.

This makes the Apollo X4 Gen 2 an excellent choice for vocalists, instrumentalists, and producers who need accurate monitoring while recording.

Limitations & Considerations.

While Apollo Monitor Correction is a game-changer, there are a few limitations to consider:

Requires a paid SoundID Reference license – Users must own SoundID Reference to create speaker calibration profiles.
Limited to Apollo X series – This feature is only available on Apollo Twin X, X4 Gen 2 and Apollo x8/x8p Gen 2.
No per-output correction – You can’t apply different correction profiles to multiple outputs simultaneously.
No multi-channel (5.1 or Atmos) support yet – The current implementation is limited to stereo monitoring.

These are minor drawbacks, but they may be dealbreakers for users needing multi-output calibration or surround sound support.

Input Level Auto Gain.

The Auto-Gain feature on the Universal Audio Apollo X4 Gen 2 automatically sets optimal preamp levels by analysing incoming audio, ensuring proper gain staging without manual adjustments. It works across mic, line, and Hi-Z inputs, supports multi-channel adjustment, and integrates seamlessly with Unison preamp plug-ins. Users simply activate Auto-Gain, perform at their loudest level, and the system sets the gain automatically, reducing the risk of clipping and improving workflow efficiency.

X4 Gen 2 Sound Performance.

Preamps – The Unison preamps deliver pristine audio capture with impressive transparency and detail. When paired with Unison-enabled plugins, the X4 Gen 2’s preamps go from clean and neutral to warm and characterful, depending on the selected emulation. Tracking vocals through a Neve 1073 plugin adds rich harmonics, while an API Vision emulation brings a punchy, forward tone ideal for guitars and drums. The flexibility and tonal versatility of the Unison preamps make them suitable for any genre or application.

Conversion – Universal Audio’s AD/DA conversion is among the best in the industry, and the X4 Gen 2 lives up to this reputation. Offering 24-bit/192kHz resolution and a dynamic range of 127dB, the X4 Gen 2 reproduces audio with exceptional clarity, depth, and detail. Bass frequencies are tight and controlled, mids are articulate and natural, and highs are smooth and extended without harshness. Whether tracking, mixing, or mastering, the X4 Gen 2 ensures your audio retains its integrity.

Real-Time DSP – The dual-core DSP provides ample processing power for running multiple UAD plugins simultaneously. Plugins like the 1176LN compressor, Capitol Chambers reverb, and Helios Type 69 EQ operate flawlessly, allowing users to shape their sound in real-time. This capability is particularly valuable during tracking sessions, as it enables artists to hear a polished sound without latency or post-production delays.

Soundstage and Imaging – The X4 Gen 2’s precise conversion and clean preamps contribute to a wide, three-dimensional soundstage. Instrument separation and placement are impeccable, making it easy to pinpoint details within a mix. This level of detail benefits both music production and critical listening.

Apollo X4 Gen 2 vs Gen 1.

The Universal Audio Apollo Gen 1 and Gen 2 interfaces differ significantly in their sonic characteristics due to advancements in technology and design. Below is a breakdown of the key sonic differences between the two generations:

No Sonarworks integration or Auto Gain in Gen 1.

First and most important difference is that Gen 1 interfaces did not have Sonarworks integration or the Auto Gain feature, both which make life so much easier. These two in fact are the prominent highlights of the Gen 2 interfaces for me personally.

Analog-to-Digital (A/D) and Digital-to-Analog (D/A) Conversion.

Apollo Gen 1 interfaces feature high-quality converters, but their dynamic range and overall clarity are slightly limited compared to newer models relatively. The Gen 1 converters provided excellent performance for their time – what now sounds like slightly warmer, a bit less detailed sound compared to Gen 2 standards. Dynamic range for Gen 1 typically falls in the 117–120 dB range, depending on the model. Gen 2 interfaces include updated A/D and D/A converters – improving dynamic range, fidelity, and detail. Gen 2 interfaces deliver a cleaner, more transparent signal with a dynamic range of up to 129 dB, allowing for slightly more headroom and a better signal-to-noise ratio. These improvements result in a slightly more open soundstage with crisper transients and greater low-level detail.

Sonic Transparency and Noise Floor.

The Gen 1 units have a slightly higher noise floor compared to Gen 2. While they still sound professional, their output is marginally less precise, particularly in complex mixes or recordings with a wide dynamic range. The lower noise floor in Gen 2 interfaces provides cleaner recordings and playback, particularly noticeable in quieter passages or high-gain scenarios. This results in a more polished and modern sonic signature.

Unison Preamps.

While the Gen 1 interfaces support Unison technology, they lack some of the nuanced hardware improvements found in Gen 2. The Gen 1 preamps perform well but are less detailed in their emulation of analog preamps, with slightly less dynamic realism in Unison-mode recordings. Gen 2 interfaces refine the Unison preamp circuitry, offering a slightly more authentic replication of classic analog gear. The improved circuitry provides better gain staging, smoother harmonic distortion, and a more “analog-like” response.

Output Monitoring.

