Soulution 501 Monoblock Amplifier: First Listen


Solid-state power amplifiers
Soulution 501 Monoblock Amplifier: First Listen

The $26,000 Soulution 520 full-function preamplifier and $55,000-per-pair 501 monoblock amplifiers are terrific. Sweet in timbre, incredibly fast, and sensationally detailed from top to bottom, they match the superlative Technical Brain EX and Constellation Performance gear in speed and resolution, although you might not guess this at first because (like their bigger, more expensive brothers, the Soulution 700 Series components) the 500s don’t have the laser-like focus of Technical Brain or Constellation. They are “bigger”-sounding than these others, and because they image slightly more diffusely (the difference is almost exactly like the difference between the imaging of a really good planar loudspeaker and a really good dynamic one) you may at first think they are going to be “politer,” more laid-back, and less detailed than the competition. I say “at first” because as soon as a hard transient comes along—like those startlingly realistic violin (and piano) pizzicatos in the second movement of George Crumb’s Four Nocturnes [Mainstream/Time] or the tremendous bass drum thwack at the start of Poulenc’s delightful Concerto for Two Pianos [Decca]—you’ll know without question that “polite” and wanting in immediacy and detail are the last things that Soulution’s new electronics are. You’ll also know without question—particularly from the Concerto for Two Pianos—that these 500 Series components have some of the most lifelike bass you’ve ever heard from mere stereo gear.

Generally I don’t get all worked up about the bottom octaves of speakers or electronics. Sure it’s swell to have “floor” and “jump” and “slam” and all those good things that you hear in an actual concert hall. The trouble is that it’s hard to get those good things in an average listening room without getting a whole bunch of bad ones along with them (like a huge midbass hump at room or port resonance, or a steep roll-off in the low bass, and/or a pronounced suckout in the power range). Getting deep, powerful midbass that also sounds “continuous” (to use Harry Pearson’s great, multipurpose word) with the octaves above and below it is a neat trick, and very few loudspeakers can bring it off without a few tricks of their own.

Take the Raidho C 4.1 that I’ve been blogging about here at This seven-driver, ribbon/ceramic-cone hybrid, D’Appolito floorstander uses four 160mm ceramic-sandwich woofers, mounted, as are all of the C 4.1’s drivers including the central ribbon, on thick anodized aluminum plates bolted to a tall, slender, tapered-in-the-back, dual-ported-in-the-front MDF enclosure. The load this large multiway presents to an amp isn’t particularly difficult (a nominal 5.8-ohm impedance with a sensitivity in the upper 80s); nonetheless, meeting the conflicting demands of such a hybrid critter isn’t all that easy for any amp to do. To come to fullest life the C 4.1 requires an amplifier that is capable of unusual power and grip in the bass and, at the same time, very low distortion, high speed, and exceptional delicacy in the upper midrange and treble (where that ribbon driver plays).

Generally speaking, most amps are going to be better at one task than at the other. For instance, in my last Raidho blog, I talked about the sound of the C 4.1’s bass as I’ve heard it via the Technical Brain and Constellation electronics. Both of these amps are speed demons, with lightning transient response, tremendous resolution of inner detail, and (given the right source) gorgeous (and highly realistic) midband-to-treble tone color and texture. However, as it turned out, with the C 4.1 neither of them was supplying the grip and impact in the bottom octaves that the Soulution 501/520 supplies.

While the Soulution 500 Series electronics didn’t exactly turn the C 4.1 into one of those “thump-in-the-chest machines” I mentioned in my last Raidho blog, they certainly increased its low-end “wow” factor substantially. With the Soulution 501/520, the midbass and upper bass (from about 45Hz to 125Hz) immediately sounded more prominent and powerful, as if the 501s had seemingly elevated those frequencies by a few dB (although it has not). Suddenly the C 4.1 had floor, jump, slam, all those good things. And yet the additional mid-to-upper bass energy wasn’t overshadowing the very low bass or cheating me of color, power, and heft in the 100Hz to 400Hz power range. Indeed, via the 501s the C 4.1’s output between 20 and 40Hz was measurably the same as its output at 1kHz, and its output in the power range just as flat.

RTA taken at the listening seat of Raidho C 4.1 driven by Soulution 501/520/540, 1/6th-octave smoothed.

Now the slightly greater prominence of mid-to-upper bass in the Raidho C 4.1 may constitute a bit of a departure from strict neutrality, but it certainly makes for a very concert-hall-like low end when the amplifier driving the Raidho is capable of supplying enough voltage at a fast enough slew rate and with enough of a damping factor to wring more information about pitch, color, intensity, and duration out of the bottom octaves than I thought the speaker was capable of. The Soulution 501 monoblock does this very thing—spectacularly well. (BTW, by printing the RTA above I don't want to leave the impression that the 501/520 is making a difference in midbass and midrange frequency response; this is simply the bass profile of the Raidho C 4.1, and RTAs taken with the TB and Constellation gear look identical. It is the difference the Soulution makes in the sound of that bass that is so impressive.)

The word “solid” best describes the Soulution low end—solid not just in a dimensional sense (although it is that), but in a continuous, “of-a-single-piece” sense. Like a chunk of black marble, the mid-to-upper bass seems to have been expertly chiseled to make a perfectly seamless fit with the power range above it and the bottom octaves below. Getting this kind of weight and sheer exhilarating impact along with superb pitch definition and astonishing resolution of timbre and texture in the bottom-most octaves and the power range isn’t usually in the cards outside of a concert hall, where you hear it all the time on orchestral tuttis or instrumental fortissimos.

