Power output with 1kHz test signal
- 8-ohm load at 1% THD: 155W
- 8-ohm load at 10% THD: 210W
- 4-ohm load at 1% THD: 244W
- 4-ohm load at 10% THD: 364W
The Stello M200 is a medium-power solid-state design with
typically wide bandwidth and output impedance higher than is usual with solid-state
amplifiers. Some of its characteristics -- for example, the way distortion varies with
power, the damping factor, and the uniformity of amount of distortion and damping factor
with frequency -- are more like those of a well-designed tube amplifier.
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. The variation
with the NHT dummy load in the audio range is of the order of +/-0.25dB.
Chart 2 illustrates how total harmonic distortion plus
noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. As
can be seen, attainable power is greater for the 4-ohm load, as is usual for most power
amplifiers. Further, the way that the distortion increases as power nears maximum is a
much softer curve than is typical for a solid-state amplifier. This indicates the
possibility of low amounts of overall feedback in the design.
Total harmonic distortion plus noise as a function of
frequency at several different power levels is plotted in Chart 3. In order to eliminate
out-of-band noise and more accurately measure the amount of distortion in this plot, the
AES-17 sharp-cutoff 40kHz low-pass filter was used instead of the usual 80kHz filter. The
amount of rise in distortion at high frequencies is very low in this design. The number of
amplifiers that I have encountered in my experience with this attribute is now countable
on two hands rather than one.
Damping factor vs. frequency is shown in Chart 4 and is
moderate but quite constant with frequency.
A spectrum of the harmonic distortion and noise residue of
a 10W 1kHz test signal is plotted in Chart 5. The magnitude of the AC-line harmonics is
quite numerous and intermodulation components of line harmonics with signal harmonics are
also very numerous and visible. The test-signal harmonics are both even and odd and don't
decline or tail-off with frequency vary quickly.