New! Introducing the Uno Compressor.

The Uno Compressor is a three board set including Sidechain, VCA and Gain Reduction meter. This page is for a set of bare un-assembled PC boards. Assembled and tested modules and board plus THAT IC options will become available.

The Uno Compressor combines a unique “near zero ripple” side chain, variable RMS timing and Wet/Dry mixing to provide versatile, fast-responding, low-distortion compression.

A “Zero Ripple” Side Chain Lowers Distortion and Compression Artifacts

Conventional dynamics side chains have rectification ripple that modulate the audio path and distort it. Maximum compression ratio and faster response are limited by the audibility of the detector.

The Uno Compressor has “near zero” ripple. Elimination of ripple-induced distortion allows faster response and higher ratio settings with less compression artifacts. Zero ripple provides more compression options.

To achieve low ripple, the side chain left and right audio are processed by a Dome all-pass filter making them phase-shifted 90° in relative quadrature. The VCA signal path is not filtered.

The two Dome filter outputs are individually RMS-detected and power-summed to produce a clean DC value with “near zero” detector ripple based on the combined signal powers of Left and Right.

Quadrature detection takes advantage of the trigonometric identity sin²ϴ⁠+cos²ϴ=1. The number “one,” equal to the aforementioned trig identity, give the Uno Compressor its name. In electronic terms the summation of the sin² and cos² outputs equaling unity implies DC without AC ripple. This technique has been widely used by designers of low-distortion fast-settling oscillators for a very long time.

In the audio production realm Blackmer’s 1972 “RMS” patent described a zero ripple compressor using log-domain RMS detection of a single input filtered to be in quadrature. The Uno Compressor takes advantage of the Dome filter's ability to weight sum and difference equally to halve the number of filters and detectors when adapting Blackmer's concept to stereo. 50-plus years later the Uno Compressor brings to fruition Blackmer’s “Zero Ripple” concept.

Zero Ripple Detection Provides Greater Ratio and Response Options

Low detector ripple allows faster attack-release response and more aggressive ratios due to less audible distortion on sustained tones and transients. Consider a 50 Hz tone compressed 4:1 with a modest 2.5 dB gain reduction and release set to the fastest 600 dB/s rate. Using the True Power Sum detection without quadrature the distortion is about 3.2%. With Quadrature Power Sum’s ripple reduction enabled, the distortion is less than 0.2%. At 1 kHz, under the same test conditions, the distortion without is 0.18% versus 0.01%. 0.01% is essentially the distortion of the VCA.

Introducing “Variable RMS”

Compressors based on dBx and THAT RMS-responding technology do not lend themselves to independent variable attack and release controls. To maintain the smooth power response that decibel-linear RMS detection provides, attack and release need to vary in lock-step.

Attack time is both program and release time dependent. Large changes in input level have shorter attack times; small changes have longer attack times.

Release rate is used to characterize overall detector speed and is specified in dB per second. Most dBx-style compressors have release rates fixed at 120dB/s.

When the release time is varied, the attack time changes with it while simultaneously remaining RMS and program-dependent.

Various attempts have been made to provide separate attack and release adjustment but they cannot be power-responding, often have electrically finicky designs and require a lot of adjustment to actually use.

Vari-RMS puts attack and release on one easy-to-adjust knob...

The Uno Compressor uses a synthetic variable timing capacitor to provide front panel release rate adjustment from 600 dB/s to 60 dB/s. Attack time remains program-dependent varying in lock-step with release while maintaining consistency with RMS power response. A low dielectric absorption film capacitor is used whose effective value is multiplied to emulate a much-larger, and more common, electrolytic timing capacitor.

The ability to adjust compressor response speed on a single knob allows the user to quickly adapt to the current circumstances and, thanks to Quadrature summing, without distortion from detector ripple.

Parallel Compression VCA

The Uno Compressor has a separate VCA board with optional Wet/Dry parallel compression.

Wet/Dry mixing can be configured to use a single 4-gang cross-fading pot, two independent 2-gang pots for Wet and Dry, or a single gang pot for Wet level. When parallel compression is not needed the builder can eliminate those components.

Relays provide switching between Process/Bypass and Inline/Parallel compression.

The Uno Compressor Sidechain has the following controls and features:

Mechanical and Power Specifications

VCA Features and Audio Specifications

To learn more about the "Uno Compressor" visit the Pro Audio Design Forum construction thread.

The evolution of the Uno Compressor is described in this Pro Audio Design Forum thread. 

Mouser Electronics Project Manager bill-of-materials are available for DIY builds:

Sidechain BOM

VCA BOM

GR-20 Meter BOM