Krell Full Power Balanced 350mc monoblock amplifier
By Jonathan Scull   •   August, 2000 With its latest series of FPB (Full Power Balanced) amplifiers, Krell is taking careful aim at the seam between classic high-power two-channel systems and quality multi-channel installations where sound is yet paramount. Nevertheless, Krell founder Dan D'Agostino was adamant: Krell's Class A components were designed for music playback. "I'm a purist, like you, Jonathan!" he told me.

The sleek, good-looking 350Mc monoblock sits in the middle of the FPB series. Specified power is 350Wpc into 8 ohms, 700 big ones into 4, and a whopping 1400W into punishing 2 ohm loads. 250W and 650W monoblocks are also available, while two-channel models with slightly different cosmetics offer 200WPc, 300Wpc and 600Wpc.

The monoblocks are available in symmetrical pairs, with left- or right-side heatsinks, and also as Theater Pack Three and Five combos. A serious two-zone preamplifier—the KCT—and a CD player—the KPS28c—join the upgraded top-of-the-line KPS25sc CD/preamp to round out Krell's Class A Series.

While both a KCT and KPS28c were on hand during the review period so that I could experiment with Krell's CAST (Current Audio Signal Transmission) system, I evaluated the 350Mc as a classic pair of two-channel audiophile amplifiers.

Core Technologies
There are a number of technological pillars on which Krell base their Class A designs. First, fully balanced circuit topology with positive and negative signal paths symmetrically referenced to ground, which results in greater noise rejection and less "cable interference."

"The primary advantage of input-to-output balanced operation is control." says D'Agostino. "Independent symmetrical circuit paths individually amplify the positive and negative signals, locking the speaker into a push-pull relationship. FPB series amplifiers exercise absolute control over the most unruly speakers, even those with exotic impedance curves and disruptive back-electromotive forces (EMF). These amplifiers overcome the speakers natural resistance to motion and forces them to perform the precise required movements." Very Krell.

Tech pillar two: Regulation is good, and the more of it, the better. According to their TechSynopsis: "With Krell's Active Regulation, any change of supply voltage is instantly corrected, whether due to variations in the AC line voltage or to changes in output stage current delivery." The paperwork goes on: "The power supply relies on large banks of capacitors to support it during demanding dynamic passages. Unfortunately, the capacitor's passive nature dictates that there will always be a time lag between the precise moment the power supply needs current and when it's supplied. The result is the output rail voltages cycle back and forth from linear to non-linear states, depending on the demands of the signal and the load."

The output stage in all FPB amps is fully and actively regulated through a "free-floating feedback arrangement" where "ultra-fast regulators" operate independently of the output stage while monitoring its condition and responding to even the smallest drop in current or voltage with "essentially instantaneous regulator response."