SAE 2401 Power Amplifier Review

The SAE “01” Series “Dual High Resolution” power amplifiers have been designed, according to their manufacturer, specifically to drive real loudspeaker loads rather than being optimized for laboratory bench measurements using resistive loads.

The three models that presently make up the series have identical electrical characteristics except for their power ratings. The most powerful, the 2401, was tested for this report.

The rationale for the design of the “01” Series amplifiers is the acknowledged fact that loudspeakers are not only reactive loads (exhibiting the properties of capacitors or inductors at different frequencies in addition to their resistive components) but are also nonlinear. Since a dynamic speaker acts as a motor, its voice-coil motion generates a “back emf” that tries to drive currents in the reverse direction through the amplifier’s output terminals. For the amplifier to be insensitive to this reverse-signal component, it must have a high damping factor (low internal impedance) over the entire audio-frequency range.

Most amplifiers have a high damping factor at low frequencies because of the considerable negative feedback employed in their circuits. At higher frequencies the damping factor decreases toward what SAE calls the “natural” damping factor, which is determined by (among other things) the number of output devices used, the power-supply capability and its regulation, and even the gauge of the connecting wire within the amplifier. According to SAE, the complex nature of a loudspeaker load makes the “natural damping factor” of the amplifier a more important consideration than its feedback-determined low-frequency damping factor.

The same factors that give an amplifier a high degree of natural damping also allow it to deliver high currents to the speaker loads. A loudspeaker generates an acoustic output proportional to the current passing through its voice coil, and this current does not have a simple relationship to the applied voltage as is the case for a linear load resistor. It is the contention of SAE that a slew rate expressed in volts-per-microsecond is not necessarily related to the ability of an amplifier to drive a loudspeaker with a current of rapidly changing level. They prefer to use “current slew rate,” expressed in amperes per microsecond, as the criterion for an amplifier’s high-frequency-power capability.

Finally, an amplifier must be able to handle asymmetric waveforms accurately. Unlike test tones, music waveforms are generally not symmetrical about the zero-voltage axis. Since the impedance of a loudspeaker may appear to be different for the positive and negative portions of the signal waveform, the amplifier must be completely symmetrical in its response to an external load. Essentially, the SAE “01” Series amplifiers have been designed to meet these requirements. They have low distortion, both statically and dynamically, with either symmetric or asymmetric signals. They have a very-high-speed current-delivering and -absorbing ability to deal with the dual problems of driving a speaker load and absorbing the return signals from its voice coils without distortion.

The SAE 2401 is rated to deliver 250 watts per channel to 8-ohm loads from 20 to 20.000 Hz with no more than 0.025 per cent total harmonic distortion. The 4-ohm rating over the same frequency range and with the same distortion limits is 375 watts per channel. The rated damping factor, referred to 8 ohms, is 65 at 100 Hz and 60 from 20 to 20.000 Hz. The rated input sensitivity is 0.14 volt for a 1-watt output or 2.24 volts for rated output, and the output noise (A-weighted) is 90 dB below 1 watt. The current slew-rate specification is 20 amperes per microsecond. The amplifier is protected against improper load or drive conditions by a combination of a relay, thermal cutoff, and a low-impedance load-sensing circuit.

The SAE 2401 is finished in black and has heavy-duty handles on its front panel, which is slotted for rack mounting. On the front panel are two parallel rows of red LEDs that show the instantaneous power output of the amplifier in 3-dB steps from rated output to —36 dB, plus a pair of LEDs that light to show that the amplifier is turned on (there are square pushbuttons on the panel to turn it on and off). Two sets of five-way binding posts on the rear apron of the amplifier supply the speaker outputs, and near each edge of the rear apron are two phono-jack inputs. For each channel there is a “normal” input whose frequency response is essentially flat down to 1 Hz and a “hi-pass” input whose response falls at 6 dB per octave below 20 Hz. The heavy-duty power cord has a three-pin grounding plug.

The power transistors are mounted along the left and right sides of the amplifier, and their heat-radiating “fins” resemble a cluster of rods extending horizontally from the transistors. Cooling air passes over them by convection, entering at the bottom and exiting from the top of the amplifier without the assistance of a cooling fan. Most of the center of the amplifier is occupied by a large power transformer and the filter capacitors. The SAE 2401 is 19 inches wide, 12-1/2 inches deep, and 7 inches high. It weighs 57 pounds. Price: $950.

Laboratory Measurements. Although the top of the SAE 2401 became quite hot at times during our tests (and its thermal cutoff shut down the amplifier a few times during our tests with 2-ohm loads), it never reached uncomfortable or dangerous temperatures. The output waveform clipped at 325 and 576 watts per channel into 8- and 4-ohm loads, respectively, at 1,000 Hz. When we measured the clipping power into 2 ohms, we were able to drive only one channel at a time. The amplifier itself was not the limiting factor; our test-bench 15-ampere line fuse blew before we reached clipping when we tried to drive both channels. This can be explained by the enormous sine-wave clipping power we measured into a 2-ohm load—925 watts per channel! Obviously, attempting to drive both channels to that power output at the same time would overtax the current rating of any home electrical wiring (15 to 20 amperes).

The IHF clipping-headroom rating of the SAE 2401 into 8 and 4 ohms was 1.14 and 1.86 dB. We also measured the dynamic-output power with the tone-burst signal specified in the IHF amplifier test standard. Into loads of 8, 4, and 2 ohms, respectively, the maximum nonclipped short-term power output was 370, 670, and 950 watts per channel. Thus, the IHF dynamic headroom rating for 8 and 4 ohms was 1.7 and 2.5 dB.

