MANUFACTURER'S SPECIFICATIONS
Power Output (each channel, both channels driven, at rated distortion, 20 to 20,000 Hz: 4 ohms, 150W; 8 ohms, 125 W; 16 ohms, 64W. Total Harmonic Distortion (at or below rated power): less than 0.1 %. Intermodulation Distortion (at or below rated power) SMPTE, any combination of two frequencies, 20 to 20,000 Hz: less than 0.1%.
Frequency Response: +0,-1.5 dB, 2 Hz to 100 kHz; ±0.1 dB 20 to 20,000 Hz. Input Sensitivity: 1.5 V. for rated power. Input Impedance: 100k ohms. Damping Factor: Greater than 100 at 8 ohms. Total Noise: Better than 106 dB below rated power into 8 ohms. Power Requirements: 120 V. a.c., 50/60 Hz, 500 W maximum.
Dimensions: 15 3/4 in. W by 6 1/8 in. H by 9 1/2 in D.
Weight: 28 lbs.
Price: $495.00.
The first thing we can say about this husky amplifier is that it is a superb unit-as would be expected from this manufacturer and at this price. Not so powerful as to be a hazard with your pet loudspeakers, yet with more than enough power for any home application, and easily enough for commercial uses where the two channels can have their inputs paralleled and their outputs driving two sets of speakers, hopefully alternated around the listening area so as to provide adequate coverage even if one amplifier should fail, although there seems to be little chance. However, in commercial applications, this possibility must always be borne in mind, for equipment failure cannot be tolerated.
The amplifier is mounted behind a heavy gold-finished panel on which are two softly illuminated meters of the 4 1/2 in. variety. Below each is a solid metal knob which actuates the switch for its respective channel. No power switch and no volume controls, since this is a basic power amplifier intended to be plugged into the control amplifier and actuated by the switch on it. The rear panel is equipped with two phono jacks for inputs, a four-terminal barrier strip for outputs, a fuse, a convenience receptacle, and the line cord. The ends of the enclosure are completely taken up by enormous heat sinks, at the center of which are the four output transistors of each channel-two complementary pairs, each with V. ratings of more than 130 volts. This indicates a wide safety range, since the maximum static d.c. voltage applied is 58.
The amplifier circuit is a fairly complex arrangement, employing a total of 45 transistors, 19 diodes, and 8 Zener diodes in the complete unit. It is divided into several sections-the amplifier proper (including the eight output transistors), the rectifier/relay board, and the meter board. Referring to Fig. 1, the input is fed to Q519, an emitter follower with current source Q518. The output of the emitter follower is fed to one base of a differential amplifier, Q501 and Q502, and its output to the base of inverter Q503 which has Q504 as its current source. The inverter is coupled to Q507 and Q506, complementary pre-drivers, and their output is fed to Q510 and Q511 which serve as drivers to feed the complementary output transistors Q802/Q804, and Q803/Q805. The amplifier is supplied with both negative and positive voltages, each 58 volts, with the common point between the two output pairs at d.c. ground potential, thus eliminating the need for coupling capacitors to feed the speakers.
A protection circuit, consisting of transistors Q516, Q517, Q505, and Q506, precludes the possibility of damage to the output transistors if excessive inputs are applied. Transistors Q516 and Q517 sense the voltage across resistors R531 and R532 (each only 0.1 ohm) and when current reaches the design maximum, corrective signals are fed to the bases of Q505 and Q506 which disable the pre-drivers on excessive output current peaks, and thus limit the current through the output transistors to a safe value. Feedback voltage is fed from the output point-the junction between R531 and R532-back to the base of the second transistor of the differential pair (Q501, Q502), and to the input to the pre-drivers to place the entire amplifier under feedback control.
Fig. 1-Simpiified schematic of the Marantz 250 amplifier.
Fig. 2--Top view with the cover removed. Note ground strap between the two
20,000-µF filter capacitors. Ground connection to exact center of this strip
is important in reducing hum.
The rectifier-relay board consists of the main power supply rectifiers, together with a three-transistor circuit which provides the delay of some two seconds before actuating the relay which connects the amplifiers to the output terminals. In addition, this current samples the d.c. voltages on the two output lines, and when there is a difference between them of 4.5 volts, it acts to de-energize the relay, thus disconnecting the speakers from the amplifier. Similarly, if there is a high-level signal below 10 Hz present, the circuit also de-energizes the relay.
Eighty-six volts a.c. is fed to the power rectifier bridge, resulting in positive and negative supplies of 58 volts being provided for the amplifier. Filtering is effected by two 20,000 uF capacitors--yes, 20 thousand microfarads. That accounts partially for the very low noise level of the amplifier.
