PrimaLuna DiaLogue Seven power amplifier
That doesn't work everywherethe arts are inhospitablebut for those with less lofty goals, it's nice to have a formula, especially a good one. The people behind the PrimaLuna line of audio electronics probably know that as well as anyone. A few years ago they combined a unique amplifier design with a similarly distinctive business model to create the ProLogue One, a product known for both value and reliability: the former by combining Chinese assembly with European (in this case, Dutch) quality, the latter for a circuit innovation that made the thing darn near foolproof.
The formula has been applied, with apparent success, to a number of subsequent products, the latest being the PrimaLuna DiaLogue Seven monophonic tubed amplifier ($5495/pair), which offers 70Wpc in ultralinear mode or 40Wpc when run as a triode ampand does so in a mildly remarkable way.
As with earlier PrimaLuna amplifiers, the DiaLogue Seven's calling card is a circuit innovation called Adaptive AutoBias. The term auto bias normally describes a circuit in which the signal grid of a power tube is referenced to ground, and the potential of the cathode is raised above ground through a cathode resistor (footnote 1), thus prompting the tube to adjust itself under operating conditions. PrimaLuna's variation of the name refers to something different: a fixed-bias amp in which the voltage applied to the signal grid is adjusted, minutely and continuously, in response to such variables as operating temperature and input-signal amplitude. Thus the Adaptive AutoBias circuit keeps the output tubes operating within their best parameters at all timeswhich, according to PrimaLuna, provides significant reductions in both distortion and tube wear. The circuit is also said to allow greater-than-usual flexibility in swapping tubes, in terms of both age and tube type, while keeping performance on an even keel. I'm told that one can even use in the DiaLogue Seven different ages or types of tubes for the two sides of a single complementary pair, for the sheer jaunty fun of it.
Another innovation distinguishes this new PrimaLuna amp from the competitionand, in this instance, from the company's humbler ProLogue series: Not only can the DiaLogue Seven be run in either triode or ultralinear mode, but the user can switch between those modes at will, with the push of a button on a remote handset (included). The late, great Peter Snell, who devised some clever ways to adjust crossover parameters from his listening seat in an effort to perfect his loudspeaker designs, is surely smiling upon this.
When I opened the DiaLogue Seven's chassis for a look inside, the key to its dual-mode design surprised me. (Had I given it just a bit more thought, I might have figured it out when I first lifted the amp from its sturdy triple cartonand noted its 71-lb-per-channel shipping weight.) Each monophonic DiaLogue Seven contains one mains transformer and two complete output transformers. In triode mode, of course, the screen grids of the KT88 pentode tubes are tied to the plates, and the tubes operate as indirectly heated triodes. But in ultralinear mode, the screen grids are tied to a portion of the primary windings of the output transformer, in a distortion-canceling scheme that functions rather like feedback. Because that requires tapping the primary at a very specific ratio, the luxury of an extra output transformer on an amp such as this is a Godsend.
In other ways, the PrimaLuna DiaLogue Seven is comparatively straightforward: solid-state rectification and a robust pi filter for the power supply, along with parallel 12AX7 tubes and a long-tail pair of 12AU7 tubes for the input/driver section. But in one other regard the amp is far from ho-hum: the quality and care with which it's made. Apart from the above-mentioned AutoBias circuit and the logic bits for the remote control, the DiaLogue Seven is completely hand-wired, point to pointand I've never seen a better-built amp. Wires were neatly trimmed and dressed, with no strand out of place. I spent a long time trying to find a single bad solder join, and could not: Someone made this as if it mattered.
Installation and setup
The DiaLogue Seven's input and output characteristics suggest good installation flexibility, the former by its 100k ohm impedance (and apparently generous gain), the latter by its choice of loudspeaker connections: 2-, 4-, and 8-ohm secondaries. As for that, I played the PrimaLunas through two very different loads: my usual Audio Note AN-E SPe HE (for High Efficiency) loudspeakers, and a borrowed pair of Wilson Audio Specialties Sophia 2s, recently and expertly installed by Wilson's Peter McGrath. My original intention was to try the DiaLogue Sevens with my rebuilt Quad ESLs, as well: Regrettably, I wasn't able to do that within this initial review period, but PrimaLuna USA is allowing me to keep the review samples a little longer than usual, during which time I'll try the Quad option.
The DiaLogue Sevens weren't fussy about placementa nice thing to say about any pair of amplifiers that together weigh more than some adultsand for the most part I kept them on my hardwood floor, without benefit of isolation tweaks. They became warm during use but not alarmingly so; their elegant-looking tube cages provided ample protection, amp from user and user from amp.
Footnote 1: The presence of a cathode resistor is not itself a sign of auto bias: A very-low-value resistor here makes it possible for a technician to measure and adjust his output tubes yet still run them in fixed-bias mode.