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Equipment Measurements

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.

Additional Data
  • 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, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.185mV, -83.7dBW; A weighted 0.088mV, -90.1dBW.
  • Output noise, 8-ohm load, balanced input, 600-ohm 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.
Measurements Summary

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.

Chart 1 - Frequency Response of Output Voltage as a Function of Output Loading

Red line: open circuit
Magenta line: 8-ohm load
Blue line: 4-ohm load
Cyan line: NHT dummy-speaker load

Chart 2 - Distortion as a Function of Power Output and Output Loading

(line up at 20W to determine lines)
Top line: 4-ohm SMPTE IM
Second line: 8-ohm SMPTE IM
Third line: 4-ohm THD+N
Bottom line: 8-ohm THD+N

Chart 3 - Distortion as a Function of Power Output and Frequency

4-ohm 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 4-ohm load


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