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.
