Theta Digital Citadel monoblock amplifier:
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There are balanced (XLR, pin 2 hot) and unbalanced switch-selectable inputs, and a separate IEC mains-in for analog power, with a separate IEC and switch for the DAC upgrade. Three LEDs above the input connectors tell you what's going on with your investment in good sound. One LED pops off when you've pounded the Citadel into thermal overload; two more indicate whether or not the positive and negative rail fuses have blown.

Top right, RJ-45 and RS-232 connectors provide communications to the outside world and allow the preamp, for example, to turn on the amp. Protocols are provided for various control sequences. And there are 5-12V triggers for Standby/Operate and Source Select when the upgrade DAC board is installed.

Float like a butterfly, sting like a bee
One design element any prospective buyer of the Citadel should be aware of is that it's a fully-differential bridged amplifier. The negative speaker terminal "floats" and does not reference ground. It should never be connected to ground, therefore.

The input is via four pairs of differential JFETs, with four MOSFETs in the driver stage. Other circuit highlights, according to Theta, are zero-feedback circuitry and carefully matched solid-state devices, plus custom-made capacitors, precision Vishay resistors, copper bus bars, and 12-gauge Litz/Teflon wiring.

The output stage uses a high-current power transformer feeding a capacitive filtering system that uses two massive chokes almost as large as the power transformer. These are fed from an ultrafast, soft-recovery, 35A bridge rectifier. Theta claims this results in a high-current power-supply line more than 800% cleaner than conventional supplies provide.

Sound
In Dido's "Here with me (Chillin' with the Family Mix)," from the Ultra.Chilled 01 Trip Hop compilation (2 CDs, UL1110-2), a woman's voice floats way back in the soundstage. The first time I heard it, I wanted to dive into that soundstage to inform this woman that I was the guy she was singing about, grab her, and...er, give her a big hug. There was a wonderful dimensionality to her voice, backed by powerful drums and a small, eclectic quirky band. The whole sonic construct floated like a butterfly: light, transparent, truly musical.

Then, from the same set, Morcheeba's "Another Chance (Afterlife Mix)": I have the original of this recording, and the remix sounds better. Again, I noted that affinity for beautiful, grainless female vocals and a full and lovely midrange, even as I noticed that the bass was a touch lighter than that of the Linn Klimax or the big Krell FPB 350Mc's—until I turned the volume up. Then the Citadels hauled off and gave it to me, top to bottom, in a manner most remarkable—the bass, which sounded rather light at lower levels, took on a dynamic, articulate, detailed, and powerful sound at higher volumes that was just splendid. Everything else stayed in place: nothing broke up, bloated out, or became strident or grainy. The soundstage and everything on it held their positions. The only profound difference was in the bass, which reared up and roared.

Also on the Ultra.Chilled 01 compilation is Zero 7's "Destiny," which not only delivers another female vocal to die for, but has a prominent guitar that showed off the Citadel's speed. "Through the Citadels there was always a beautiful bloom to music, with no artificial flavoring. In fact, everything on this album simply flew at me from deep behind the speakers and way off to the left and right, behind and to the sides—very, very expansive, and superbly transparent.

Lingering in the world of Trip Hop, I switched to another compilation disc, Paris Lounge (Wagram 3069692), a faster, more dynamic recording than Ultra.Chilled 01. Although I earlier described the Citadel's bass as "somewhat light," this nimble CD set everything straight in that department with powerful, deep, very pacey, fast, and—again—ultra-articulate bass. Nothing missing down there, I can tell you. In "Sporto Hontes," a plaintive horn blows behind what must be the hottest female vocal I've ever "seen" between my speakers. The entire musical presentation was nicely balanced; my feet tapped with no particular control from my brain. The atmosphere the Citadels re-created had me imagining that I was in Pamplona, and tomorrow was the Running of the Bulls.