Retro Instruments 176 Limiting Amplifier

A modern recreation of one the best sounding variable-mu tube limiters of all time



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SKU: RET176 Categories: , Brand:


While some pro audio recreations use similar sounding names to the model they emulate, the Retro Instruments 176 limiting amplifier isn’t trying to hide its actual namesake, which is the UA 176 as opposed to its later incarnation, the ubiquitous 1176. Recreating the lush sound of the original Universal Audio 176, the Retro 176 is a true variable-mu limiter that brings modern features, functional convenience, and reliability to the vintage party. Designed for versatility, the Retro 176 enhances dimension, depth, and detail, due to the way tube compression handles dynamics.The Retro 176 is most at home on a main stereo bus, adding glueand gloss to a 2-track master. However, it’s also great for tracking and mixing. More than just for the 2-bus, the Retro 176 puts vocals in velvet cushion of sound like no other compressor, and adds euphonic warmth and fullness to bass, piano, and acoustic guitar, all the while enhancing tone in a way that no EQ can.

With notable users, such as Grammy-winning mixer Chris Lord-Alge and multi-platinum producer, Michael Wagener, the Retro 176 should find its way into the racks of all those who prize lush vocals and a sound that can’t be found anywhere else.

Retro Instruments 176 Limiting Amplifier in one take:

• Authentic variable-mu tube design
• Precision knob scales for easy recall-ability
• Continuously variable attack and release time
• Four settings of compression ratio from soft compression to limiting (2:1, 4:1, 8:1, 12:1)
• Vocal asymmetry switch selects full-wave or half-wave detector operation
• Wide-range sidechain high-pass filter
• Hard-wire bypass switch
• Interstage transformer bypass switch
• High-quality VU meter switchable between input, output, and gain reduction
• XLR input and output connections
• 115/230V AC Mains
• Integral tube balance test
• Easy stereo strapping of two or more units
• Selected NOS tubes and components

INSIDE THE BOX—Retro Instruments Limiting Amplifier

True tube compression

Like the Fairchild compressor, the original UA 176 used a dual triode tube (the GE 6386) to control gain reduction. Due to the difficulty and expense of obtaining reliable 6386 tubes, Retro employs 6BC8 tubes, which are far more reliable and much less expensive. However, tubes are just one small part of the Retro 176’s allure, which is all about it’s sound. Variable mu compression (mu is a Greek symbol for the amplification factor of a tube) is not as responsive to transients as other compression circuits,but it does add a rich harmonic bed for sound to luxuriate in, and therein lays its charm for overall mix compression and vocals in particular.

One of the unique design features of the Retro 176 is its switchable inter-stage transformer, which separates the gain reduction stage from the amplifier stage. Apart from providing electronic isolation between the two stages and feed to the output stage, it also providesa specific impedance load for the variable gain stage to work into. Alternatively, the inter-stage transformer can bypassed, which changes the tone and character of the compression. Contrary to what you might expect, with the inter-stage transformer switched out of the circuit, the sound of the 176 is thicker and more “round” sounding. Switched in, it appears to add air in the upper frequencies, with a brighter, more open sound.


Like the Teletronix LA-2A, the Retro 176 has a fixed threshold. Gain reduction is adjusted by the Input Level control. The input is optimized for standard +4dBm line levels to provide 0dB to 20dB gain reduction.


The ratio can be adjusted in four steps from 2:1 to 12:1. Lower ratios will put a compression signature over a wider dynamic range. Higher compression ratios are best to hold the signal level down. The ratio control also changes the threshold. The threshold axis point is at 10dB gain reduction. This results in similar gain reduction at the various ratio settings when doing approximately 10dB of gain reduction, however at less amounts of gain reduction the lower ratio settings will start compression sooner than higher ratios.

Extended attack and release

However, not to be limited, so to speak, the Retro 176 has extended adjustmentsfor both its attack and release characteristics. Slow Attack lets more impact through. Fast Attack holds down peaks and sounds more stressed. A fast release adds energy and density, andslower settings provide gentle control, retaining more of the dynamics of the source material.