As practically everyone knows, the rectifiers of a VU meter connected across any audio circuit introduce a measureable amount of distortion, so the metering circuit board serves to isolate the meter rectifiers from the output circuits. Each channel employs two transistors, three Zener diodes, and six diodes. The transistors serve to provide a push-pull signal from the single-ended circuit of the speaker line, and this push-pull signal drives the bridge rectifier which furnishes the d.c. for the meter movement. Two of the Zeners are connected back to back to protect the meter against overloads, and two regulate the positive and negative voltages from the 58-volt supply to a more-usable 13 volts. Two of the diodes serve to prevent any excessive voltage spikes from reaching the meter amplifier circuit. Each meter is illuminated by two lamps, and a pleasing blue glow results from the filtering of the glass on the meter front. The switches have three positions--off, "0", and "+20," with the "0" representing an output of 0.75 watts, and the "+20" representing an output of 75 watts.
Performance
Fig. 3--THD and IM distortion vs. power output. Fig. 4--Square
waves at A, 20 Hz; B, 1000 Hz, and C, 12,000 Hz.
Fig. 5--Distortion vs. frequency at two power output levels.
This is an amplifier which needs very little in the way of P performance curves. The frequency response is flatter than water on a plate from the claimed 2 Hz to 70,000 Hz, drooping slightly-1 dB-at 100 kHz. Power bandwidth, not specified by the manufacturer, was measured at 4 to 65,000 Hz. Distortion--both harmonic and intermodulation-was well under 0.1 percent up to the rated output, and, in fact, IM began to rise at an output of 150 watts. At the 1-watt level, harmonic distortion was below the residual of our measuring equipment--0.04 percent. Frequency response could be drawn with a ruler on the usual 20-20,000 Hz curve paper. Distortion curve' shown in Fig. 3, and square-wave photos are shown Testing an amplifier of this quality strains the test equipment to its limits. For instance, with a signal-to-noise ratio of over 100 dB, it must be remembered that this represents a signal of about 0.3 mV measured across 8 ohms. An output of 125 watts across 8 ohms implies an rms voltage of 31.6, and 100 dB below that is the 0.316 mV. It doesn't take much of a ground loop to introduce that much of a signal in the measuring apparatus. We did measure less than 1 mV of noise, however, and that is better than -90 dB. This amplifier, therefore, could well be represented by the classical "equivalent circuit," which is usually that of an input voltage times the gain, with nothing else added.
Power measurements at 4 and 16 ohms confirmed the manufacturer's claims for those load impedances. Then testing with the standard load which consists of a resistor, an inductance of 20 µH series with a noninductive load resistor of--in this case--9.26 ohms which is paralleled with a 2µF capacitor, and again we found the square waves at 10 kHz at 10-watt and 100-watt outputs to be identical with those at 1000 Hz, indicating complete stability in the amplifier.
(The 9.26 ohms was arrived at by a simple way of making a noninductive resistor. We paralleled two center-tapped 75 ohm wire-wound resistors, then connected one lead to the paralleled center taps, and the other lead to both ends of the paralleled resistors. Connecting to the center tap and to the two ends of a wire-wound resistor results in practically no inductance-in our case, a measured 0.1 µH. It would have been simpler if we had a single 35-ohm wire-wound 50-watt resistor which we could center-tap, but that is not a standard value.)
Subjective Listening
In order to evaluate the Marantz 250 at close to full output, we again resorted to the expedient of placing a 300-watt, 8 ohm resistor in series with a 1-ohm resistor, across which we hung our speaker, doing the same for both channels. This resulted in approximately 1.25 watts fed to our speakers while the amplifier was putting out 125 watts-protecting the speakers yet permitting us to listen when the amplifier was working at its maximum. The sound was not much different than we were accustomed to from our usual amplifier, clean and smooth, even on peaks which actually reached the 125 watts. Then we connected the speakers direct to the amplifier as anyone would normally do, and although we could not have stayed in the room with full power, we were most pleasantly surprised at the overall quality of the sound. We are fortunate in not living in an apartment, so we were able to get up to levels in the vicinity of 110 dB SPL and the resulting sound was still excellent. If we were looking for a fine high-power unit, with top drawer performance, we would not have to look further than this one.
With the kind of performance the Marantz 250 offers and added to it the ease of converting to 220 volts if we ever moved out of the U.S.A., there is no doubt that this unit is sure to be acclaimed as one of the finest amplifiers ever to reach the market. (The conversion to 220 volts is a simple operation which shouldn't take more than ten minutes.)
-C. G. McProud
(Audio magazine, Jun. 1972)
Also see:
Marantz Model Thirty Console/Amplifier (Oct. 1970)
Marantz Model 250 Stereo Power Amplifier (ad, Aug. 1972)
Marantz Model 2500 Stereo FM/AM Receiver (Equip. Profile, Feb. 1978)
Marantz Model 22 AM/FM Receiver (Apr. 1970)
Marantz Model 1150D Stereo Control Amplifier (Mar. 1976)
Marantz PM-94 Integrated Amplifier (Dec. 1988)
Marantz Model 50308 Stereo Cassette Deck (Dec. 1978)
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