Apollo Gen 1 headphone and monitor outputs are capable but lack the extended clarity and precision found in the Gen 2 models. Users may notice a slightly less defined stereo image and less articulate low-end reproduction. The redesigned monitor and headphone outputs in Gen 2 units provide superior audio quality for critical listening. Bass is tighter, midrange is more focused, and highs are more detailed, making it easier to make precise mix decisions.

DSP Power and Plug-in Performance.

Both generations rely on UAD DSP for real-time plug-in processing, but the Gen 1 hardware predates some of the more demanding plug-ins and complex workflows of modern production and its DSP can sometimes struggle with resource-heavy plug-in chains. While DSP power itself remains similar in terms of core count, Gen 2 units are optimised for handling more complex sessions and workflows, benefiting from software and firmware improvements. The improved converters also allow plug-ins to operate with greater fidelity, resulting in more realistic analog emulation.

Summary of Sonic Differences.

Aspect	Apollo Gen 1	Apollo Gen 2
Dynamic Range	117–120 dB	Up to 129 dB
Noise Floor	Higher	Lower
Detail & Clarity	Warm but slightly less defined	Cleaner, more transparent
Unison Preamps	Good but slightly less refined	Slightly more detailed
Monitor Outputs	Good but less precise	Crisper clarity and soundstage
Bass Reproduction	Slightly warmer	Tighter and more defined
Stereo Imaging	Slightly narrower	Wider and a bit more accurate

Final Thoughts on Gen 2 vs Gen 1.

The Apollo Gen 2 interfaces are a step forward in sonic performance, providing improvements in dynamic range, noise floor, and overall clarity. These advancements make the Gen 2 models better suited for critical listening, detailed recordings, and complex productions. While the Gen 1 remains a solid choice for many users and is nowhere close to being a significant step down, let alone obsolete, those seeking the highest fidelity, precision and having their purchase future proof will benefit from going with the new Gen 2 units, even more because of the Sonarworks compatibility.

Comparisons with other similar interfaces.

Apollo Twin X.

The Apollo Twin X is a more compact and affordable option with two Unison preamps and fewer I/O options. While it’s ideal for solo musicians or small setups, the X4 Gen 2’s expanded I/O and DSP make it better suited for multi-instrumentalists and collaborative workflows.

Antelope Audio Zen Tour.

The Zen Tour offers comparable connectivity and onboard effects but lacks the analog authenticity and plugin ecosystem of UA’s Unison technology. The X4 Gen 2’s superior preamps and UAD plugins give it a clear edge in tonal flexibility and sound quality.

RME Fireface UCX II.

RME is renowned for its stability and ultra-low latency, but the Fireface lacks the real-time DSP and analog modelling capabilities of the Apollo X4 Gen 2. While the UCX II excels in precision and reliability, the X4 Gen 2 offers a more inspiring and creative workflow.

Pros and Cons of the Apollo x4 Gen 2.

Strengths.

Enhanced Audio Quality: The improved converters and upgraded preamps ensure studio-grade audio capture and monitoring.
Versatile Connectivity: With analog and digital I/O, the Apollo X4 Gen 2 is suitable for a wide range of recording setups.
Real-Time UAD Processing: The ability to use UAD plug-ins during tracking without latency offers unparalleled flexibility and professional-grade results.
Modern Features: Innovations like Auto-Gain, Sonarworks room correction, and bass management position the Apollo X4 Gen 2 as a forward-thinking device.
Software Bundles: The included UAD plug-ins and instruments, particularly in the Studio+ package, add tremendous value for users looking to expand their sonic palette.

Weaknesses and Considerations.

Additional Costs: Some key features, such as room correction, require the Apollo Monitor Controller add-on and the SoundID Reference software, which are not included. This increases the overall cost of ownership.
Power Adapter – I wish Universal Audio had designed a IEC C13 style power supply for the Twin and X4 series of interfaces instead of the typical thin wire adapter. C13 power supply is much more robust and reliable.
Thunderbolt Cable Not Included: As with other Universal Audio interfaces, the Thunderbolt cable must be purchased separately, which can be inconvenient given the unit’s premium price. I fail to understand why a $50 TB3-4 cable can’t be included in the asking price.
DSP Limitations: While the onboard DSP allows for real-time plug-in processing, users working with larger sessions may find the DSP resources insufficient, requiring them to offload processing to their computer.
Price Point: At its price, the Apollo X4 Gen 2 is targeted at professionals, making it less accessible to hobbyists or beginners.

Conclusion.

The Universal Audio Apollo X4 Gen 2 is a masterpiece of audio engineering, offering unparalleled sound quality, UA Console user experience, routing flexibility and overall usability. Its combination of portability, Unison preamps, real-time UAD processing, Sonarkworks Correction implementation and pristine conversion makes it an invaluable tool for any serious creator. While its premium price, expensive plugin pricing and limited I/O compared to what other brands provide for half the price may give pause, the overall usabililty, experience and performance justifies the investment for a serious studio user. For musicians, producers, and engineers seeking a compact yet powerful interface that bridges the gap between analog warmth and digital precision, the Apollo X4 Gen 2 is a standout choice. It’s not just a tool; it’s an enabler of creativity.