Time constant

The original UA 175B and 176 had a single time constant. However, the Retro 176’sattack knob has a pull switch that disables the program-controlled time constant. The program-controlled time-constant (with the knob IN) makes the Retro 176 gain reduction more natural, controlling dynamics with less distortion.

Asymmetry control

Another feature unique to the Retro 176 is its asymmetry switch, which provides very useful functionality for the detector. The asymmetry switch determines whether the detector reacts full-wave, positive-going half-wave or negative-going half wave. Musical waveforms are very complex, and as such, rarely symmetrical. Particularly in the case of multi-mic recording, the leading edge of an instrument’s waveform can either be positive-going or negative going.On occasion, a wave will spike in one direction and be smooth in the other. When half-wave is selected, the detector only reacts to one side of the waveform, and the asymmetry switch determines which edge of the wave the compression will ride on.

Side chain high-pass filter

The Sidechain HPF is a very effective tool to eliminate pumping on low frequency material. It allows the compression to work more effectively controlling dynamics in the midrange frequencies. Bass frequencies will pass through with little or no compression, which makes it very useful for bass-heavy music, such as hip-hop, or maintaining punch for kick drum and bass in pop and rock styles, etc. The sidechain high-pass filter is also ideal for vocals.

OUTSIDE THE BOX—Retro Instruments 176 Limiting Amplifier

The front panel of the Retro 176 has the distinctive look and feel of the vintage analog gear of yesteryear, with the addition of precision knob scales for easy recall-ability. Also like its ancestors, the 176 has boat-anchor heft and build quality, with tubes and transformers mounted on the rear panel, which means it should be rackmounted with care and space around the unit. Once mounted, the Retro 176 invites use and will perform flawlessly.

Retro 176 Limiting Amplifier front-panel controls include:

Power Switch, Indicators and Interlock (upon initial installation, apply power;the red indicator and metershould light and the front door should be securely closed

Amplifier Active/Bypass Switch (provides a hard-wired bypass of the 176 for instant A/B comparisons)

Input Level Control

Output Level Control

Compression Ratio Control

Attack and Release Controls

Sidechain High Pass Filter

Asymmetry Switch

Meter-Select Switch (Provides metering of the input and output levels, and gain reduction

Stereo Linked Operation

Who’s Using Retro Instruments?

Notable users of Retro gear including Grammy-winning and platinum-selling producers and engineers, such as Chris Lord-Alge (Retro 176), Joe Chiccarelli (Retro 2A3), Silvia Massey (Sta-Level), Ross Hogarth (Sta-Level, 2A3), Jacquire King (Retro 176), Michael Brauer (Retro 2A3), Glenn Rosenstein (Retro OP-6, Sta-Level), and many more world-class producer-engineer-mixers and artists.

In over fivedecade, there hasn’t been a significantly better sounding compressor than the 176. If you’re looking for gear that is truly future-proof, you have to go Retro. For more information, call or chat with your Westlake Pro audio representative today.

Retro 176 Specifications:

• Frequency Response:Flat within 0.5dB from 20-20,000Hz
• Harmonic Distortion: Below 1% from 20-20,000 Hz at 0-15 dB Gain Reduction
• Noise Level: Greater than 76 dB below normal operating level
• Gain Reduction: 20dB of available gain reduction
• Maximum Gain:32dB with input and output controls set maximum
• Input Impedance: 600 Ohms – floating transformer balanced
• Output Impedance: 600 Ohms – floating transformer balanced
• Minimum Input Level:–26dBm@2:1 ratio; –12dBm@12:1 ratio
• Normal Operating Level:+4dBm
• Maximum Operating Level:+20dBm
• Maximum Operating Temp:55° C
• Physical Dimensions:Standard 19″ rack mounted. 2U, 3-1/2″ high, 9-1/2″ deep
• Power Requirements:40 watts at 115/230 VAC, 50/60Hz

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