Sharp SM-A75 Audio/Video Amplifier Review

Sharp SM-A75
Sharp SM-A75

Sharp's SM-A75, part of the Optonica line of audio components, is a compact, versatile surround-sound amplifier and audio/video control center. Each of its four channels is rated for at least 25 watts into an 8-ohm load with no more than 0.8 percent total harmonic distortion when all four are driven simultaneously, as they would be in a typical surround-sound system. The SM-A75 can also be operated in a standard two-channel stereo mode, in which the power rating increases to 35 watts per channel. The specified frequency range at the rated power output and distortion level is 30 to 20,000 Hz for the front channels and 30 to 15,000 Hz for the rear ones.

The preamplifier section of the SM-A7 5 has inputs for a turntable, a CD player, a tape deck, a tuner, a TV set, two auxiliary sources, two VCR's and a videodisc player. There are also recording outputs for the audio and video tape decks. The front panel has two small tone-control knobs and a pair of pushbutton volume controls. Digital circuits vary the gain in 2-dB steps. A small dubbing mode button connects the two VCR's for recording in either direction or for videodisc dubbing to VCR 1. RCA phono jacks on the front panel duplicate the VCR 2 jacks on the rear of the amplifier, simplifying the temporary connection of a second video recorder to the system. There are also two stereo headphone jacks, one for the front channels and the other for the rear.

A key feature of the Optonica SM-A75 is its surround-sound processor. The front-channel signals can be combined in a matrix to produce left and right difference signals (L - R and R - L). Each of these signals can then be processed through its own 16-bit digital delay system, which is variable in 1-millisecond steps from 1 to 92 ms. An adjustable fraction of each delayed signal can also be recirculated through the delay system ("feedback") to provide a more spacious sound. After amplification, the delayed signals drive the rear (surround) loudspeakers.

Digital memories in the SM-A75 store the key parameters (delay times, amount of feedback, frequency response, and relative level of the rear channels) of eleven preset surround modes and up to three user-programmable modes. The factory presets include three modes for music programs, hall, live, and stage; two for movies, THEATER and SF (presumably meaning "science fiction," a genre in which unusual sound effects might be expected); three for sports, stadium, ringside, and game; two "basic" surround modes, matrix and presence; and standard Dolby Surround. The preset parameters are stored in a nonvolatile (permanent) memory bank. After any mode, preset or user-programmed, has been selected, its parameters can be varied, but the changes are not saved when the unit is switched off.

Obviously, such an extensive signal-modification system requires considerable user adjustment and selection, yet the front panel of the SM-A75 is almost free of controls. The supplied remote control serves as more than a mere convenience; it is actually the system's primary operating control, used for switching the amplifier on or off, selecting its input, and adjusting volume and speaker balance. It is also used to put the amplifier into its "pass" mode, which bypasses the delay system and silences the rear speakers, to select any one of the surround modes, or to vary a surround mode's delay times and amount of feedback. Other buttons on the remote control switch between mono and stereo, mute the audio, and provide a fixed bass boost called "Super Bass." Two video programs (main and sub) can be sent to different monitors and interchanged whenever desired.

Most of the front panel of the SM-A75 is devoted to a multifunction display, obviously a necessity in view of its operating complexity. At the far left of the display window, the name of the selected program source appears in large (half-inch) fluorescent letters, and beside it is a similar display of the selected surround-sound mode (or pass in the stereo mode). The middle portion of the display shows the left and right rear-channel delay times together with a number from 0 to 9 indicating the amount of feedback selected.

On the right are graphic indications of output levels, balance adjustments, and relative overall volume setting. A number of other indicators appear in the display window as required so that the complete operating status of the amplifier is shown at all times. The entire display flashes on and off if the speaker-protection circuit shuts down the amplifier.

The rear apron contains all the signal input and output phono jacks. A pair of front pre-out jacks can carry the front-channel signals to an external power amplifier, leaving the SM-A75 to drive only the rear channels. The speaker connectors accept stripped wire ends. A slide switch changes the sensitivity of the balance/output-level display, and an adjustment is provided for channel balancing in the Dolby Surround mode. One of the three AC convenience outlets is switched.

The Optonica SM-A75 measures 17 inches wide, 12-5/8 inches deep, and 3-3/4 inches high. It weighs 15-1/2 pounds. Price: $600.

Lab Tests

Our basic measurements, such as frequency response, power, and distortion, were made on the front channels only. In addition, we measured the rear channels' frequency response and phase shift relative to the corresponding front channels in each of the eleven factory-set surround modes.

