Manufacturer's Specifications
Power Output: 250 watts per channel into 8-ohm loads, 20 Hz to 20 kHz; 375 watts per channel into 4-ohm loads.
Rated THD: 0.025 percent.
Rated SMPTE IMD: 0.025 percent.
Clipping Headroom: 0.5 dB.
Damping Factor: 60.
Frequency Response: 20 Hz to 20 kHz, ±0.25 dB. S/N: 100 dB, A weighted, re: 1-watt output.
Input Sensitivity: 2.24 V re: rated output, 0. 14 V re: 1-watt output.
Slew Factor: 3.
Current Slew: 20 amps/µS.
Dimensions: 19 in. (48.3 cm) W x 7 in. (17.8 cm) H x 12 1/2 in. (31.7 cm) D.
Weight: 52 lbs. (23.4 kg).
Price: $1050.00.
In introducing their "01" series of amplifiers (of which this one is the second highest powered), SAE published a widely distributed "white paper" which restated several basics some of us have realized only too well for a long time. Amplifiers, says SAE, do not operate into resistive static loads under normal use, and therefore the static measurements that most of us make (including this reviewer) do not tell us how an amplifier will sound. Further on in the white paper they go on to say that, "These conventional tests do provide insight as to the quality of the engineering of the product under test but not an absolute measurement of the sonic quality." And, further on again, "The point is that the current concept of distortion is not wrong, it is just not relevant to the whole system process unless it is set up to deal with this process. Unfortunately, THD, IM, TIM and the like are basically a one-dimensional view of a multi-dimensional event." The SAE paper then goes on to describe how the company designed a series of amplifiers which dealt with the problems of reactive loads that constantly vary. They offer arguments why these amplifiers were a.c.-configured (rather than following the current vogue of all-d.c. design) and speak of their high current slew rate (which they feel is more relevant than the popularly quoted voltage slew rate of so many volts per microsecond), as well as the steps taken to insure a high natural damping factor (as opposed to the "artificial" damping factor, which is a function of the amount of negative feedback in the amplifier and tends to fall off above bass frequencies). I have no particular quarrel with any of the statements in the SAE paper, but I had hoped that as I read further I would be confronted with some new and magical method of measurement which I might apply to the A-501 in order to verify the claims made for its sonic superiority. Aside from an experiment in which we are asked to compare the amplifier output when an asymmetric pulse is fed to it, first when the amp is hooked up to a static (resistive) load and next when hooked up to a loudspeaker, the only other suggestion is that we listen to the amplifier, in addition to measuring its static performance characteristics (which, as you see from the specs listed at the outset of this report, SAE continues to do as well).
Somewhat disappointed at the lack of a new amplifier measurement technique in the SAE white paper, I therefore proceeded to measure the A-501, examine it, and listen to it, not necessarily in that order. The massive looking all-black front panel is configured for 19-inch standard rack mounting and is fitted with a pair of handles that are, considering its weight, more than ornamental. Separate power-on and power-off square push buttons are at the lower center of the panel. Above these are a rather elaborately calibrated series of LEDs (one row per channel) which serve as peak power output indicators. Calibration is in watts (referred to 8-ohm loads) and in dB below rated output of 250 watts per channel.
The rear panel of the A-501 power amplifier is equipped with color-coded, five-way binding posts for speaker cable connections, and, at either end of the rear panel, there are pairs of input jacks. The two jacks associated with each channel's input are not wired in parallel. One of them gives direct access to the amplifier's first stage and provides ultra-wide band operation.
The other input, identified by the words "high pass," provides 6 dB per octave roll-off below 20 Hz and is intended for use in a bi- or tri-amplified system or where extended low-frequency response has proven to be hazardous to speaker performance or has caused acoustic feedback between speakers and turntable.
The rear panel also contains a line fuseholder.
Circuit and Construction Highlights
The A-501 uses multiple output transistors and a very large power supply, including a high-current toroidally wound power transformer and large high-current power storage capacitors. All power handling circuits are hand-wired using #14 gauge multi-stranded cable for minimal internal impedance, instead of being part of circuit board paths. As can be seen from the photo of its internal construction, the A-501 has extensive heat-sinking, which is one of the reasons why it is as heavy as it is. The amplifier is equipped with a relay protection circuit. If, under any circumstances (internal or external), d.c. or subsonic frequencies appear at the output, the amplifier will disconnect from the load and remain disconnected until the problem has been corrected. The sensitivity of the protection circuit is adjusted so that commonly encountered transient signals, such as the thump of a tuner's muting circuit or the act of dropping a stylus on a record surface, will disconnect the loudspeakers momentarily. In such cases, the amp will not shut down completely but will allow a low-level signal to be heard. Part of the relay design is a contact[...]
Fig. 1--Power output vs. THD at 1 kHz, solid line, and at 20 Hz and 20 kHz, dashed
line, into 8-ohm loads.
Fig. 2-Distortion vs. frequency at the rated output of 250 watts per channel, both channels driven into 8-ohm loads.
(Audio magazine, Nov. 1981)
Also see:
SAE Mk IIICM Basic Amplifier (Equip. Profile, Jan. 1975)
SAE Model 2922 Integrated Amplifier (Mar. 1979)
SAE Mk XII Speaker System (May 1973)
SAE Model 5000 Impulse Noise Reduction System (June 1977)
Robertson Sixty Ten Amplifier (Jan. 1986)
MXR Model 139 Linear Preamplifier (Nov. 1981)
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