(NOTE: This is the technical sidebar that will appear with my review of the PrimaLuna Seven in TAS #199. While there are undoubtedly valid reasons to refrain from posting the sidebar in advance of the review, I thought readers would appreciate a preview, at least in part, of this remarkable amplifier)
The design team at PrimaLuna faced a formidable challenge; namely, how to match (or exceed) the sonic signature of its DiaLogue Two in a set of monoblock amplifiers with twice the power. This was no small feat because the DiaLogue Two possessed a natural and highly engaging midrange, outstanding soundstaging, extended and articulate bass and highs, and a musicality reminiscent of a live performance. As many designers can attest, it’s quite easy to take a step backwards rather than forwards when creating a more powerful amplifier.
PrimaLuna’s Chief Engineer, Marcel Croese, was no stranger to design challenges. As Goldmund’s principal engineer for several years, he was credited with many innovative designs, including the Mimesis 29 and 28 power amplifiers, and the MM29.4 and MM28.4 monos, among others. He also developed PrimaLuna’s innovative, and highly effective, Adaptive AutoBias circuit.
While many may have used larger transformers to double the output power, Marcel ruled that approach out because big output transformers don’t behave as well or sound as good as their smaller brethren. He opted to use the same outstanding wide-bandwidth output transformers as in the DiaLogue Two (described in Issue 175), employed two discrete audio channels on one chassis, and paralleled them at the loudspeaker terminals so that the outputs would be summed, resulting in double the output power. He also added a 16-ohm tap to each transformer, which when paralleled resulted in an 8-ohm output.
However, the summing approach created other challenges, since a percentage of power from one free-running transformer was pushed into the other, due to output voltage differences caused by circuit tolerances and tube aging. To deal with this problem, Marcel developed a specific cross-coupled positive/negative feedback scheme to balance out both channels perfectly. It also enabled Marcel to offer a 2-ohm tap to drive very low sensitivity/low impedance loudspeakers, as well as some electrostatic loudspeakers.
Since less total gain was needed than in the integrated amplifier, Marcel was able change the pre-stages in a way that improved the drive capacity to the power tubes, resulting in even lower distortion, without the need for extra negative feedback. The circuit was inherently more stable and virtually immune to drive imbalance and its accompanying distortions with tube aging.
This innovative twin-channel-summing approach with cross-coupled positive/negative feedback not only resulted in surprisingly low harmonic distortion, but also matched (and improved upon) the sonic signature of the smaller DiaLogue Two. JH