Field Test: Thermionic Culture The Phoenix Compressor

Apr 1, 2004 12:00 PM, By Michael Cooper

Variable-Mu Unit Delivers Classic Tube Sound


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Thermionic Culture's The Phoenix is a pretty rare bird. It's a variable-mu tube compressor — the first type of compressor ever made. Few manufacturers offer variable-mu compressors these days, Manley Labs and Pendulum Audio being notable exceptions. Yet those in the know seek out these unique processors for their idiosyncratic sound.

Variable-mu compressors use a tube as the gain control element. They are inherently soft-knee compressors: the harder you drive the gain control tube by cranking the input level, the higher the compression ratio becomes, which causes an increasing densification of the processed sound as the gain control tube approaches saturation. Because tubes generally have a more limited dynamic range than other gain control elements, they usually can't produce more than 15 dB of gain reduction. Generally faster in action than opto-electronic compressors but not as fast as those that use VCAs or FETs, variable-mu compressors produce a unique dynamic response.

This 2-channel, 3U-high unit can be operated in either dual-mono or linked in stereo. Each channel has a continuously variable channel (input) gain, attack time, release time, threshold, output trim controls, a toggling bypass switch and large VU meter. A link control is provided for stereo operation.

There are no ratio controls — boosting a channel gain control simultaneously adjusts input gain and drive level for the PCC85 tube, which serves as the gain control element. The Phoenix's initial compression ratio is 1.2:1, increasing to 5:1 with 15 dB of compression. Attack times range between 4 and 120 ms, and release times vary from 60 ms to 2.2 seconds. Factor in its wide-ranging threshold control, which actually precludes compression when in the fully clockwise position, and you've got a unit that offers both subtle and dramatic processing.

The rear panel has balanced XLR I/Os. Huge air vents on the chassis' top, rear and sides keep it cool. Knob positions and meter readings are easily seen from a distance. The Phoenix has a bit of a homegrown look that should appeal to boutique-audio enthusiasts. All controls have a solid feel, and internal componentry looks tidy.

Hash marks on the rotary controls are denominated using arbitrary numbers that range from 1 to 11; decibels and units of time would be more informative. The front panel does not display if the link and two bypass switches are activated when in the up or down position. I found myself adjusting a channel gain or output trim knob to tell (by listening) whether a channel was bypassed.

The effect of all rotary controls is removed from the audio path when a channel's bypass switch is engaged, yet the channel's VU meter still registers the amount of gain reduction that would be affected if the channel was activated. This allows you to dial in reasonable settings in front of clients before switching in the processing. Unfortunately, the VU meters are not backlit, and the Phoenix does not offer any sidechain inputs.

Specs are good: Inherent noise floor is 95 dB below the maximum operating level, and frequency response varies less than 1 dB between 12 and 56k Hz. Maximum gain is 30 dB, making it compatible with +4dBu and -10dBV nominal levels.


I recorded various sources using a Millennia HV-3D mic preamp. The Phoenix consistently smoothed each track's fluctuating dynamics in an unobtrusive manner. Compressing male lead vocals (recorded with an AKG TLII mic) 4 to 5 dB, I did not hear pumping. The track sounded much fatter and less clinical. The Phoenix also seemed to broaden the low-mid frequencies, which was not always a plus.

With an Aguilar DB 900 DI on the front end, 2 to 4 dB of gain reduction smoothed an electric bass track's dynamics nicely over the instrument's range. For this track, I set The Phoenix's attack control to “7” (fairly high) to avoid losing presence. With careful tweaking, the compressed track sounded great.

I made a kick drum track pop pretty hard by applying 2 dB of gain reduction. (I'm sure the actual amount was far greater, but it didn't fully register due to the VU meter's inherently slow response.) Setting attack and release times to their fastest settings did not produce any audible distortion on the processed track.

I got great results applying 2 to 3 dB of gain reduction to an arpeggiated, stereo acoustic guitar track. The Phoenix delivered the pleasing density for which variable-mu compressors are sought. The Link control kept the stereo image rock-solid. However, the channel gain and output trims are not linkable. The best results for stereo operations are achieved when using matched settings for these controls.

The Phoenix really impressed me when used as a stereo bus compressor. Compressing a rock ballad 3 to 4 dB on peaks, the result was intoxicatingly creamy and bold, with beautiful density and sustain. The Phoenix rounded the mix's edges to produce a softer sound with moderated detail. Bass and low-midrange frequencies were broadened, adding thickness and size to digital tracks. I could easily crank the mix up to aggressive levels without causing audible pumping. Like other analog compressors, when using this much compression, my only complaint was a slight loss of depth. But on balance, the overall sound was extremely flattering.

The Phoenix rises to meet most challenges gracefully. Althought it is expensive ($4,295), if you want that characteristically dense and creamy variable-mu sound, The Phoenix is worth a serious listen to.

Thermionic Culture (dist. by Unity Audio), 44/1440/785/843,

Michael Cooper is the owner of Michael Cooper Recording, located in Sisters, Ore.

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