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Equipment Measurements

October 2004

Nightingale ATS-90 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.
  • Power output and distortion plotted with one channel driven (this is a mono amplifier).
  • Gain: 45.9x, 33.2dB.
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm input termination: wideband 5.0mV, -55.0dBW; A weighted 0.150mV, -85.5dBW.
  • AC line current draw at idle: 3.3A.
  • Output impedance at 50Hz: 3.4 ohms.
  • This amplifier inverts polarity.
Measurements Summary

Power output with 1kHz test signal (on 8-ohm taps)

  • 8-ohm load at 1% THD: 60W
  • 8-ohm load at 10% THD: 100W

  • 4-ohm load at 1% THD: 30W
  • 4-ohm load at 10% THD: 61W


The Nightingale ATS 90 is a medium-power monoblock tube power amplifier operating fully class A up to full power. The design utilizes six 7581A output tubes (an enhanced-characteristic 6L6) in push-pull parallel.

The first surprise with this amp in measuring it came in the open-circuit frequency-response test. The amp becomes unstable at low frequencies shortly after the load is removed. This prevented displaying the curve for this condition. In Chart 1, the value obtained before the LF instability started was +3.1dB. This would produce a curve of this value over most of the audio frequency range. Other than that, the data from the chart and the +3.1dB open-circuit value reveal an output impedance on the 8-ohm output of about 3.4 ohms. The high-frequency rolloff above 50kHz is unusually sharp and seems to be caused by the notch characteristic just above 100kHz. Needless to say, this is typically caused by the characteristics of the output transformer. Frequency-response deviation for the NHT dummy-speaker load is about +/-2dB -- a bit high, but in the typical range for tube power amplifiers. Chart 2 displays the 1kHz THD+N and SMPTE IM distortion as a function of power output for 8- and 4-ohm loading on the 8-ohm output. Not shown is the 1kHz THD+N for a 16-ohm load which indicate, with the data on the 8- and 4-ohm loading, that the best performance is with the rated output tap load (i.e. , 8 ohm on the 8-ohm output and 4 ohm on the 4-ohm output). Chart 3 illustrates the THD+N versus frequency at four different power levels. Damping factor versus frequency is plotted in Chart 4. In Chart 5, the spectrum of the THD residue is shown for a 10W 1kHz signal with 8-ohm loading on the 8-ohm output. Of interest is the rising characteristic of the noise at low frequencies, again indicative of a subsonic peak in the response even with the output loaded.

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

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: 90W
Blue line: 30W
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 a 4-ohm load


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