|Marantz Model 9 mono power amplifier|
AT A GLANCE: The Marantz Model 9 is a single-channel power amplifier rated at 70 watts output, with built-in option of running it at 40 watts. Performance is outstanding. Price: $324.
IN DETAIL: To those who know of the performance and quality of previous Marantz amplifiers and preamplifiers, it is enough to say of the Model 9 that it represents "pure Marantz at its best." For those who may not know, as well as those who want to know why, here are some details.
To begin with, the Model 9 is built like a battleship and shows conservative operation of all components. Four EL-34 tubes are used for the output stage; these tubes are literally loafing and should have a very long life. The other three tubes, used in the early stages, run quite cool. Inspection shows the use of high-grade parts, such as telephone-quality electrolytic capacitors, low-noise deposited carbon resistors, and so on. Complete metering facilities are provided, with adjustments for each tube, including bias and balance of the AC signal to the output tubes. Evidence of attention to controlling phase shift is seen in the use of factory-sealed trimmer capacitors. The power supply uses silicon rectifiers which improve voltage regulation and reduce heat dissipation.
The power transformer and output transformer are huge, and are unique in that they run absolutely cool. After three hours of operation, much of it at full power output, the power transformer was faintly warm to the touch and the rest of the unit, including the chassis, was actually cold. The 70-watt rating of the Marantz Model 9 is actually quite conservative. Our tests show that even at 20 cps it will deliver 70 watts at 0.3% distortion. At higher frequencies the distortion is literally unmeasurable up to the point of overload. The IM distortion curve is very similar to the 20-cps harmonic distortion curve, with some 80 watts output at, well under 1% distortion. To show the power capability of this amplifier when the usual IHFM standards are applied, we measured its output at the 1% distortion point over the audio frequency range. There is no point in plotting it, but it delivers 80 watts at 20 cps, about 85 watts through most of the range, and well over 90 watts between 15 and 20 kc.
Another unique feature we found was that with a 3-mfd capacitor across the output, the power available at 10 kc was not reduced in the least. We have never before seen an amplifier which would deliver even half its rated power under this test condition.
The triode connection, which may be selected on a switch, reduces power about 40%. No other characteristics are affected.
Stability is absolute. Square wave response is virtually perfect. Frequency response is flat within one or two tenths of a db from 20 to 20,000 cps. A low cut filter is provided, switch-controlled from the panel. It cuts response below 20 cps, and has no effect above that point. Such a filter in an amplifier of this power can prevent blowing speakers with switching transients, or other subsonic disturbances.
Hum and noise are totally inaudible and hardly measurable, 91 db below 10 watts. Sensitivity is relatively low, needing about 0.4 volts for 10 watts output. This would suggest that the Model 9 be used with a preamplifier of suitable output and quality of signal, such as Marantz's own Model 7 control unit. Accordingly, a new model of this unit was used in listening tests with the Model 9. As far as we can determine, the Model 7 remains the same excellent device we tested some two years ago. At all listening levels, and with any low level or high level input, absolutely no hum, hiss, or other extraneous noise could be heard. In fact, hum and noise measurements made on the new Model 7 preamp produced results that, in most cases, were 2 or 3 db better than with the former unit. These, of course, could represent a normal variation among production units.
In sum, the Model 9 is an amplifier built to the highest of standards. With two needed for stereo, it is only for the most exacting and well-to-do audiophile. Little comment is needed on listening quality which is as splendid as its measured performance indicates. Our final impression is one of the extreme reliability and obvious potential for long, trouble-free life built into this amplifier.
H. H. LABS.