Power output with 1kHz test signal
- 8-ohm load at 1% THD: 413W
- 8-ohm load at 10% THD: 506W
- 4-ohm load at 1% THD: 658W
- 4-ohm load at 10% THD: 821W
General
The Belles 350A is a high-power linear solid-state
amplifier representing the culmination of David Belles’ 20-year amplifier research
and design experience.
Chart 1 shows the frequency response of the amp with
varying loads. As can be seen, the output impedance, as judged by the closeness of spacing
between the curves of open-circuit, 8-ohm, and 4-ohm loading, is quite low over the audio
range, and only above perhaps 50kHz can one see any sign of a change with loading. The NHT
dummy speaker load is not plotted here as the variations with frequency would not be
observable with this vertical scale factor. Needless to say, difficult, impedance-varying
speaker loads are not going to affect the flat-delivered response from this amplifier.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and 4- and 8-ohm loads. As can
be seen, attainable power is greater for the 4-ohm load, as is usual for most power
amplifiers. Furthermore, the attainable power is quite a bit more than the rated power for
this amp. I contacted Mr. Belles to check on this and he said something to the effect that
he "likes to be conservative." Well, that is the most conservative amplifier
power output rating I have seen! Unusual for these measured data is that the IM distortion
vs. power is so close to the 1kHz harmonic distortion vs. power. This is actually
indicative of the very low distortion of this design, with actual distortion rising above
the noise only at the onset of output clipping.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. There is a moderate
rise in distortion with frequency at the higher power levels.
Damping factor vs. frequency is shown in Chart 4 and is
very high at low frequencies, but as is typical for most amplifiers, it starts to decline
above a few hundred Hertz. But note, since the damping factor is so high, it is still
greater than 100 at 20kHz!
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal for 4-ohm loading is plotted in Chart 5. Even though the measured
output noise of this amp was low, the nature of the noise is mostly pulses when the
power-supply rectifiers conduct and, consequently, there are a lot of higher harmonics of
the AC-line frequency in this spectrum. The 1kHz signal distortion consists of a quickly
declining series of even and odd harmonics.
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