mbl Reference 9007 power amplifier
By Michael Fremer   •   September, 2006 Best known for its omnidirectional loudspeakers, the German manufacturer mbl also produces three complete lines of high-performance electronics that, despite being large and built to jewel-like perfection, are sometimes lost in the shadow cast by the dramatic-looking—and -sounding—Radialstrahler 101E, which I reviewed in October 2004.

While mbl's least expensive line, the Basic models, is limited to one each of a preamp, stereo amp, integrated amp, CD player, tuner, and loudspeaker, the Noble and Reference lines are crowded with products from which numerous complete systems (minus a turntable) can be assembled.

The top line, the Reference series, includes three fully balanced monoblock amplifiers capable of being configured as unbalanced for stereo use. The newest of the three is the Reference 9007 ($13,300 each), claimed to deliver 440W RMS into 8 ohms, 570W into 4 ohms, or 800W into 2 ohms. Stereo operation cuts the power outputs significantly, to 130Wpc, 200Wpc, and 290Wpc, respectively.

What prompted this review was my auditioning of McIntosh Laboratory's C1000 balanced preamp (the review was published in the August issue). While the three-box C1000 can run single-ended, I felt it deserved a hearing in balanced mode. In Las Vegas during CES, I was asked by mbl if I had any interest in reviewing their new Reference 9007 amp, also balanced. I took them up on the offer.

Designed from scratch
Founded in 1979 by engineer Wolfgang Meletsky to produce the original Radialstrahler loudspeaker, Berlin-based mbl has since grown into a fully integrated company. Everyone at mbl with whom I spoke emphasized that the company's electronics, inside and out, are designed and built in-house, as are the 9007's gleaming curved faceplate (in black/gold or Arctic Silver) and beefy, solid-copper speaker terminals.

In early June, when I sat down with Meletsky at Home Entertainment 2006 to get a rundown on the Reference 9007's design, he emphasized that the DC servo-coupled amp is built using all discrete components—there are no integrated circuits or microprocessors. Low-tolerance, low-induction resistors and megahertz-bandwidth capacitors are used throughout, as are multilayer FR4 glass-epoxy circuit boards featuring ultrashort signal paths, heavy copper planes in the output section, and galvanic gilding to prevent oxidation. Other premium parts include "superfast" ring-emitter (bipolar) transistors and custom-built toroidal transformers for the driver and power stages. Energy storage is via a bank of custom-built, 132,000µF capacitors from Sweden, while separate power supplies feed the driver and output stages. The power transformer is mounted on an epoxy-encased aluminum shield.

The circuitry includes mbl's exclusive Direct Push/Pull (DPP) design, which is claimed to minimize delay by shunting part of the signal directly to the output instead of routing it through various stages along the way; and mbl's Isolated Gain Cell (IGC) technology, which requires less current and lets the amp run cool while offering class-A–like sound, though it actually runs in class-A/B.

Wolfgang Meletsky described to me in great detail the care that goes into choosing and loading the power capacitors used in the 9007; he said they generate only 1% of the distortion found in "normal" caps. The power supply also uses "fast" rectifiers to match the charging current to the capacitor's capabilities. According to Meletsky, if the capacitor is charged too quickly, there's ringing on the supply rails; too slow, and there's ripple. The charge and discharge times must be the same, he told me, also to prevent ringing.

He also explained why the 9007's full sonic potential is available only in balanced monophonic mode, but his explanation couldn't jump the language barrier and the gaps in my technical knowledge. One thing that was clear is that the 9007's frequency response is claimed to extend to 320kHz in balanced mono mode but to only 200kHz in single-ended stereo mode.

The gleaming exterior's fine, watch-like detail and fit'n'finish are equally impressive, and despite the 9007's relative compactness, it weighs 75 lbs. Whether or not you appreciate the oversized, mbl logo of shiny gold on the front is a matter of taste. I think a bit more understatement is called for.

$26,600 is a lot to pay for a pair of 440W monophonic amplifiers—some might even think it an exorbitant amount—but in terms of design elegance, circuit topology, parts and build quality, and its lustrous, satin-smooth finish, it's clear that no expense was spared in designing and building the Reference 9007.

Setup and use
Because the Reference 9007 can be used as either a balanced monoblock or a single-ended stereo amplifier and has provisions for biwiring and biamping, its rear panel has six output terminals and a potentially confusing cable-routing schematic etched onto it. The written instructions tend to increase rather than reduce the confusion. Also on the rear is a balanced XLR connector, a stereo pair of RCA jacks, an IEC jack, and an illuminated power switch.

Speaking of confusion, below the large mbl logo on the front is a function-indicator light flanked by two switches, Sleep and Standby/On. I never did fully comprehend the difference. The instructions tell you to first press Sleep to put the amp in "warmup mode." Then you're told to push Standby to turn the amplifier on. Push again and the amp is in Standby mode, and ready to operate when you push Standby again.

A heading in the manual labeled "Sleep Mode" reads: "When the amp is in 'sleep mode' only the sleep function is operating." Say what? To clarify, below that is a three-column grid labeled "Sleep Switch," "Standby Switch," and "Mode." When the Sleep and Standby switches are both off, you're in Sleep mode. When Sleep is on and Standby is off, you're in Standby mode. When Sleep is off and Standby is on, you're also in Sleep mode. When Sleep is on and Standby is on, you're in Operating mode.

Understand all that? I didn't—not for the entire time I had the 9007s for review. I just left them on all the time and paid the electric bill.

The Reference 9007 grips the road
For the McIntosh C1000 review, I ran the preamp balanced into the mbls and single-ended into my reference Musical Fidelity kW monoblocks—balanced because that's how most owners of the C1000 will probably use it, and single-ended for the benefit of unbalanced owners—er, I mean, owners of unbalanced gear.

Simultaneously inserting two new components into an audio system makes reviewing either tricky, but I'm confident I understood the personalities of the McIntosh C1000 and mbl Reference 9007, as well as the contributions each made to the exceptionally fine sound they together produced during the two months I had them in my system.