Thermionic CULTURE Phoenix Review
Reviewed by Michael Cooper for Mix

Thermionic Culture's The Phoenix is a pretty rare bird. It's a delta-mu tube compressor, which was the first type of compressor ever made. Few manufacturers besides Thermionic Culture offer delta-mu compressors these days, Manley Labs and Pendulum Audio being some of the other notable exceptions. Yet those in the know seek out these unique dynamics processors for their idiosyncratic sound. Delta-mu compressors use a tube for their 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. This 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 (such as optos, VCAs or FETs), they usually can't produce much 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, delta-mu compressors produce a rather unique dynamic response. That's what makes The Phoenix so intriguing.

Come Fly With Me

Towering 3U high, the 2-channel unit can be operated in either dual-mono or linked in stereo. Each channel sports its own continuously variable channel (input) gain, attack-time, release-time, threshold, and output trim controls, as well as a toggling bypass switch and large VU meter. A link control is provided for stereo operation. A Beefy power switch and its associated large green status lamp finish off the unit's front panel.

As inferred earlier, there are no ratio controls-boosting a channel-gain control simultaneously adjusts input gain and drive level for the PCC85 tube that serves as the gain-control element. There is one PCC85 and one 12AT7-the latter serving the unit's output - each channel's audio path, as well as a 6AL5 used for each channel's sidechain detector. Earlier units-ones that don't have a "P" suffix in their serial numbers-used 6BQ7A tubes in lieu of PCC85s. 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, while release times vary from 60 ms to 2.2 seconds. Factor in The Phoenix’s wide-ranging threshold control, which actually precludes compression when in the fully clockwise position, and you've got a unit that is capable of both subtle and dramatic processing.

On Phoenix's rear panel you'll find ins and outs on balanced XLRs, an IEC power connector and a fuse receptacle. The unit sports huge air vents on the top, rear and both sides of the chassis, keeping it relatively cool. Knob positions and meter readings can be easily seen from a distance and when viewed at reasonable angles, thanks to white and black color schemes and muted light reflectivity. The build of the meters and the way the top chassis is offset from the floor of the unit give The Phoenix a bit of a homegrown look that should appeal to boutique-audio enthusiasts. Thankfully, all controls have a solid feel, and internal wiring and componentry look tidy.

What's Missing

All of The Phoenix's rotary controls have hash marks that are denominated using arbitrary numbers that range from 1 to 11; decibels and units of time would have been more informative. Also, the front panel is not marked to inform you whether the link and two bypass switches are activated when in the up or down position. It turns out that down is active. I often found myself adjusting a channel-gain or output-trim knob simply to tell (by listening to the result) whether or not 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 associated VU meter still registers the amount of gain reduction that would be effected if the channel were subsequently activated given the current positions of the various controls. That's a nice touch, because it allows you to dial in reasonable settings in front of clients before switching in the processing. Unfortunately, the VU meters are not backlit.

The Phoenix does not offer any sidechain inputs. To perform ducking or frequencyconscious compression with The Phoenix, execute the following workaround: choose, for example, channel 1 for the audio path and disarm compression by turning its threshold control fully clockwise, feed your key signal into channel 2 and activate the link function. Set up this way, channel 2's threshold, attack and release controls will govern channel 1's dynamics processing. The downside is that you can only process one (mono) channel of audio with this setup.

Specs look quite good, especially considering The Phoenix's tube-based design. The unit’s maximum operating level is +24 dBu into 10 kilohms (+19 dBu into 600 ohms), inherent noise floor is 95 dB below the maximum operating level, and frequency response varies less than 1 dB between 12 and 56k Hz. The maximum gain is 30 dB, making the unit compatible with both +4dBu and -IOdBV nominal levels.

First Flight

I made recordings of various sources, using my Millennia HV-3D mic preamp, and listened to each track both with and without The Phoenix engaged. The Phoenix consistently smoothed each track's fluctuating dynamics in a very unobtrusive manner.

Compressing male lead vocals (recorded with an AKG TLII mic) 4 to 5 dB, I heard no pumping. The track sounded much fatter and less clinical than without compression applied. The Phoenix also seemed to broaden the low-mid frequencies somewhat, which was not always a plus.

Using an Aguilar DB 900 DI box on the front end, 2 to 4 dB of gain reduction smoothed an clectric bass guitar track's dynamics nicely across the instrument's range. For this particular track, I found I had to set The Phoenix's attack control to "7" (i.e., fairly high) in order to avoid losing presence. With some careful tweaking, the compressed track sounded great.

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

Igot 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 "vari-mu" compressors are sought. The unit's Link control kept the stereo image rock-solid. However, I discovered that The Phoenix's channel-gain and output-trim controls are not linkable; the best results are achieved for stereo operation when matched settings are used for these controls.

I was most impressed with The Phoenix when using it as a stereo-bus compressor. Compressing a rock ballad around 3 to 4 dB on peaks, the result was intoxicatingly creamy and bold, with beautiful density and sustain. The Phoenix rounded the edges of the mix to produce a softer sound with moderated detail. Bass and low-midrange frequencies were especially broadened, adding thickness and size to digital tracks. I could easily fight the "level wars" and win, cranking the mix up to aggressive levels without causing any audible pumping. As is the case with other analog compressors I've heard when using this much compression, my only complaint was a slight loss of depth. But on balance, the overall sound was extremely flattering.

By the Time I Get To...

The Phoenix rises to meet most challenges with grace. At a list price of $4,295, however, the unit is extremely expensive. But if you're willing to splurge for a tube compressor that delivers the characteristically dense and creamy variable-mu sound, The Phoenix is worth a serious listen.