Vacuvox U23m Mastering Vari-Mu Compressor/Limiter

The Vacuvox U23m is a mastering version of the U23 vari-mu compressor/limiter building upon the legacy of the Vacuvox CL-2 for which the Rohde & Schwarz U-23 broadcast limiter served as a basis. Several improvements have taken the design and functionality of the CL-2 a step further while preserving the euphonic sound of the original. The Vacuvox U23 has an all Class A vacuum tube audio path, and side-chain path based upon the original circuit of the Rohde & Schwarz U23. These ensure unparalleled performance and provide unrivaled headroom and transparency.

DESIGN AND FUNCTIONALITY

The Vacuvox U23m mastering compressor/limiter is assembled entirely by hand and uses high-quality modern components. The custom-made transformers are faithful recreations of the original, sonically excellent, Rohde & Schwarz transformers. The four audio transformers are made of permalloy with 80% nickel and 20% iron content. The chokes and power supply are made of SiFi. The power transformer is upgraded for better noise performance and stability. A front panel layout, a modification of the output gain and the thresholds and two extra gain stages are the most significant improvements that have been incorporated in the Vacuvox U23m.

21-stepped switches for input, output, attack, release, and threshold are used for the U23m mastering version. These extend the use of a pair of U23m compressor limiters to critical stereo applications for many different genres of music. Even jazz and classical material can be mildly compressed with a perfect stereo balance and without sacrificing the sense of dynamics and producing unwanted artifacts.

The various gain structure options from the Pad, Input, Gain and Threshold offer the possibility to create different kind and levels of saturation subtly. Therefore the sound can be varied between transparent, dense and fluid by lowering the threshold, driving the input transformer or driving the tubes.

FEATURES:

• Four custom-made wide-band audio transformers with permalloy core (80% nickel and 20% iron content)
• Two audio chokes with SiFe core
• One power supply transformer with SiFe core (adjustable for 120 and 240 V)
• One power supply choke with SiFe core
• Ceramic Tube sockets throughout
• Tube complement of two EAF42, three EF86, one ECC40 and one EAA91
• Combination of selected and stereo matched NOS tubes of Philips, Valvo, Siemens and/or Telefunken
• Simpson Electric metering
• Polypropylene film capacitors from Mundorf and Wima throughout
• High quality carbon-film and 1% metal-film resistors from Kiwame and Koa Speer throughout
• High-quality Blore-Edwards and Grayhill switches, fitted with precision components, throughout
• Input pad of -10 dB
• 3-position tube Gain switch
• 21-step variable threshold
• De-esser centered around 6 kHz as part of the side-chain
• Switchable stereo link
• Balanced input and balanced output
• Input impedance: 10k Ohms
• Output impedance: 30 Ohms/100 Ohms
• Minimum load impedance: 300 Ohms
• Frequency response: 20 Hz - 22k Hz (±0.5 dB)
• Noise: -93 dB (20 Hz - 22k Hz, unweighted, RMS) @0 dB unity gain
• Signal to noise ratio at +14 dBu = 100 dB (20 Hz - 22k Hz, unweighted, RMS)
• Distortion in amplifier mode: THD <0.15% @1 kHz and 42 dB output gain
• Power consumption: 60 Watt (120/240 VAC)

The above features are included with both the U23 and U23m, the following are specific to the U23m:

• Ratio: 7-step rotary switch with ratios 2:1, 3:1, 4:1, 5:1, 7:1, 9:1, 12:1
• Input: 21-step attenuator switch with 1 dB per step
• Attack: 21-step rotary switch with time settings between 1 mS and 50 mS
• Release: 21-step rotary switch with time settings between 0.2 S and 1.2 S
• Output: 21-step attenuator switch with 2 dB per step for the first 5 steps and 1 dB per step for the last 15 steps
• Selectable output level (normal or +6 dB) of the output transformer