One of the secrets to both the 500 Series’ and the 700 Series’ sonic success is standard-settingly lower distortion, achieved by slightly unusual means. Where every other tube and solid-state amp designer I can think of flees from a large amount of negative feedback like the plague, Soulution has rushed to embrace it. It is Soulution’s contention that negative feedback in itself has gotten a bad rap; it isn’t feedback but the time it takes to send the “corrected” signal from the output to the input (technically, propagation delay) that causes the problems. Unless that propagation delay is zero or close to zero, applying feedback will add time-domain errors to the musical signal, since as time goes by (and music doesn’t sit still for a portrait shot) the feedback loop will be “comparing apples at the output to oranges at the input” (in the words of Cyrill Hammer, Soulution’s CEO).

In the 700 Series, Soulution’s, uh, solution to the propagation delay problem was to increase the amp’s bandwidth to the megahertz region and, thereby, increase the speed with which the feedback loop does its thing by a factor of 1000. Most amplifiers and preamplifiers work with a propagation delay of 1–5 microseconds; Soulution’s amplifiers and preamplifiers work with an overall propagation delay of 5–10 nanoseconds, reduced to 1–2 nanoseconds in the voltage amplification stage—where most of the negative feedback is applied. To quote Hammer again: “Since the timing errors of the Soulution amplifiers are negligible, we have the opportunity to apply as much negative feedback as we need wherever it is required in the amplifier without reducing sonic performance. This is how we can lower distortion to never-before-seen levels.”

Lower distortion certainly helps explain the Soulution 501/520’s terrific top-to-bottom clarity and resolution. But the phenomenal distortion numbers alone (THD is less than 0.001% and SNR greater than 120dB) can’t explain why Soulution’s new 500 Series electronics sound so world-beatingly “real” (and exciting) in the bottom octaves. You might think that its power delivery was a matter of sheer watts, but, while more than capable of driving the C 4.1s to louder-than-life levels, the 501s are only rated at 125W into 8 ohms, 250W into 4 ohms, and 500W into 2 ohms. Voltage, amperage, and peak power, however, are different stories.

At a glance, you would never think it—the 501s are actually quite compact, about a quarter the size of the humongous 700s and nothing to write home about in the looks department—but these little Soulutions put out 70V RMS, 45A, and 5000W of impulse power at a slew rate of 900ns with a damping factor that exceeds 10,000!

The sheer, seemingly inexhaustible flow of power from these small, rather utilitarian things (fed by the every-bit-as-outstanding 520 preamplifier—about which I will blog separately) may have something to do with “improvements” that Soulution has made to the power supplies. Unlike the 700 Series amplifiers, the 500 Series uses “switch-mode” supplies—two of them, electrically isolated from each other (and from the audio circuit) by opto-couplers and transformers, “high-performance-filtered” for noise at the inputs and outputs, and high-speed voltage-regulated. Each of these switch-mode supplies is capable of delivering 600VA, and Soulution claims that, together, they “deliver considerably more stable power than any conventional, transformer-based technology.” (To be clear, the Soulution 501 is not a Class D amp. Though it uses a switch-mode power supply, its gain stages run in Class AB, heavily biased toward Class A. In addition to the switch-mode supplies, the 501 also uses four linear power supplies for other functions.) 

The upside of switch-mode supplies as I understand it (make that “as Robert explained it to me”) is that they keep the power supply constantly and fully charged no matter what the signal-demands; they can also be power-factor-corrected (so that wattage and voltage are not slightly out of phase, as they are in conventional supplies). I know there are switch-mode naysayers, who point out that, even if filtered and shielded, the strong noise (chiefly RF) of the digital switching signal can be radiated throughout the circuit. All I can tell you is that at this early stage of listening I’m too wowed by the 501/520’s bass response to hear this issue (if it raises its ugly little head I’ll let you know). What I am hearing is that when an amp has 47,000µF of power capacitance, as the 501 does, with no droop or phase shift in the supply at any level with any signal, the net effect seems to be equivalent to plugging your speakers directly into a wall socket.

There is no question in my mind that it is the combination of exceedingly low distortion, astounding speed, almost limitless voltage, amperage, and peak power, and tremendous damping that allows the Soulution 501s to bring the Raidho C 4.1s to such incredible life in the bottom octaves where grip, damping, transient response, and unlimited power are all-important.

But what about the treble octaves? What about the delicacy, speed, and sweetness they require?

I will answer these questions after further listening in a second Soulution blog that will also cover the marvelous 520 preamplifier, which has built into it a truly great phonostage, making it one of the very few “full-function” preamplifiers still on the market (and, considering the quality of that phonostage, one very very very good deal as ultra-high-end products go). But rest assured that, unless things change dramatically over the next few weeks, the 501/520 appears to be able to do mid-to-treble-range delicacy and sweetness with the best of them.

Though they aren’t the only goodies in Santa’s bag, in the sector of the market in which they compete the new Soulution 500 Series (which also includes the 540 CD/SACD/DAC) are unquestionably must-audition products, but then if you’ve heard them in Munich driving Focal Stella Utopias to previously undreamed-of heights you already know that.