Chord SPM 650 power amplifier
The key to producing the SPM 650's output power130Wpc into 8 ohms, 170Wpc into 4 ohms, and 200Wpc into 2 ohmsis Chord's replacement of the usual heavy, bulky power transformers and energy-storing capacitor banks with a compact, lightweight, high-frequency, switched-mode power supply based on technology developed for aerospace telecommunications and data processing.
While some of us spent the 1980s listening to hair bands, John Franks, Chord's founder and chief designer, spent that decade adapting and attempting to perfect high-speed switching technology for use in high-quality power amps. His labors culminated, in 1989, in the launch of Chord's SPM 900 stereo amplifier.
Chord's present power supply is a self-contained, self-monitoring device fed by a mains input filter claimed to reject incoming noise as well as noise generated by the amplifier. The filtered AC is directly rectified to generate high-voltage (300350V) DC, which is stored in a bank of high-voltage capacitors. This DC voltage is then "chopped" via high-voltage MOSFETs running at 80kHz, and fed to a "very special" ceramic-core, high-frequency transformer wound with individual multistrand Litz wire designed, according to Chord, to avoid high-frequency skin effect losses. The size a power transformer needs to be shrinks as the AC frequency increaseshence Chord's small, lightweight transformers, compared with the big, bulky ones found in conventional power supplies that run at 50 or 60Hz.
The small transformer's output feeds ultrafast rectifiers, which are used to reconvert the signal to DC and smoothed by a small coil and filter caps. This voltage can be kept constant or varied as needed by controlling the timing of the rectifiersall without the need for "inefficient" voltage regulators. All of this processing takes place at frequencies well above human hearing, which, Chord claims, "should be good for audio circuitry." Chord also claims that "enormous reserves of power can be instantly reproduced" by such a power supply design.
The SPM 650's output stage uses MOSFETs designed exclusively for Chord that are biased to operate in class-A during normal use, changing class-B under more demanding situations. The result, according to Chord, is an amplifier that "sounds simply effortless."
Chord claims ultra-high reliability for the SPM 650, thanks in no small part to its sophisticated output protection circuit, which monitors the magnetic flux differential between the power-supply rails and uses an algorithm to determine whether power should be supplied or withheld. If the circuit sees a short circuit or other anomalysuch as serious static discharge from an LP, as I found outthe amp switches to standby mode.
Setup and Use
Chord advises against using power conditioners with its power amplifiers, so I didn't. They also recommend breaking in the SPM 650 with 1520 hours of continuous play, after which it can be switched off when not in use, with a subsequent warmup of about 10 minutes before each listening session. Before doing any serious listening, I gave the review sample far more break-in than that: Just before going away for a few days, I set my Sooloos music server to Swim mode and let it rip.
While most Chord products exude blingoften too much, in my opinionthe SPM 650 has an understated, uncharacteristically modest front panel with but a single Power LED, which glows red in standby mode, green when active. Other than the nameplate, the only distinguishing design elements that let you know you're looking at a Chord product are the pairs of aluminum tubes that run along the side panels from front to back. The SPM 650 sits on four metal columns that end in small rubber feet, the columns affixed to a rather flimsy bottom plate that will surely rattle in the presence of speaker output if left undamped. I first listened without any kind of tweaking other than placing the SPM 650 on a Symposium Acoustics platform.
The rear panel is uncomfortably crowdedthe unbalanced RCA jacks are less than half an inch to either side of the IEC AC jack, and the gold-plated, five-way speaker binding posts are placed at such an angle that access to the ground post is partially blocked by the vestigial cooling fins protruding from the top of the jack panel. The result is insufficient clearance, should you choose to use typical audiophile unbalanced interconnects fitted with oversized RCA plugs and typical audiophile-style AC cords. I used TARA Labs Zero interconnects, which forced me to use whatever skinny-jacked AC cord I could findand even then, it was a tight fit.