The output clipped at 37.8 watts into 8 ohms and at 49 watts into 4 ohms (for which the amplifier is not rated). When we attempted to drive a 2-ohm load with a continuous signal, the protection circuit shut the amplifier down at a relatively low output. In dynamic power measurements, however, it delivered about the same power into 2 ohms and 4 ohms (63.9 watts).

The amplifier's sensitivity for a 1-watt reference output was 50 millivolts (mV) for a high-level input and 1.85 mV for the phono input. The phono input overloaded at 90 mV at 1.000 Hz, but the overload point fell to 23 mV at 20 Hz and 63 mV at 20.000 Hz. Because of the SM-A75's electronic volume-adjustment circuit, it is possible to overload the high-level inputs as well, but the CD input overloaded at a safe 5 volts.

The total harmonic distortion (THD) plus noise was about 0.05 to 0.06 percent over most of the audio range with outputs from 3.5 to 35 watts into 8 ohms. It rose slightly to 0.1 or 0.12 percent at the frequency extremes of 20 and 20,000 Hz. At 1.000 Hz into 8-ohm loads, the distortion decreased from 0.2 percent at 0.1 watt to 0.045 percent at 10 to 30 watts. With a 4-ohm load, the readings were slightly higher.

The tone-control characteristics were conventional, and the Super Bass circuit boosted the output below 1,000 Hz by a maximum of + 8.5 dB in the 40- to 100-Hz range. The basic frequency response of the front channels was +0, - 2 dB from 20 to 20,000 Hz. The RIAA phono-equalization error was +0.65, - 1 dB from 20 to 20,000 Hz.

Our phase and amplitude measurements on the left front and left rear channels revealed that only three different basic characteristics were used for the eleven preset surround modes. Two of the music modes, live and stage, as well as all three sports modes shared the same amplitude and phase responses. The rear channels were in opposite phase relative to the front channels, within a ± 45-degree variation, from 20 to 20,000 Hz. The amplitude response, referred to the 1,000 Hz level, increased to +6 dB at 100 and 15,000 Hz.

The third music mode, hall, had an essentially flat response, down 2 dB at 20 and 20,000 Hz, and its phase-variation curve remained within the same limits as the others. The two basic surround modes, matrix and presence, and the Dolby mode were essentially identical to the HALL mode in their amplitude and phase characteristics. A third characteristic was used for the two movie modes, theater and SF, in which the frequency response was rolled off above 5,000 Hz to -10 dB at about 15,000 Hz.

Since the three basic matrix characteristics were used with a variety of time-delay and feedback combinations, the eleven modes had distinctly different sound characters. The time delays ranged from a minimum of 1 millisecond for matrix to a maximum of 85 microseconds (μs) for stadium. The feedback settings ranged from 0 for the matrix, Dolby Surround, and theater modes to a maximum of 9 for SF.


For our use tests, we connected the Optonica SM-A75 to four speakers, a tuner, a CD player, a VCR, and a TV monitor. We played a number of Dolby-encoded videocassettes and CD's as well as listening to fm broadcasts. With a minimum of difficulty, we were able to use the SM-A75 effectively to decode Dolby Surround video programs and to enhance a variety of stereo music programs. We did not use every feature, but all those we did try out worked properly. Its effect in the Dolby mode was as dramatic as one could hope for, given the constraints imposed by a temporary four-channel speaker setup in a room that is far from an ideal audio/video environment.

Although we customarily use amplifiers rated from 100 to 350 watts per channel, the Optonica SM-A75 never ran out of power or made us feel that we were listening to a "low-powered" amplifier. Its four channels were easily capable of delivering dynamic peaks of over 200 watts, more than enough for most users. The only measured characteristic that was definitely substandard was the phono-input overload. Tolerable at 1,000 Hz and still marginally acceptable at 20,000 Hz, it measured only 23 mV at 20 Hz-a clear invitation to distortion when playing a record with any deep bass content.

Most of the surround-sound amplifiers we have used were larger, heavier, and harder to operate than the Optonica SM-A75. We frequently find that excessive complexity discourages full use of a product's capabilities. Sharp has achieved an ideal compromise, we feel, in this deceptively simple-looking unit. It is not the full equivalent in performance (or price!) of a system comprising a separate digital sound processor, a full-featured preamplifier, and a pair of stereo power amplifiers, but it should come close enough to satisfy many people. Considering its modest size and price, its ease of use, and the degree of listening enjoyment it can provide, the Optonica SM-A75 earns high marks.