July 2005
Stello M200 Mono
Amplifiers: Measurements
All amplifier measurements are performed
independently by BHK Labs. Please click to learn
more about how we test amplifiers there. All measurement data and graphical
information displayed below are the property of SoundStage! and Schneider
Publishing Inc. Reproduction in any format is not permitted.
 Measurements were made with 120V AC line voltage with one
channel driven (this is a mono amplifier), driving the unbalanced inputs unless otherwise
noted.
 Gain: 39.4x, 31.9dB unbalanced input; 11.0x, 20.8dB balanced
input.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 0.185mV, 83.7dBW; A weighted 0.088mV, 90.1dBW.
 Output noise, 8ohm load, balanced input, 600ohm input
termination: wideband 0.169mV, 84.5dBW; A weighted 0.081mV, 90.9dBW.
 AC line current draw at idle: 0.76A.
 Output impedance at 50Hz: 0.25 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
 8ohm load at 1% THD: 155W
 8ohm load at 10% THD: 210W
 4ohm load at 1% THD: 244W
 4ohm load at 10% THD: 364W
General
The Stello M200 is a mediumpower solidstate design with
typically wide bandwidth and output impedance higher than is usual with solidstate
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 welldesigned 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, 8ohm, and 4ohm 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 4ohm 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 solidstate 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
outofband noise and more accurately measure the amount of distortion in this plot, the
AES17 sharpcutoff 40kHz lowpass 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 ACline harmonics is
quite numerous and intermodulation components of line harmonics with signal harmonics are
also very numerous and visible. The testsignal harmonics are both even and odd and don't
decline or tailoff with frequency vary quickly.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
Chart 2  Distortion as a Function
of Power Output and Output Loading 
(line up at 20W to determine lines)
Top line: 4ohm SMPTE IM
Second line: 8ohm SMPTE IM
Third line: 4ohm THD+N
Bottom line: 8ohm THD+N
Chart 3  Distortion
as a Function of Power Output and Frequency 
4ohm output loading
Cyan line: 200W
Blue line: 70W
Magenta line: 20W
Red line: 2W
Chart 4  Damping Factor
as a Function of Frequency 
Damping factor = output impedance divided into 8
Chart 5  Distortion and
Noise Spectrum 
1kHz signal at 10W into a 4ohm load