At 1,000 Hz, the distortion with 8-ohm loads was less than 0.004 per cent up to more than 270 watts output, reaching 0.165 per cent at 300 watts. The distortion was slightly higher when driving 4 ohms, measuring between 0.0045 and 0.0085 per cent for power outputs between 1 and 500 watts per channel. Even 2-ohm loads did not result in objectionable distortion; it was about 0.01 per cent up to 500 watts and reached 0.09 per cent at 700 watts (this was with only one channel driven, for the reasons stated earlier).

Across the full audio-frequency range the distortion varied only slightly with power and hardly more with frequency. Typically, it was about 0.003 per cent from 20 to 2,000 Hz and rose smoothly to about 0.01 per cent at 20,000 Hz at power outputs from one-tenth-rated to full-rated power into 8 ohms. The sensitivity of the amplifier was exactly as rated, with a 0.14-volt input driving it to 1-watt output, and the output noise level (A-weighted) was better than our — 90-dB measurement limit.

The power-output LEDs responded fairly well to peak levels, although their response time seemed to be closer to that of a standard VU meter than to genuine peak-responding indicators. With 300-millisecond tone bursts (the standard test for VU-meter ballistic response) the LEDs read 3 dB low, and with 20-millisecond bursts they read 12 dB low. On continuous signals, their calibration was approximately correct. The amplifier’s frequency response was fiat from 5 to 20,000 Hz, down 1 dB at 60 kHz, and down 3 dB at 125 kHz. Its IHF slew factor was 3, and the IHF overload recovery time was less than our 2-microsecond resolution.

SAE suggests certain measurements and tests to show the superiority of their amplifier over others when driving real speaker loads. These involve the use of asymmetrical pulse signals, which we did not have available. However, we were able to infer some of the special qualities of the amplifier from a standard IHF two-tone IM measurement, using equal-amplitude input signals at 19,000 and 20,000 Hz. When their combined peak-output amplitude was equivalent to that of a 250-watt sine-wave signal, the third-order IM product at 18,000 Hz was 89 dB below 250 watts, which is very close to the residual distortion of our spectrum analyzer. Even more impressive and revealing was the total absence of a second-order (difference frequency) component at 1,000 Hz. That distortion component would show the degree of asymmetry in the amplifier (whose circuits are said to be completely symmetrical throughout). This was the first amplifier we have tested which did not produce any detectable second-order distortion in this test (down to our measurement “floor” of less than — 100 dB). These tests would seem to confirm SAE’s claims for high linearity and symmetry of the amplifier’s response to both positive- and negative-going signals.

Comment. Like most manufacturers of high-performance amplifiers, SAE urges that their products be judged by listening. As we have said on many occasions, the audible distinctions between power amplifiers tend to be so subtle and (to us, at least) so insignificant that we cannot use this as a basis for judging amplifier quality. We did make A-B comparisons against another amplifier of roughly similar ratings (which was not at all comparable to the SAE 2401 in its current-output capability) using good speakers that could absorb the full output of either amplifier. We did not hear any differences at all between the two amplifiers (nor did we expect to—which may account in part for why we did not, but that is another story!).

Disregarding sound quality for the moment, it was much easier to appreciate the advantages of the SAE 2401 from our experience in testing and using it as a part of a music system. Part of this appreciation concerns what it does not do. It has absolutely no vices or quirks that we could find. There is no fan to degrade ambient noise level; it runs completely (and literally) cool in normal operation, yet is not fazed in the least by treatment brutal enough to make its top cover quite hot. It makes no sounds, either electronic or acoustic, transient or otherwise, to mar the simple act of turning it on or off (there is a few seconds’ delay on turnon before the internal relay connects the speaker outputs). Its colorful LED power readout actually gives a useful indication of the amplifier’s power output rather than merely serving a cosmetic function, as is so often the case.

The SAE 2401 is an extremely powerful amplifier in terms of what it can actually deliver to real speaker loads. Lacking a schematic diagram, we do not know whether it has any of the usual current-limiting circuits to protect its output transistors, but its behavior does not show any sign of such circuits. The fact that SAE says it will deliver a 20-ampere-per-microsecond current slew rate into a 1-ohm load suggests to us that only the output transistors and the power supply limit the current it can deliver to its loads. When we test amplifiers as powerful as this one, we tend to operate them cautiously, having had too many of them self-destruct or at least burn out inaccessible fuses when pushed to their limits or beyond. We were impressed by the fact that the SAE 2401 never winced or faltered even when pouring almost a kilowatt of clean power from one channel into a 2-ohm load, and by the fact that the only failure during our tests was of the 15-ampere line fuse in our test-bench wiring.

Naturally, we would not attempt to judge the validity of. SAE’s claims solely on the basis of limited listening with only a couple of types of speakers. That is one of the problems with subjective evaluation of any audio component—somewhere there may be a unique combination of program material, loudspeakers, amplifier characteristics, and/or room acoustics that will reveal some special merit (or fault), but it is not practical to have on hand, or to search out and set up, such special conditions from among the near infinity of possible acoustic circumstances and component combinations.

Still, our experience with the SAE 2401 indicates that it is as near to being an unbreakable amplifier as we have seen, capable of clean power outputs beyond any reasonable listening requirements (and probably beyond the ability of most speakers to absorb, so caution is advised). In spite of this, it can be picked up by one man (though not too casually), does not require a special power line unless you plan to drive both channels to full power into loads of 2 ohms or less, and is completely silent in operation. All things considered, even the 240l’s moderately high price does not seem at all unreasonable for what it delivers. It is clear that the SAE 2401 is a most impressive amplifier by any criteria.

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