Quicksilver Audio Horn Mono monoblock power amplifier Measurements part 2
Fig.8 Quicksilver Horn Mono, 4 ohm tap, spectrum of 50Hz sinewave, DC-1kHz, at 13W into 4 ohms (linear frequency scale).
Fig.9 Quicksilver Horn Mono, 4 ohm tap, HF intermodulation spectrum, DC-24kHz, 19+20kHz at 1.5W into 4 ohms (linear frequency scale).
The Horn Mono gives out moderate power levels, of course, which will not be an issue with high-sensitivity horn speakers, nor with more typical speakers like the inexpensive Paradigms RD also used in his auditioning. The Horn Mono overloads in a very "soft" manner, the distortion rising in proportion to the power level.
Fig.10 shows the THD+N percentage changes with continuous output power from the 8 ohm tap. The amplifier doesn't actually "clip" until after the normally defined "clipping point" of 1% THD+N is reached. At this 1% point it gives out 20W into 8 ohms from the 8 ohm tap (13dBW), but less power into lower impedances, of course, which is typical of an amplifier with an output transformer. At the true clipping point from this tap, which can be just seen as the "knee" in the right-hand trace just below the top of the graph, the Horn Mono puts out 36.3W into 8 ohms (15.6dBW). However, it takes an input of 6.64V for the amplifier to reach this level, which might be beyond the capabilities of some preamplifiers.
Fig.10 Quicksilver Horn Mono, 8 ohm tap, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.
From the 4 ohm transformer tap (fig.11), the 1% THD power was 27.3W into 8 ohms (14.4dBW); slightly less, 22W, into 4 ohms (10.4dBW); and a still useful 10W into 2 ohms (4dBW).
Fig.11 Quicksilver Horn Mono, 4 ohm tap, distortion (%) vs continuous output power into (from bottom to top): 8 ohms, 4 ohms, 2 ohms.
I used the Miller Audio Research Amplifier Profiler to examine how much power was available from this unassuming amplifier, using a toneburst signal more typical of the transients to be found in music: 10 cycles of 1kHz followed by 400 cycles of silence. The results from the 4 ohm tap are shown in fig.12. Into 16 ohms (red trace), the THD (no noise, as this FFT-based measurement actually sums the levels of the actual harmonics) is astonishingly low for a tube amp, before rising to reach the 1% THD point at 12W (13.8dBW). Into 8 ohms (black), this point is reached at 23.6W (13.7dBW), while into 4 ohms (blue), the relevant figure is 44.9W (13.5dBW).
Fig.12 Quicksilver Horn Mono, 4 ohm tap, distortion (%) vs 1kHz burst output power into: 16 ohms (red trace), 8 ohms (black), 4 ohms (blue), 2 ohms (green), 1 ohm (magenta).
The very small difference in the measured dBW rating means that, into impedances that are equal to or greater than the specified transformer tap, the amplifier behaves very close to being a pure voltage source with this transient signal—which is more typical of a high-performance solid-state design!
This graph shows that the maximum power available from the 4 ohm tap with this transient signal is no less than 62.7W into 2 ohms (green trace, 12dBW), equivalent to an RMS output current of 5.6A! Note, however, the fairly rapid rise in distortion at lower output powers into this low impedance. And current limiting sets in into impedances lower than 2 ohms, limiting the maximum 1 ohm power (magenta trace) to 29.6W (5.7dBW), which is again around 5.5A current.
I was frankly astonished by the measured performance of the Quicksilver Horn Mono, given that it uses just one small-signal tube and two of that workhorse output tube, the EL34. The use of a sensible amount of overall negative feedback gives very low noise, excellent linearity, and a low output impedance, all of which, in combination with its excellent power delivery, will contribute to the good sound it gave with the very revealing Avantgarde Unos.
Yes, there are the well-damped but still noticeable ultrasonic parasitic resonances, and the amplifier is not as tolerant as cost-no-object tube models when it comes to driving speakers with loads that drop well below the specified impedance of the output transformer tap. But some compromises have to be made for an amplifier that retails for just $1595/pair, and provided the purchaser has sensitive speakers (or more normal speakers in a small room), the Quicksilvers will work well.—John Atkinson