Sharp SM-SX100 digital integrated amplifier Page 2

The Skin
The amp's dramatically Deco-ish looks are self-evident, but a photo can't convey the cool, sleek, solid feel of this compact 40-lb product. The rear panel tells you Sharp is serious: there are two pairs of ultra-high-quality WBT speaker binding posts, five digital inputs including ST link glass-optical and TosLink optical, BNC (true 75 ohm) and Neutrik RCA coax connectors, and an SACD bitstream input jack. Analog ins include two pairs of Neutrik RCA jacks (Neutrik female jacks have the same neat, spring-loaded "safety" ground rings as the male plugs) and a pair of XLR analog ins. There's a TosLink digital out and a pair of Neutrik analog outs—and, of course, an IEC AC jack.

The front panel is 7/8"-thick aluminum, with a V-shaped, green-tinted acrylic window wedged in the center above the amplifier's one and only knob: a large volume control. There's a pushbutton On/Off switch and an array of seven blue LED input buttons. Switch the amp on and it defaults to the SACD DSD bitstream input. Blue LEDs indicate the selected source and a blue numeric display in the plastic window indicate volume settings from "0" to "128."

The angled side panels, finished in an ebony composite, form integral pointed feet. (The SM-SX100 is also made in Cocktail Blue and Peacock Green, but Sharp America imports only the black.) Near the center rear of the chassis bottom is a more familiar-looking conical point. Sharp provides spacer cups on which to set the amplifier.

The Guts
The SM-SX100's internal construction includes separate power-supply and 1-bit sections to minimize noise interference, hand-picked parts including gold-plated connectors and metal-film resistors, a gold-plated wire-brush volume pot, a copper-plated chassis, and gold-plated, oxygen-free-copper PCB traces.

I was honored to have Mr. Kiyoshi Masuda, the SM-SX100's chief of design, pay a visit to explain how it all works. Unfortunately, Mr. Masuda's technical brilliance far outweighed his command of English, and my mathlexia kept me from understanding and digesting the voluminous technical and conceptual information he threw at me, most of which dealt with the principles of delta-sigma modulation (1-bit digital encoding) and the SM-SX100's operation.

Fortunately, through a series of e-mails that included diagrams, I later (much later) came to understand how delta-sigma modulation works and how it is used in digital-domain amplification in the SM-SX100.

The heart of the Sharp amplifier is a delta-sigma analog/digital converter running at a very high frequency, 2.8224MHz or 64Fs, where Fs is 44.1kHz. It is not coincidence that this sample rate is the same as that used in the DSD encoding used in Sony and Philips' Super Audio CD system. John Atkinson covered the theory behind delta-sigma conversion in his sidebar on DSD in the November 1999 issue (pp.96-97).

In very simple terms, rather than use a conventional multibit PCM quantizer to describe the level of the analog signal at each clock instant, a 1-bit quantizer merely outputs a "1" if the signal is positive-going, a "0" if negative-going. The raw output of this quantizer is an ultra-high-frequency stream of codes corresponding to the amplitude of the signal, but with very high quantizing noise and very low resolution. A high-order noiseshaping feedback loop around the quantizer—Sharp uses seventh-order noiseshaping—is therefore used to increase the audioband resolution and push the vast majority of the quantizing noise above the audioband and up to the RF region. All of this circuitry is built into a single chip in the SM-SX100.

The DSD input goes directly to the seventh-order delta-sigma modulator chip, as do the analog signals. Multibit PCM signals from CD, DAT, or DSS satellite dishes (44.1, 48, or 32kHz) are sample-rate-converted into 1-bit datastreams by a Philips chip. According to the service manual, before being routed to the chip, analog and digital signals pass through separate sections of a double-ganged high-quality volume-control potentiometer that adjusts the levels of both stereo analog and digital bitstream voltages.

The bitstream output of the delta-sigma chip directly drives the SM-SX100's output stage, called by Sharp a "1-Bit Amplifier." Apparently a pulse width modulation (PWM) stage, this consists of four power-MOSFET switches controlled by a crystal-controlled driver. These MOSFETs switch between the ±32V power-supply rails, with the timing controlled by the input data. The more 1s in the input data, the longer the switches stay open at the positive voltage; the more 0s, the longer they stay open at the negative voltage. The service manual is coy about the actual switching frequency used by the output stage; according to the specification, it appears to be the same as the delta-sigma chip's 2.822MHz.