Kora Electronic Cosmos monoblock power amplifier Measurements
Stereophile Guide to Home Theater editor Thomas J. Norton used to dread measuring tube amplifiers when he was this magazine's technical editor. I know how feels, the Kora Electronic Concept Cosmos proving a little temperamental on the test bench—three of its 1A output-tube fuses blew at various times during the testing, and one of the 6AS7G tubes suddenly glowed brighter than it should have. After I turned the amplifier off, let it cool down, then turned it on again, this tube had no further problems, however.
The Cosmos had identical sensitivities from both its unbalanced and balanced inputs, with a sensible 27.1dB gain into an 8 ohm load. An input voltage of 125mV results in an output of 1W into 8 ohms. The amplifier didn't invert signal polarity. The unbalanced input impedance measured 81k ohms at 1kHz, this dropping to a still high 70k ohms at 20kHz. The input impedance via the balanced jack was 100k ohms at 1kHz.
The output impedance was reasonably low for a tube design at 0.62 ohm across most of the audioband, this rising to 0.75 ohm at 20kHz. Modification of the frequency response due to the interaction between this source impedance and the way the loudspeaker's impedance changes with frequency was moderate, at around ±0.5dB with Stereophile's simulated speaker load (fig.1, top trace). However, this graph also shows a small amount of top-octave rolloff, though the actual -3dB point lies at an excellent 100kHz. As a result of this wide bandwidth, the Kora's reproduction of a 10kHz squarewave had an excellent shape, with short risetimes (fig.2).
Fig.1 Kora Cosmos, frequency response (from top to bottom at 2kHz): 1W into 8 ohms, 2.83V into dummy loudspeaker load, 2W into 4 ohms, 4W into 2 ohms (0.5dB/vertical div.).
Fig.2 Kora Cosmos, small-signal 10kHz squarewave into 8 ohms.
The Kora's unweighted, wideband signal/noise ratio with the input shorted was okay at 64.5dB ref. 1W/8 ohms, this increasing to 80dB when A-weighted. The Cosmos is not particularly linear as amplifiers go, as can been seen in fig.3, which plots the THD+noise percentage against frequency into various load impedances. This graph's primary distinguishing feature is the rise in THD in the bass—due, I assume, to output-transformer limitations. In the midrange and treble, and into 8 ohms, the distortion at this output level (2.83V) remains below 0.2%—which is okay, considering its low-harmonic nature (see later). But there are significant rises in THD as the load impedance drops; I would not recommend using the Cosmos into speakers with impedances that drop below 4 ohms.
Fig.3 Kora Cosmos, THD+noise (%) vs frequency at (from top to bottom at 10kHz): 4W into 2 ohms, 2W into 4 ohms, 1W into 8 ohms, 2.83V into simulated loudspeaker load.
Fig.4 reveals that the distortion content at moderate levels for a midrange signal is almost pure second harmonic, which, if not carried to excess, will "fatten" the perceived sound. At lower frequencies and higher output powers, higher harmonics make an appearance, as can be seen in fig.5, which shows the spectrum of a 50Hz tone at 12.5W into 8 ohms. The second harmonic lies at a high -36dB in this graph (1.5%), but has been joined by other harmonics. Mitigating their appearance, however, is the fact that they regularly decrease in level with increasing order, something that, nearly a quarter-century ago, French engineer and writer Jean Hiraga demonstrated was associated with a subjectively pleasing sound.
Fig.4 Kora Cosmos, 1kHz waveform at 1.9W into 4 ohms (top), distortion and noise waveform with fundamental notched out (bottom, not to scale).
Fig.5 Kora Cosmos, spectrum of 50Hz sinewave, DC-1kHz, at 12W into 8 ohms, one channel driven (linear frequency scale).
There is a caveat, however, in that the presence of this harmonic distortion does not lead to excessive intermodulation distortion. Fig.6 shows the spectrum of the Cosmos' output while driving an equal mix of 19kHz and 20kHz tones at 12.5W into 8 ohms. Only the 1kHz difference product rises above the -60dB line, at an indicated -54dB (0.2%), which is probably acceptably low. However, at the same voltage level into 4 ohms (fig.7), the intermodulation products rise to levels that start to make me feel uncomfortable.
Fig.6 Kora Cosmos, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 15.2W into 4 ohms (linear frequency scale).
Fig.7 Kora Cosmos, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 15.2W into 4 ohms (linear frequency scale).
Under continuous drive, the Kora amplifier doesn't quite meet its specified power output at the usual 1% THD+noise point, at least into 8 ohms (fig.8, bottom trace). I got just 79W into 8 ohms at this limit, which is 1dB below the 100W figure. I measured 103.3W of continuous power at 1% THD into 4 ohms (17.1dBW) and 115W at a more relaxed clipping definition of 3% THD, which means the Cosmos will probably just achieve its specified power into 6 ohms. Note that the actual "knee" in the fig.9 traces occurs after the distortion has already risen above 1% for the 4 and 2 ohm load conditions.
Fig.8 Kora Cosmos, distortion (%) vs continuous output power into (from bottom to top at 1W): 8 ohms, 4 ohms, 2 ohms.
Finally, I tested the Kora's output power with a low-duty-cycle 1kHz toneburst, using the Miller Audio Research Amplifier Profiler software, which reveals—better than would a continuous sinewave—how an amplifier will behave under the more dynamic conditions typical of music (fig.9). At 1% THD, the Cosmos still managed only 90W (19.6dBW) into 8 ohms (red trace), but gave 144W into 4 ohms (18.6dBW, blue) at the same distortion level. The latter is equivalent to an RMS output current of 6A, which is good for a tube amplifier. Slightly more power, 156W, was available into 2 ohms, but at the cost of 1% THD at almost all power levels above 1W. As I stated before, loads of less than 4 ohms are probably best avoided with this amplifier. Surprisingly, the amplifier seems a little less happy into 16 ohms (black trace) than it does into 8.
Fig.9 Kora Cosmos, distortion (%) vs 1kHz burst output power into 16 ohms (red trace), 8 ohms (black), 4 ohms (blue), 2 ohms (green), and 1 ohm (magenta).
These measurements don't gainsay Michael's very positive listening impressions, but as always it will be important to audition this amplifier with your own speakers before making a final purchase decision, I feel.—John Atkinson