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Equipment Measurements

May 2005

darTZeel NHB-108 Model One Stereo Amplifier: 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 both channels driven, driving the unbalanced inputs unless otherwise noted.
  • Gain: 20x, 26dB.
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 0.602mV, -73.4dBW; A weighted 0.062mV, -93.2dBW.
  • Output noise, 8-ohm load, balanced input, 600-ohm input termination: wideband *6.41mV, -52.9dBW; A weighted 0.088mV, -90.1dBW. (* There was a small amount of approximately 90kHz low-level spurious signal in the left channel that the manufacturer indicates that some units do exhibit and is deemed harmless to the sound.)
  • AC line current draw at idle: 1.3A.
  • Output impedance at 50Hz: 0.29 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 142W

  • 4-ohm load at 1% THD: 158W

General

The darTZeel NHB-108 is a medium-power solid-state design with typically wide bandwidth and output impedance a bit higher than is usual for solid-state amplifiers. Some of its measured characteristics are similar to those of a tube amplifier, such as relatively high measured distortion and a modest damping factor. Both are suggestive of little or no overall negative feedback in the design.

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 +/-0.25 dB.

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. Amount of distortion is relatively high for solid-state designs, but the way the amp goes into clipping is more like a typical solid-state amplifier.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Interestingly, the distortion amount vs. frequency for the lower powers is virtually constant, whereas at higher powers the distortion does rise a bit at the high end of the audio band.

Damping factor vs. frequency is shown in Chart 4, and is moderate but reasonably constant with frequency, again not usual for solid-state designs.

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 just visible near the noise floor. The test signal harmonics are both even and odd and don't decline or tail-off with frequency very fast.

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 10W 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


8-ohm output loading
Cyan line: 100W
Blue line: 40W
Magenta line: 10W
Red line: 1W

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 an 8-ohm load

 

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