Solid State Power Amp Reviews

It is unusual to begin a review with a detailed discussion of setup. But setup protocol for the Moon 861 power amplifier ($22,000 each), the top-level amplifier in the North Collection from Moon, which I reviewed bridged in mono, proved crucial to its sound.

The setup saga began when Moon co-owner Costa Koulisakis traveled to Port Townsend from Quebec to help remove two 861 amplifiers from their sturdy flight cases and set them up. (We also set up the Moon 891 Network Player/DAC, which I'll review next month, with and without the 861s.) First, he noted that Moon designed the 861 to rest on amp stands, supported by the amp's specially designed feet, which contain a suspension system composed of O-rings, a poron damping pad of specific density, and a contact point composed of dense ABS (like Legos). "Besides absorbing some external and internal vibration, these new footers also help level the amplifier when it is placed on an uneven surface," he said. "Each foot will compress a different amount to prevent the amplifier from rocking in any direction."

Koulisakis explained that Moon voiced the 861 three different ways: on HRS amp stands, on a granite surface, and directly on the floor of their main listening room, which has very low-pile carpet glued directly to a concrete floor. "The HRS stand works best, but the 861's elaborate footing system already does a portion of the work that a good amp stand will do."

Koulisakis lamented that my Grand Prix Monza amp stands were too small to accommodate the 861s. Not because the 861 is gargantuan; far from it. Rather, my amp stands are smaller than the norm to enable them to fit in tight spaces. In addition, each stand's four support pillars extend higher than their bamboo shelves and are situated more or less where the Moon 861's support feet would make contact. No way, José.

The solution seemed simple enough. I customarily place Wilson Audio Pedestals between all my components and the Monza system's bamboo shelves. When I mentioned that the Pedestals would raise the amps higher than the stands' corner pillars (which are hardly of Greek-temple proportions), Koulisakis cautioned that the Moon's bottom cover was not designed to support the weight of the amplifier on its own. As much as he lamented that placing the amps directly on my bamboo flooring (which sits atop cement, with a very thin requisite layer of felt in between) might subject them to undue vibrations, he saw the floor as the only practical solution.

Once the amps had warmed up, their sound was not what I'd expected after hearing them in Chicago and Munich (footnote 1). Nor, I sensed, was it what Koulisakis expected. After discussing alternatives, we decided that the way forward, which I'd pursue after he left, was to obtain two sturdy 19" × 21" pieces of particle board, which would be far less resonant than either plywood or many hardwoods; place them atop my bamboo shelves with Wilson Pedestals in between; and use the 861's suspension feet to support the amps on the particle board. Think British double-decker bus with an observation tower atop.

Port Townsend may be a small, rural community, but it's also a Victorian seaport arts colony and the wooden boat capital of the United States. After exploring a basic lumber shop and a specialty hardwood supplier, I visited an artisan cabinetmaker. I knew I'd found the right place when I saw above the cabinetmaker's desk a CD of Glenn Gould playing Bach. It got even better when his assistant showed me his homemade speaker/amplifier rig. I returned home with two heavy, 1" thick slabs of high-density particle board. Friend Scott and I repositioned the amps and found the sound so opposite to what we'd initially heard that I felt I owed both Moon and readers a third, middle-ground alternative.

After many days of pondering, I initiated a WhatsApp chat with Koulisakis. "I've been thinking and thinking, and like one of the Wise Men of Chelm, I think I've come up with a marvelous solution."

Koulisakis agreed that if I positioned the Pedestals beneath the 861s' solid heat fins, no damage would be done to the amp's bottom panel. If I moved the amps back on the stands so that their fronts rested behind the corner support pillars and their rears extended several inches beyond the stands, I could safely support them by positioning Pedestals under either end of their heat fins. Which is what the husband and I proceeded to do. Suffice to say that what I heard most resembled what I'd expected to hear.

