Soulution 711 Stereo Amplifier, 701 Monoblock Amplifier, and 725 Full-Function Preamplifier

Simply The Best

Equipment report
Solid-state power amplifiers,
Solid-state preamplifiers
Soulution 701,
Soulution 711,
Soulution 725
Soulution 711 Stereo Amplifier, 701 Monoblock Amplifier, and 725 Full-Function Preamplifier

Soulution, the Swiss company with the whimsical name, has been on a genuine tear lately. Its “more affordable” 5 Series electronics—the $55k 501 monoblock amplifier and $26k 520 full-function preamplifier, which I raved about in Issue 235—were in certain key ways (the bottom octaves, timbre, and large-scale dynamics) considerable improvements over its original, far-pricier 7 Series offerings (the $115k 700 monoblocks, the $40k 710 stereo amp, and the $40k 720 preamp, which I also raved about back in Issue 199). Now, with the release of its even more costly second-generation 7 Series components—the $155k 701 monoblocks that Robert will comment on, and the $65k 711 stereo amplifier and the $50k 725 full-function preamplifier that I will discuss—Soulution has put the horse back before the cart, producing what are, by a fair margin, the best solid-state electronics I’ve heard in my system. Indeed, the best electronics I’ve heard period, tube or solid-state.

I’m sorely tempted to leave it at that and save you the ordeal of reading paragraph after paragraph of backstory (and me the trial of writing them), but since claiming something is “the best” is close to meaningless without an explanation of what “the best” signifies, we’ll take the scenic route.

As it’s been a while since I reviewed the original 7 Series product, let me start by going over the ground that sets all Soulution gear apart from other solid-state, tube, and hybrid gear—the company’s unorthodox use of negative feedback.

I first heard about Soulution (the name is a concatenation of “soul” and “solution”) from solid-state amp maven and loudspeaker-designer Alon Wolf back in 2008. At the time I knew nothing about the brand, and when I looked it up on-line and discovered it was owned by another Swiss company called Spemot that specialized in building electrical motors and refrigeration units for the automotive industry, I was not enthralled. Shades of Crown, thought I. Then I chanced upon a rave review of a Soulution product—the 120Wpc dual-mono Soulution 710 stereo amp—in the tough-minded German hi-fi magazine Stereo, and got more interested. You see Stereo had pronounced the 710 a sonic and technological wunderkind. Indeed, the amp had tested so unprecedentedly low in distortion, so high in channel separation, so superbly well in S/N ratio that the magazine’s chief technician hung the test results in a gold frame above his bench.

Of course, some of us (at least some who go back that far) remember those Japanese solid-state amps from Sansui and others that also boasted record-low THD figures—but sounded like crap. The trouble was that to achieve such stellar specs the Japanese engineers had to ladle on so much global negative feedback that their amps were virtual TIM (transient intermodulation distortion) and SID (slew-induced distortion) generators. Feeding back the signal from the output in order to compare it to the signal at the input (and thus fix any errors that may have accrued as it made its way through the circuit) works fine if that feedback process is instantaneous, but feedback is a disaster if the amp takes too long to make its corrections. After all, the musical signal coming into the amplifier doesn’t hold still for a portrait; it is constantly changing; and if too much time elapses (and we’re talking nanoseconds here), the signal that the feedback circuit is comparing at the output is no longer the same signal that is being seen at the input. Think of it as a worst-case “jitter” scenario, albeit in the analog realm. Ever since the “specs wars” of the late Sixties, the received wisdom about solid-state has been that negative feedback is a bad thing—only to be applied sparingly and locally—while shorter signal paths and fewer parts are good ones.

With its 7 Series electronics Soulution turned this conventional thinking on its ear. In concert with the company’s owner and CEO, Cyrill Hammer, Soulution’s engineers decided that it wasn’t feedback itself, but the speed at which the feedback loop operated that was the problem.

