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The art of audio compression has seen many legendary units throughout history, but few are as revered as the Rohde & Schwarz U-23 broadcast limiter. Often called the "German Fairchild" for its similarities to the iconic Fairchild 660/670, this rare piece of broadcast equipment found new life through Vacuvox's innovative reimagining.
Vacuvox first captured the magic of the U-23 with their CL-2 tube compressor/limiter, which quickly became a favorite among top producers and engineers, including members of Oasis, Coldplay, and Madonna. Building upon this success, Vacuvox introduced the U23m—a mastering-grade evolution that maintains the soul of the original while adding precision controls essential for modern production.
Let's explore how this masterful recreation brings the classic sound of broadcast compression into today's mastering environment.
At its core, the U23m is a mastering version of Vacuvox's acclaimed U23 vari-mu compressor/limiter. While preserving the rich character that made its predecessor famous, the U23m introduces several crucial refinements for mastering applications.
The unit features an all-Class A vacuum tube audio path and side-chain circuitry based on the original Rohde & Schwarz design. Custom-made transformers faithfully recreate the original units, with four audio transformers crafted from 80% nickel and 20% iron permalloy, plus SiFi-core chokes and power supply components. An upgraded power transformer delivers improved noise performance and stability.
The U23m's mastering credentials shine through in its precise stepped controls. Twenty-one-position switches for input, output, attack, release, and threshold settings ensure perfect recall and precise stereo matching—essential for critical mastering work across any genre, from delicate classical pieces to hard-hitting rock productions.
The unit's variable gain structure allows engineers to craft different flavors of saturation. By adjusting the pad, input gain, and threshold controls, you can achieve everything from transparent dynamics control to subtle harmonic enhancement or more aggressive coloration.
For this "Hear The Gear" session, Vintage King Audio Consultant Nikhil Suresh put the U23m through its paces across a variety of sources, demonstrating its versatility in both subtle and more aggressive applications.
The U23m's high gain setting on vocals introduces subtle harmonic enhancement, adding depth and presence without obvious distortion. The moderate 4:1 ratio provides consistent level control while maintaining the natural vocal dynamics.
Super-fast attack (1ms) and release (0.2s) times ensure transparent compression that doesn't pump or breathe, while the de-esser at 3.5 kHz smooths out harsh sibilants without dulling the top end.
In the second variation, shifting the de-esser to 10 kHz offers a different approach to high-frequency control, maintaining more presence while still taming peaks.
Same settings but with de-esser frequency shifted to 10 kHz.
On acoustic guitar, Nikhil uses a straightforward compression approach to bring out natural detail while maintaining dynamics. The high gain setting adds subtle harmonics that complement the guitar's acoustic resonance.
A 4:1 ratio with medium attack (3 o'clock position) allows the initial string attack to pass through while controlling the sustained notes, creating a more consistent yet dynamic performance. The moderate release time prevents any obvious pumping effects while maintaining natural decay.
The second variation introduces the de-esser at 10 kHz to control string brightness without losing detail, which is particularly useful for aggressive strumming patterns.
Same settings as above, but with the de-esser engaged at 10 kHz for extra high-frequency control.
The U23m shows remarkable versatility across different drum sources. On kick drum, the high gain setting adds weight and density while the 4:1 ratio with a slightly delayed attack (12:30 position) preserves the initial transient impact. The fast 0.2s release maintains punch while preventing unwanted low-end build-up.
For variations 2 and 3, the gentler 1.5:1 ratio with different gain staging approaches demonstrates how subtle compression can enhance the kick's natural tone without obvious processing.
On snare, the high gain setting paired with medium attack (50ms) preserves the initial crack while the 4:1 ratio and fast release (0.2s) enhance sustain and room tone. The moderate gain reduction of 4-6 dB helps the snare sit perfectly in the mix without losing impact.
The drum room variations showcase the U23m's character on complex sources. The "punchy" setting uses the de-esser at 6 kHz to control cymbal wash while maintaining attack, while the "aggressive" setting with faster attack (1ms) and longer release (1.2s) creates a more dramatic, explosive room sound.
Moderate compression for punch:
Two variations using gentler 1.5:1 ratio settings with different gain staging approaches for subtle enhancement.
Same base settings with:
The U23m's low gain setting on bass provides cleaner compression while still maintaining warmth. The aggressive 9:1 ratio with lightning-fast attack (1ms) creates consistent level control perfect for modern productions.
The lowpass filter at 180 Hz focuses the compression on the fundamental frequencies, preventing unwanted pumping from high-frequency content. The second variation tightens the bottom end further by dropping the filter to 90 Hz, ideal for genres requiring precise low-end control.
Same settings but with the lowpass filter at 90 Hz for a tighter bottom end.
Applying compression on the piano demonstrates the U23m's ability to handle complex dynamics while maintaining clarity. The high gain setting adds subtle harmonic enhancement while the moderate 3:1 ratio provides musical compression.
The combination of fast attack (1ms) and slow release (1.2s) creates a smooth sustain while preserving the piano's natural decay. The lowpass filter at 180 Hz prevents the compression from responding too heavily to lower registers, maintaining consistent response across the entire keyboard range.
Electric piano through the U23m showcases the unit's ability to enhance both attack and sustain characteristics.
The high gain setting adds subtle tube warmth while the 3:1 ratio provides musical dynamics control. The moderate attack and release times (10 and 12 o'clock respectively) preserve the Rhodes' natural envelope while adding consistency and sustain.
The second variation's modified timing creates a different dynamic envelope, demonstrating the U23m's flexibility in shaping electric piano sounds.
Modified attack and release times for a different envelope shape.
The U23m proves equally capable on brass instruments, with three distinct approaches demonstrating its range.
The first variation uses high gain for added harmonics with a gentle 2:1 ratio that provides dynamic control while maintaining expressiveness. The de-esser at 3.5 kHz helps tame harsh overtones without dulling the horn's natural brilliance. Fast attack (1ms) and release (0.2s) times with heavy gain reduction (16 dB) create a controlled yet musical performance.
The subsequent variations with different filtering and timing settings showcase how the U23m can adapt to different playing styles and arrangement needs.
Variations with different filtering and timing settings showcase the unit's range on saxophone.
The Vacuvox U23m masterfully bridges the gap between vintage character and modern precision. Its mastering-grade controls and meticulous attention to component selection make it equally suited for subtle dynamic control and more aggressive tonal shaping. Whether you're looking for transparent mastering compression or hoping to add some tube warmth to your tracks, the U23m delivers with the flexibility and precision that modern production demands.
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