Whether reading this leaves you fascinated, bored to tears, or shaking your head at how much time I spent on setup, please hear me out. Three different, totally plausible setup options yielded three distinct sounds. Setup may be less crucial when the 861s are paired with components less capable of producing the fine gradations of timbre, nuance, dynamics, and shading than those in my reference system, but it can make a major difference when you're setting up components whose sound justifies their five- and six-figure price tags. Even though it took multiple pairs of hands and a fair amount of sweat to satisfactorily position the 861s, it's what was necessary to give them their just due in audiophile court.

To learn the verdict of judge and jury—that's me—please read on.

The evidence
Everyone who has ever attended a presentation by Costa Koulisakis knows that the man speaks with completeness. Simaudio's 13-page user manual (footnote 2) describes the 861 as an "MDCA—MOON Distortion-Cancelling Amplifier" in dual-mono configuration with no global feedback, special MOON transistors, a monaural-mode toggle switch, MOONLink (a proprietary wiring system for use with other Moon products), and multicolor LED indicator. Koulisakis expanded on this in our hour-long interview, following up with a summary document in which he reiterated what makes the 861 unique.

Compared to the Moon 860A v2 amplifier I reviewed in November 2022, the 861 is what Koulisakis termed a "revolutionary" design that evolved from its predecessor. "Thanks to what we learned from the 860A v2, we developed so many new ideas that we could only implement them if we redesigned the circuitry from scratch," he said.

"We focused on signal correction and feedback. As an audio signal passes through an amplifier's gain stages and is amplified, it accumulates a certain amount of distortion. Amplifiers typically employ some form of signal correction to reduce that distortion. Since the 1990s, our amplifiers have reduced distortion without using a global feedback loop, which feeds the amplified signal back to the input. Because global feedback loops can affect sound quality, and the local feedback that's often used to correct the individual gain stages also has its downsides, our approach utilizes different and more accurate, 'real-time' distortion correction."

Simaudio's new Moon Distortion-Cancelling Amplifier (MDCA) technology dispenses with all forms of feedback. Instead, it performs signal correction in a special proprietary parallel circuit path that is outside the path of the audio signal. A small "correction" signal, applied only to the output stage, is said to reduce distortion by almost a factor of 10 at high power. Simaudio (the company that produces Moon-branded hi-fi products) claims that this type of signal correction is more efficient than any type of feedback loop because, among other reasons, it does not rely upon a high-gain, high-power signal path.

"When signal correction is determined outside the signal path, we can keep the audio path simpler and free of unwanted side effects that result from correcting within the signal path," Koulisakis said. "The 861's input stage is also very special and unusual because it duplicates the incoming signal of each channel and then routes one of the two signals of each channel for signal correction. Having parallel signal paths also helps lower output impedance and increase the damping factor.

"We use a precision comparator circuit that determines the amount of distortion and injects a correction signal into the signal path at the output stage. This is not an audio signal in the normal sense; it's a minute signal that offsets the accumulated distortions in all the gain stages. Because MDCA determines the distortion component outside the signal path, this technology works better than any feedback loop within the signal path that attempts to reduce distortion by re-injecting the audio signal back into it. Our signal correction also works much faster as it compares the input and output signals and determines the required minute signal corrections in real time; this enables the amplifier to respond faster and produce a cleaner, lower-distortion, and more accurate output signal."

That's the "revolutionary" bit. In addition, the size of the 861's power supply and chassis have increased, as have output power and weight. Its dual-mono design, balanced circuitry, low-impedance proprietary Moon output transistors, and parts quality are similar to its predecessor's. To meet European safety standards and allow a more secure fit, the company has replaced WBT binding posts with Furutech binding posts that accept bigger spades. Currently, however, the narrow opening for spade lugs is only on the bottom. That works well for people who don't want their speaker wires to show but less well for owners with low racks who want to prevent cabling from touching the floor.

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Footnote 1: Per Stereophile's reviewer policy, I did not share what I heard.

Footnote 2: See simaudio.com/wp-content/uploads/2023/05/20240506-User-Manual-861-_En.pdf.