The design team at PrimaLuna faced a formidable challenge: How to match (or exceed) the sonic signature of its integrated and stereo amps, in a set of monoblock amplifiers with twice the power. This was no small feat because the stereo amps possessed a natural and highly engaging midrange, outstanding soundstage, extended and articulate bass and highs, and a musicality reminiscent of live performances. As many designers can attest, it’s quite easy to take a step backwards rather than forwards when creating a more powerful amplifier.
While many designers may opt for larger transformers to double the output power, the PrimaLuna team decided against that approach knowing big output transformers sometimes don’t behave as well or sound as good as their smaller brethren. They opted instead to use two discrete audio channels in one chassis, and paralleled them at the loudspeaker terminals. Building an amp with two output transformers per monoblock amp is almost unheard of due to one simple reason. Cost. (Note: We know of one company doing it in their $15,000 per pair amps)
The sonic benefits are significant. This design adds a 16-ohm tap to each transformer, which when paralleled in this circuit design, resulted in an 8 Ohm output. You now have three sets of speaker taps: 2 Ohm, 4 Ohm, and 8 Ohm; to get a perfect "handshake" from the amps to your speakers. The ability to drive planer and ribbon speakers is unmatched.
This approach created other challenges. You risk a percentage of power from one free-running transformer pushing into the other, due to output voltage differences caused by circuit tolerances and tube aging. To deal with this problem, the team developed a specific cross-coupled positive/negative feedback scheme to balance out both channels perfectly, while distortion is lowered significantly in the slipstream. In addition, newly designed pre-stages improved the drive capacity to the power tubes, resulting in even lower distortion levels, without the need for extra negative feedback. The circuit was inherently more stable and virtually immune to drive imbalance and the distortions that accompany tube aging.
This innovative twin-channel-summing approach with cross-coupled positive/negative feedback not only resulted in surprisingly low harmonic distortion, further pushing the envelope of performance of all PrimaLuna mono amps.