As I’ve already noted, to eliminate the time-related distortion, graininess, and edginess that feedback engenders, you have to make those feedback loops correct errors instantaneously. This means that circuits and power supplies must operate at incredibly high speeds (which translates into incredibly high bandwidths) and with very high precision. Forgetting about shorter signal paths and fewer parts (the 710 amplifier used over 3000 components!), Soulution found ways to do this very thing, reducing propagation delay times (the amount of elapsed time it takes to correct a signal via feedback) to 5–10 nanoseconds (billionths of a second), where big solid-state amps typically had propagation delay times of 1–5 microseconds (millionths of a second). This thousand-fold increase in speed allowed for a huge increase in local negative feedback (and a drastic lowering of THD levels), without the usual price paid in time-domain errors.

The measured results of Soulution’s ingenious, high-speed, high-local-feedback circuit were phenomenal. In the 710 stereo amp, for example, THD was well below 0.0006%, signal-to-noise ratio well above 108dB, channel separation an astounding 86dB, damping factor greater than 10,0000, slew rate 330V/ns, while power bandwidth went from DC to 1MHz. (The monoblock amps measured substantially better!)

The sonic results were just as astounding. Suddenly you could hear…everything, and hear it with unprecedented clarity, speed, and neutrality.

I will never forget my first listen to the Soulution 710 stereo amplifier. It just so happened that, at the time, I was using what remains the most finely detailed transducer I’ve reviewed, the then-brand-new MartinLogan CLX electrostats. In concert, that amp and those speakers set a standard of transparency and resolution that had never before been approached and has never since been equaled in my system. The sheer number of previously inaudible details about the performance, the music, the venue, and the engineering they brought to light on record after record—and these were records I thought I knew by heart—was simply mind-boggling.

Hearing exactly how, oh, Joni Mitchell’s vocal harmonies on Blue had been separately recorded in a sound booth and then potted into the mix to create a plethora of Joni’s in the background (backing up Joni in the foreground) simply thrilled a “fidelity-to-sources” listener like me, for whom the experience was like peering over the shoulder of the mastering engineer and observing how he’d mixed the various tracks down to two-channel work parts. However, I could also see where so much “non-musical” detail might be off-putting to a listener for whom the color and drama of music comes first. After all, not everyone wants to watch the man behind the curtain twiddling dials and adjusting sliders each time he sits down to enjoy his favorite tunes.

Indeed for some listeners (not me, mind you) the 710’s presentation was overly “analytical”—dreaded word—in that it revealed artifices (like tape splices, mike preamp clipping, and overdubs, as well as every mechanical noise that a musical instrument is capable of making when it is played close by the diaphragm of a microphone) that other, less transparent amps and preamps glossed over, and that the musicians and recording and mastering engineers probably didn’t want or intend you to hear so plainly. While I reveled in this wealth of detail for all the previously unheard subtleties it revealed and for the greater sense of realism it brought to well-recorded sources, some members of my little listening panel weren’t so sanguine. A few of them felt the 710—particularly in combination with the CLXes—was just a bit too revealing.

Moreover, it could be argued (and was in other mags) that the 710’s super-high resolution was being purchased at a cost in lifelike density of tone color. Even a fan like me would have to admit that listening through it did require a bit of a sonic adjustment. The 710 was not an amp one would ever call warm and inviting—or cold and off-putting, for that matter. It just didn’t have a color of its own; like glass or water it had the color of what you saw through it or reflected by it, be that the source or the speakers. Some critics (and some on my listening panel) chose to view this colorlessness as the absence of tone color, and by tube or tube-hybrid amplifier standards the amp was a bit lean in the all-important midbass and power range (100Hz–400Hz), where so much of music’s drive, body, and natural warmth originate.

My bottom-line conclusion about the 710 was that if you were an “absolute sound” or “fidelity-to-source” listeners like me, it was a no-brainer must-listen. However, if you preferred an inherently warmer, richer, more gemütlich sound, then the 710 probably wouldn’t be your cup of resistors and capacitors.

This is where things stood with Soulution gear until the introduction, in 2012, of the company’s new 5 Series electronics, wherein everything changed—dramatically.