Symphonic Line Kraft 400 monoblock power amplifier Measurements

Sidebar 1: Measurements

A full set of measurements of the Symphonic Line Kraft 400 was made in its unbalanced mode. Balanced measurements were not made, for reasons explained below.

Following the 1/3-power, one-hour preconditioning test, the Kraft 400's heatsinks were barely warm to the touch—so cool, in fact, that it seemed that the heatsinks are far larger than they need to be.

The input impedance measured a low 9.8k ohms. Some care should be exercised to avoid matching the Kraft 400 with a preamp having an unusually high output impedance. The output impedance was 0.04 ohms or less at low and mid frequencies, and under 0.07 ohms at 20kHz—which should not affect the system's frequency response into different loudspeaker loads. Voltage gain into 8 ohms measured 33.1dB, and signal/noise (ref. 1W into 8 ohms) measured 103.4dB over a 22Hz–22kHz bandwidth, unweighted (but see below). The Kraft 400 is noninverting in the unbalanced mode.

Fig.1 shows the frequency response of the Kraft 400 at 1W into 8 ohms; the result into a 4 ohm load was virtually identical. The 10kHz squarewave shown in fig.2 is an excellent result, with only the slightest reduction in risetime to distinguish it from a virtually perfect result. The 1kHz squarewave, not shown, was virtually perfect.

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Fig.1 Symphonic Line Kraft 400, frequency response at 1W into 8 ohms in unbalanced mode (0.5dB/vertical div.).

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Fig.2 Symphonic Line Kraft 400, 10kHz squarewave at 2W into 8 ohms.

Fig.3 shows the THD+noise vs frequency curves. It is here that my problems with the Kraft 400 are first visible. When the amplifier was first turned on for its 1/3-power pretest, there was a slight but unmistakable odor of burning insulation or paint. It only lasted a few seconds, the amp appeared to be functioning normally, so I thought little of it. Later, as I was about to make the THD+noise vs output measurements, I noted a much stronger, similar aroma. While the amplifier was briefly subjected to a 1V input into a 2 ohm load at about the same time, this was only for slightly over a second, and should not have caused a malfunction in such a large, powerful amplifier. In any event, I turned the amplifier off briefly, after which it appeared to function normally.

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Fig.3 Symphonic Line Kraft 400, THD+noise vs frequency at (from top to bottom at 20Hz): 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms, after "incident;" 4W into 2 ohms, 2W into 4 ohms, and 1W into 8 ohms, before "incident."

To be certain nothing was amiss, I went back and remeasured the THD+noise vs frequency. Before and after measurements are shown in fig.3. The "after" results, while still quite good, are noticeably higher. When I remeasured the S/N, it had deteriorated to 74dB. As the amplifier was otherwise functioning properly, I elected to complete our test series to see if anything else unusual turned up. I did encounter a subsequent pattern of fuses blowing on the power-supply rails, though I was able to restore operation several times to complete the measurements presented below.

The THD+noise waveform is shown in fig.4. Note that the result is heavily second-harmonic—always a good sign for an amplifier's sound. The spectrum of the Kraft 400's output while it drove 50Hz into 4 ohms is shown in fig.5. While this is a respectable result—about 0.5% (–45.4dB) at 100Hz, and just over 0.08% (–61.5dB) at 150Hz—this was for an output of just over 55W. The distortion rises significantly at only a few watts higher, with the waveform showing quite visible, and intermittent, distortion. Most good amplifiers can get reasonably close to 2/3 power into 4 ohms (with the 400, just over 400W) before any sign of clipping becomes visible.

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Fig.4 Symphonic Line Kraft 400, 1kHz waveform at 1W into 8 ohms (top); distortion and noise waveform with fundamental notched out (bottom, not to scale).

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Fig.5 Symphonic Line Kraft 400, spectrum of 50Hz sinewave, DC–1kHz, at 55.6W into 4 ohms (linear frequency scale). Note that the second harmonic at 100Hz is the highest in level, at &#15 45.4dB (0.5%).

Fig.6 shows the output spectrum with the amplifier reproducing a combined 19+20kHz signal at an indicated 258W into 4 ohms. This is a more respectable result, with only three artifacts greater than –60dB (0.1%). The result was slightly better still at 135W into 8 ohms (not shown). With this input signal, it is much more common that an amplifier clip prior to reaching two-thirds its rated power.

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Fig.6 Symphonic Line Kraft 400, HF intermodulation spectrum, DC–22kHz, 19+20kHz at 258W into 4 ohms (linear frequency scale; "0" = –6dB).

The 1kHz, THD+noise vs output power curves for the Kraft 400 are shown in fig.7. The rise in distortion around 100W—well short of actual clipping—may have been due to the incidents reported earlier, but this could not be verified. At any rate, this amplifier does put out substantial power, clipping at approximately 250W into 8 ohms, 470W into 4 ohms, and 780W into 2 ohms. And the line voltage was slightly low when these measurements were made—about 107V! Because of this, I decided to finish the rest of the measurements the next day.

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Fig.7 Symphonic Line Kraft 400, distortion (%) vs output power into (from bottom to top at 10W): 8 ohms, 4 ohms, and 2 ohms.

But the next day the Kraft 400 refused to respond. It turned on, and all the LEDs lit up, but there was no output. It wasn't a fuse, but if it had been, this would have been the sixth rail fuse blown in the test—sufficient reason to discontinue further measurements. Whatever had caused the intermittent problems of the day before was now clearly plotting to prevent the completion of the tests. Remaining incomplete were the balanced measurements, measurements into our simulated real-world load, discrete clipping tests, and checking the DC offsets.

Had JS not experienced any service problems with the Kraft 400, we would have hauled our second sample up to the test bench despite the heroic efforts required to move this amplifier (ropes, pulleys, and a chorus of "Yo, heave ho!"). But Jonathan had had problems. We called it a test.

Aside from our difficulties in keeping the Kraft 400 operating properly, I had several problems with the amplifier's physical design. The internal parts appear to be of good quality, but there's a lot of dead air inside the amplifier—more than half its volume, in fact. I estimate the total weight of the internal parts to be no more than a third, or perhaps as little as a fourth, of the amplifier's total weight. The rest is the heatsink and the massive steel plates of the case. The output transistors are all mounted in a square at the center of the heatsink, another indication (along with the cool operation) that the heatsink is considerably overdesigned.

The case itself of the review samples is not well finished, with unpolished, butt-joined edges. Certainly it's not up to the superior fit'n'finish we've seen in earlier Symphonic Line products. The square, sharp edges of the amplifier, combined with its weight and lack of any handles or convenient handholds, make it a dangerous design to move around.

Given the asking price, these are not trivial considerations. The amp should be about half as large as it is, and about half as heavy. But then, it wouldn't be muy macho, and the asking price would have to be reduced. But I guess that's just the cynic in me talking.—Thomas J. Norton

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