Taken from 'High Fidelity Magazine', May 1965

THE EQUIPMENT:
Sherwood S-9000, a solid-state stereo combination preamplifier/power amplifier. Dimensions: WA by 12'/2 by 4 inches. Price: $299.50. Optional cabinet, $8.50. Manufacturer: Sherwood Electronics Laboratories, Inc., 4300 North California Ave., Chicago, 111. 60618.

COMMENT:
A recent entry into transistorized audio is the new S-9000 by Sherwood - a gleaming, compact integrated amplifier in the medium-to-high power class. Its neat, logical styling and its fine performance both are reminiscent of former Sherwood equipment. The S-9000 comes in a metal case which, with the legs supplied, may be placed on a shelf "as is," or further dressed up in the wood-grained leatherette case available. If a panel cut-out installation in custom cabinetry is used, the amplifier may be mounted in any position from horizontal to completely vertical (with the face plate up).

The operating controls are laid out in a straight line across the centre of the front panel and include, from left to right: a four-position program selector switch (tape head, phono, tuner, auxiliary); a five-position stereo/mono selector switch (reverse, normal, channel I, channel 2, mix); a bass control (for left and right channels simultaneously); a similar type treble control; a loudness (volume) control combined with the ON/OFF switch; and a stereo-balance control. Below these controls are a tiny phono-gain control (tape head and phono input levels are adjustable) and six slide switches used for: tape monitor, high frequency (scratch) filter, low frequency (rumble) filter, loudness contour, phase reverse, speakers ON/OFF. With the loudness switch at the IN position the volume control functions as a loudness contour control and provides effective bass boost at lower volume levels. Also included on the front panel is a low-impedance stereo headphone jack. Headphones may be used simultaneously with speakers, if desired.

At the rear of the S-9000 are five pairs of stereo inputs: three pairs are for high level sources (tuner, auxiliary and tape monitor) and two are low level inputs (phono and tape head). For feeding signals from the S-9000 to a tape recorder there is a pair of signal output jacks. Also at the rear are two accessory AC outlets, one of which is switched; a grounding terminal; a fuse-holder; and the AC power cord. The pair of left and right speaker terminals will accommodate speakers having an impedance between 4 and 16 ohms.

The circuitry of the S-9000 contains eleven silicon transistors in each channel and four silicon rectifiers in the power supply. Two transistors are wired as push-pull outputs and are fed by a driver transformer. One power output transistor in each channel has strapped to its body a heat-sensitive circuit breaker that will open the AC line voltage to the power transformer in the event that the speakers are accidentally shorted. In addition to this protection, Sherwood cautions the user against short-circuiting the speakers. According to Sherwood the circuit breakers will open when the temperature of the output transistors reaches 210° F, and will close again when the temperature drops to 170° F.

In tests conducted at United States Testing Company, Inc., the S-9000 met its important specifications with ease, providing high power with very low distortion across the audio band. With both channels operating together (the most rigorous test), it shaped up as a very clean 40-watts-per-channel amplifier, with ample reserves for driving the lowest efficiency speakers.

The frequency response of the amplifier was flat to within 1 db out to 35 kc, and down only 3 db at about 9 cps and 70 kc (with the volume control at the 12 o'clock position). While observing the 10-kc square-wave response of the amplifier on an oscilloscope, USTC testers adjusted the volume control until the square wave exhibited optimum high frequency response. At this position (10 o'clock), the response - already excellent - became even more outstanding, and was down only 0.8 db at 100 kc. These measurements are further reflected in the two 10-kc square-wave response photos corresponding to these two volume control settings. Actually, either one is very good, and indeed above average for a combination amplifier. The 50-cps square-wave photo shows some phase distortion, which is typical of most integrated amplifiers.

Damping factor of the S-9000 was a very favourable 40. Its equalization was quite good for the RIAA (disc) playback position. The NAB equalization characteristic showed some loss below about 40 cps - not very important inasmuch as most tape playback is done through a tape deck's own preamp. Both the scratch and rumble filter circuits were well designed, resulting in good filtering action. Sensitivity at all inputs was suited to today's program sources and the amplifier's signal-to-noise ratio was good - as was its stability.

The IM characteristics of the S-9000 were excellent, and – incidentally - different from those of other transistor amplifiers recently encountered. In most transistor amplifiers, the IM distortion has measured higher at low power levels than at high power levels. In the S-9000, the IM is very low at low power output, rising as the amplifier approaches its maximum power output - which is the pattern, of course, of tube-type amplifiers. In any case, IM distortion - with the amplifier driving 4-, 8-, or 16-ohm loads - was very low.

Well-built, neatly styled, and very easy to listen to, the Sherwood S-9000 is a topflight integrated amplifier, offering in a compact format the kind of performance formerly associated with bulkier separate preamp and power amplifier equipment.