Retro Instruments 176 Tube Limiting Amplifier

New Take On a Classic Tube Compressor

The Retro Instruments 176 takes its inspiration from one of the holy grails of tube compression, Bill Putnam's classic 176 — forefather to the solid-state 1176 (which became an iconic industry standard). Retro designer Phil Moore's 176 remains largely faithful to the original UA schematic, while sporting embellishments that bring it in line with the requirements of modern music production. If you want exacting dynamics control, while juicing your tracks with scrumptious tubular warmth and air, take it from Sweetwater: Retro Instruments' 176 is an ideal way to do it.

Offering a versatile palette of tonal options, Retro Instruments' 176 can produce great results when used for mild compression, making any signal smoother, denser, and more even. But you can also push it further and unleash the fearsome creature that's lurking behind its seemingly benign "limiting amplifier" classification. In most cases this will add an urgency and brightness to your source, and the results will remain quite civilized until you drive the 176 to its extremes. After that, the gloves come off, and you can deploy the 176 to craft all manner of sonic mayhem. However you use it, though, the 176 will reward your efforts to thoroughly understand its sophisticated controls and circuit topology. Think of it as a powerful beast with impeccable manners.

We all thrive on sonic variety, and here's another area where the Retro Instruments' 176 really shines, with an inter-stage transformer (located between the gain-reduction element and the output stage) that you can switch in and out of the signal path. Another unique feature is the 176's "asymmetry" function, which determines whether the detector reacts to the full waveform, or only to the positive or negative sides. As the comprehensive product manual explains, it "determines which edge of the wave the compression will ride on." This may assist in "opening up" the compression on signals with the asymmetrical waveforms that are characteristic of certain brass or vocal tracks.

Most professional audio equipment from the era of the original 176 boasted weapons-grade build quality, and with its no-nonsense battleship-gray livery, so does the Retro 176. Just like the original, the Retro sports a bottom-hinged front panel, and all the tubes and transformers are easily accessible on the back panel. Most of the unit's substantial weight comes courtesy of the "iron," that is, the massive power transformer, the input and inter-stage audio transformers (made by CineMag), and the specially designed output transformer.

Now, about that output transformer. As were virtually all compressors back in the day, the Retro 176 is a feedback design, in which a detector signal is tapped after the gain-reduction element, and its custom output transformer plays an important role in this scenario. The primary coil has four taps for the compressor's individual ratio settings. With this arrangement, a low-driving impedance to the detector helps maintain quick attack times, and - equally vital - the four ratio taps counteract the threshold, so that every ratio setting occupies the compressor's "sweet spot."

Whether or not a compressor has a dedicated sidechain input, every dynamics processor has a "sidechain" — separate from the main signal path — that functions as an actuation trigger. Although the Retro Instruments 176 doesn't have a separate sidechain input, it does offer a highpass filter that acts on its sidechain. If you are compressing say, your snare drum, this allows you to roll off kick drum leakage that could cause the compressor to clamp down inappropriately on your snare signal. It's just one example, but you can see how this feature (which, as you might guess, was not on the original 176) will be invaluable in your quest for exacting dynamics processing.

Retro Instruments 176 Limiting Amplifier Features:

• Authentic tube circuitry
• Vintage warmth, impeccable modern specs:
  • 20dB of available gain reduction
  • Signal to noise ratio of greater than 76dB
  • Flat frequency response within 0.5dB from 20-20,000Hz
  • Harmonic Distortion of less than 1% from 0-15dB gain reduction
• Precision Knob scales for easy recallability
• Continuously variable attack and release times
• Four compression ratio settings (2:1, 4:1, 8:1, 12:1)
• Delivers subtle compression, limiting, and everything in between
• Wide-Range Sidechain Highpass Filter
• Vocal Asymmetry Tool
• Hard-wire Bypass switch
• Interstage Transformer Bypass switch
• High Quality VU Meter switchable between Input, Output and Gain Reduction
• XLR input and output connections
• Integral tube balance test
• Easy stereo strapping of two or more units
• Selected NOS tubes and components
• 115/230V AC mains; IEC power connector
• Hand-crafted in Retro's Northern California factory