Power output with a 1kHz test signal
- 16-ohm load at 1% THD: 24.7W
- 16-ohm load at 10% THD: 32.4W
- 8-ohm load at 1% THD: 43.3W
- 8-ohm load at 10% THD: 56.9W
- 4-ohm load at 1% THD: 76.0W
- 4-ohm load at 10% THD: 93.2W
General
Here we have a very rare animal: an honest-to-God true
class-a amplifier. True in the classic sense that the AC input power stays constant up to
output clipping into 8-ohm loads. For lower loads, the mode of operation does change to
class AB, but at reasonably high powers -– higher than where the kernel of most of
the music resides at reasonable playing volumes.
This amp gets quite HOT when warmed up. I measured about
140° F at the center of the heat sinks and about 120° F on the front panel, top plate,
and rear panel. Surprisingly, when the top cover was removed, the internal temperature of
the components was less -- a good thing for the life of these components.
Chart 1 shows the frequency response of the amp with
varying loads. The high-frequency response is moderately wide with an approximate 3dB down
point of 100kHz. Output impedance, as judged by the closeness of spacing between the
curves of open-circuit, 8-ohm, and 4-ohm loading, is quite low in the audio band and even
up to 200kHz. The usual NHT dummy-load curve is not shown as the variations in the
response would not show. The variation with the NHT dummy load in the audio range is of
the order of +/-0.005dB -– a truly negligible amount.
Chart 2 illustrates how THD+N (total harmonic distortion
plus noise) vs. power varies for 1 kHz and SMPTE IM test signals and amplifier output
load. Amount of distortion is noise dominated up to about 1W and starts to rise smoothly
up to clipping.
THD+N as a function of frequency at several different power
levels is plotted in Chart 3 for 4-ohm loads. Typical of most power amplifiers, the THD+N
rises with frequency above 500–1000 Hz. In this design, the rise is quite pronounced.
Results for this test were similar for 8-ohm loading.
Damping factor vs. frequency is shown in Chart 4 and is of
an unusually high value up to about 400Hz and, again, typically, falls off above this
frequency. Still at 20kHz, it is greater than 170.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5 for 4-ohm loading. The AC-line harmonics are
complex but reasonable in magnitude and caused mostly by filter-capacitor-charging current
pulses getting into the signal-ground circuitry. The signal harmonics fall off nicely with
order at this power level and test frequency.
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