Continent of Mu

The Manley can really fatten up a mix.

The first compressors ever made incorporated a Variable-Mu design. Those use a vacuum tube—such as a dual triode or pentode—for the gain-control element. Technically oriented readers will be interested to know that this is a fully differential, push-pull design in which one side of the tube handles the positive waveform phase and the other side the negative phase.

Variable-Mu compressors do not offer an adjustable ratio control. What people love about the Variable-Mu compressors is that they continuously increase their ratio the harder they’re pushed (the higher above threshold that the input signal rises), resulting in an increasing desensification of sound. Though Vari-Mu compressors offer faster attack and release times than optos, they are not as fast as VCA designs, and therefore they’re not as effective at handling peaks as VCA-based units. Also, as a class, Vari-Mu compressors cannot produce as much gain reduction as other types of compressors because the employed tube typically runs out of dynamic range sooner than other types of gain-control elements. A Vari-Mu usually gets 12 to 15 dB of gain reduction and sometimes considerably more.

The Manley Stereo Variable-Mu Limiter/Compressor features an all-tube audio path that is optimized for pro (+4 dBu nominal) levels.

Few Variable-Mu models are currently on the market. I am aware of high-end units made by boutique manufacturers such as Manley Labs and Pendulum Audio. Because of design differences, Pendulum Audio’s Variable-Mu compressors typically offer faster attack times than Manley’s.

The Manley Stereo Variable-Mu Limiter/Compressor ($4,000) sounds awesome on bass and electric guitar; it imparts a fat, lush tone with plenty of presence and clarity. When set to limit mode, the Manley also sounds great on snare drum. However, the unit is intended primarily as a stereo-bus compressor for processing an entire mix. In this application, I had to keep the attack time near its slowest setting to avoid pumping. But properly dialed in, the Manley Stereo Variable-Mu can really fatten up a mix.

FET in the Cap
FET-based compressors are scarcely available now. They use an FET (Field Effect Transistor) as the gain-control element, which has advantages and disadvantages. On the plus side, FETs offer attack and release times that are much faster than optos can provide, and even faster than many VCAs. The downside is FETs have a somewhat limited dynamic range. In traditional FET designs, very hot levels can cause amplitude-modulation artifacts (for example, pumping) and waveform distortion.

The single-channel Universal Audio 1176LN Limiting Amplifier ($2,295) is a faithful reproduction of the late ’60s-era, FET-based compressor prized for its lightning-fast attack and crystalline sound.

Crane Song’s single-channel Trakker ($2,550) and dual-channel STC-8 ($4,450) compressors use a unique Pulse Width Modulator (PWM) FET design to solve the distortion problem. Unlike traditional FET designs, a PWM FET does not modulate the gain reduction, so there is less distortion at high gain. In technical terms, the PWM is essentially a switch that turns the audio signal on and off at a 1.1 MHz frequency, letting only a percentage of its energy through at any given time to control the gain. A filter rids the signal of switching artifacts. Because the signal is either on or off, it doesn’t modulate the gain-control element’s resistance or resulting gain reduction. The result is that the distortion content is linear as gain reduction increases. In addition, attack time can be as fast as a few microseconds. Unfortunately, I have not worked with any Crane Song compressors in my studio.

BACK | NEXT

Reprinted with permission from Magazine, February, 2001
© 2001, Intertec Publishing, A Primedia Company All Rights Reserved