|The Unison 3.1 is a DC-coupled differential voltage gain amplifier without global feedback. Very unusual for an integrated? It's biased extraordinarily high into Class A. It operates on that vaunted plateau for up to 35 watts (i.e. for most listening levels with most speakers) before sliding/slipping into Class A/B for the remainder of its 100w power envelope, which then cleanly doubles into 4 ohms for 200 wattage servings. Say watt? The Class A input stage uses the BurrBrown PGA2310 digital attenuator as the sole active device for a S/N ratio of >100dB, with transformer taps independent from the output stage.
The overbuilt drive section uses the large 2kVA power transformer and 18 paralleled bipolar output devices per channel. This adds up to 2000 watts of theoretical max power, 75 amperes of potential current and an ultra-wide 5Hz to 10MHz band-width. 48,000uF of total capacitance make up the power supply, visible in the fourth image as the large blue caps mounted face-down below the input board. Slew rate specs out at 50Volts/ microsecond, input impedance as 20 kOhms, and output impedance as 0.08 ohms. Finally, the overall dimensions are 17" wide by 5.5" tall by 14" deep, with an accompanying weight of 55 lbs.
The anodized front panel sports three triangulated grooves with two outer push buttons for the upper two, three for the lowest. From left to right and top to bottom, these trigger input selection (toggling through CD, tuner, video and aux as confirmed by a row of four blue miniature LEDs); level up; monitor on/off with blue indicator; level down; subwoofer on/off with blue indicator; standby; and level select. The latter is a clever implementation of balance control that assigns offsets (displayed in dB) for the left, right and subwoofer channels.
The central ovoid inside the top-most groove contains the infrared remote eye; displays the overall output level in blue; and doubles as the balance calibrator while in the "level select" mode. Alas, no provisions for display dim/off. The smaller ovoid display below is the Continuum badge, though not backlit as the power amp's.
The back panel sports coaxial pre- and sub outs, one monitor loop and four line-level inputs as well as the obligatory IEC power inlet, the mains switch and a convenient voltage selector. The internal architecture is a poster child for clean and neat, eschewing the proverbial rat's nest of flying-leads point-to-point wiring by concentrating instead on modern surface-mount technolgy to keep path lengths and noise to a minimum.
Having requested a preconditioned unit to avoid break-in delays and hasten turn-arounds, I was ready to rumble from the get. Which brings me to the accompanying Stage 3.3 amplifier with its fetching silver face plate. The photo below shows the Unison's and stand-alone amp's innards to be virtually identical, indicating that prospective owners of the former get a no-compromise power amp that so happens to include gain and input switching provisions.
Was I shipped said amp to go bi? Kinda. Coda's marketing director Mark Ward wanted the sonic advantages of high Class A biasing elucidated in print. You see, the Stage 3 stereo amplifier, like the Unison by special order, is available in 3.1, 3.2 and 3.3 iterations. These differ in output power, with the number behind the decimal point indicative of 100, 200 or 300 watts respectively. It explains the apparent overkill of output devices for my 100-watt Unison loaner: The basic architecture is designed for 300/600w into 8/4ohm duty. Is it then simply "rev-limited" for the lower-power units? Not exactly. The spare headroom is used for idling at higher Class A operation instead. The 100-watter remains in Class A for 35 watts; the 200-watter for 15; the 300-watter for 7. Without cost differential, the tough choice facing the prospective owner thus entails: Higher raw or lower refined horsepower?
Clever? You betcha. But more than that, it would afford me a unique opportunity thus far available only to tube amp owners.
How so? The base design's inherent flexibility would facilitate direct and easy juxtaposition of different biasing schemes using otherwise identical circuitry. After all, how often have you heard that Class A is inherently superior? More than you can shake a shtick at. Were you ever empowered to actually compare sonic signatures of different bias settings though? My point precisely. Hence, I rather anticipated this final portion of the assignment with gathering momentum. For once, I'd perform not the tired old A/B comparisons but full-throttle A/ABs. I'd finally become somebody. Halleluja. Naturally, to compel the 3.3 amp's decline/descent into lowly Class A/B action, I would ultimately require something other than my ultra-efficient horns which, as the Art Audio micro-power PX-25 proved, cruise all day on less than 6 watts while making plenty o' noise. Enter nOrh's excellent but underpriced 87dB SM6.9s. To hamstring the Unison and duplicate the potential handicap of the extra interconnect for the 300-watter fed from its pre-out, I'd jumper the integrated's monitor loop i/o ports with the very same cable. Preliminaries tied up nearly as neatly as the Unison's innards, I was ready for my aural class action suit. Incidentally, the Unison's or Stage 3 stereo amplifier's power/bias scheme can be retro-actively converted at the factory. Coda merely replaces the emitter resistors, changes the transformer taps and resets the bias. Owners are thus assured non-obsolescence should their power demands -- up or down -- change over time.