|Marantz Model 8B stereo power amplifier|
AT A GLANCE: The Model 8B by Marantz is a top quality dual-channel basic amplifier. In tests conducted at United States Testing Company, Inc., it easily met or exceeded its specifications, was capable of delivering clean power well above its rated output, and had amazingly low distortion. In addition to being an excellent performer, the Model 8B is superbly built and is designed for long, trouble-free service. Dimensions are: 13½ inches wide, 7¼ inches high, 10½ nches deep. Price is $264; optional grille covering, $9.00. Manufacturer: Marantz Company, 25-14 Broadway, Long Island City 6, N. Y.
IN DETAIL: The Marantz Model 8B amplifier is, in all respects, a clean performer, conservatively rated and magnificently constructed. Most of its fifty pounds of weight is in its heavy-duty transformers which are shielded in their own housing atop the chassis. Indeed, the clean and simple lines of the Model 8B strike a note of quality when the unit is first seen after being un- packed, and this impression is borne out by subsequent examination and tests. Craftsmanship also is evident on the underside of the chassis where the wiring and layout of parts are among the neatest yet encountered.
The circuitry of the Model 8B, in each channel, contains a 6BH6 pentode voltage amplifier, a 6CG7 dual-triode driver, and a pair of EL34 pentodes as the push-pull output stage. Feedback is used from the secondary of the output transformer back to the cathode of the 6BH6 amplifier. The 8B uses semiconductor rectifiers for its high voltage supply and for AC filament voltages. An auxiliary octal socket is provided for powering a preamp and supplies a B+ voltage of 435 volts (open circuit), 6.3 volts AC, and 12.6 volts AC. There is no AC power switch on the Model 8B which therefore will be "on" whenever it is connected to a hot AC outlet. However, the unit would logically be controlled from an external preamplifier when in use. All connections, including the 4-, 8-, and 16-ohm speaker terminals, are on the rear apron of the chassis.
The Model 8B has two signal input jacks per channel,
one set for connecting to a preamplifier and the other for testing purposes.
The "test" input provides flat response below 20 cps, while
the "preamp" input introduces a low frequency filter that attenuates
frequencies below 20 cps. In the manufacturer's view, this filter is desirable
to restrict subsonic signals that will not contribute to the audible range,
and that—if reproduced—could be wasteful of the amplifier's
power output as well as introduce distortion. This last factor, Marantz
explains, could result from the reproduction of low frequency pulses that
tend to force a speaker cone to make a very wide excursion, known as "speaker
breathing." These pulses, while contributing nothing desirable to
the reproduced sound, could be superimposed on higher frequencies normally
reproduced and thus create IM distortion. The 20-cps filter, at the input
to the amplifier, is intended to minimize such effects.
The power bandwidth for rated distortion of 0.5% in the 20-cps to 20-kc range was measured and was found to extend well beyond that range. The amplifier's power bandwidth was, therefore, measured on the basis of clipping (or visible distortion on an oscilloscope) and was found to extend from 10 cps to 75 kc. At the 35- watt level per channel, harmonic distortion was extremely low, remaining at about 0.1% across most of the range and rising to only 0.29% at 20 cps. At half- power (17.5 watts), distortion was less, being under 0.1% over most of the range and coming up to 0.25% at 20 cps, and to 0.13% at 20 kc. Intermodulation distortion also was found to be very low over the entire usable power range of the amplifier. At 20 watts, the IM distortion was only 0.2%, while at 40 watts it still remained under 0.5%. These all are excellent figures and attest to the fine performance of the Marantz.
Measured through the "test" input, the amplifier's frequency response was flat within ±0.2dB from 5 cps to 20 kc, and was down to -1dB at 36 kc, and to -3dB at 65 kc. When measured through the "preamp" input, the response was down 1dB at 20 cps, and 3dB at 10 cps, reflecting the attenuation introduced by the low-frequency filter described earlier. Nonetheless, the response curve still is virtually a straight line over the audio range.
Square-wave response at 50 cps, taken through the "test" input, was excellent. Taken through the "preamp" input, the 50-cps square wave showed the high amount of phase distortion expected as a result of the low-frequency filtering of subsonic components. According to Marantz, this phase distortion is of little consequence. Those who disagree with this approach can, of course, connect a preamp into the "test" input on the 8B and thus run it "wide open" and flat down into the 5-cps region. Although Marantz does not feel that this extreme low-end response contributes anything worthwhile to the reproduced sound, the option of using it is provided.
The 10-kc transient response, as evidenced by the square-wave pattern, was very clean, showing no signs of ringing. Rise-time of this wave was 8 microseconds. The amplifier's damping factor was a relatively high 16, and its stability with reactive loads (such as electrostatic speakers) was very good. Input sensitivity for full output was found to be 1.3 volts. Signal-to-noise ratio, with respect to rated output of 35 watts, was 101.5 db, which is fantastically good.
In USTC's view, the Marantz 8B is one of the best power amplifiers available today. With its extremely low distortion, clean transient response, stability, freedom from hum and noise, and clean power reserves, it should provide top performance in any reproducing system. Too, judging from its appearance and construction as well as from past experience with other Marantz products, the Model 8B should require little or no servicing over the longest possible time.