October 2004
Nightingale
ATS90 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.
 Power output and distortion plotted with one channel driven
(this is a mono amplifier).
 Gain: 45.9x, 33.2dB.
 Output noise, 8ohm load, unbalanced input, 1kohm 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.
Power output with 1kHz test signal (on 8ohm taps)
 8ohm load at 1% THD: 60W
 8ohm load at 10% THD: 100W
 4ohm load at 1% THD: 30W
 4ohm load at 10% THD: 61W
General
The Nightingale ATS 90 is a mediumpower monoblock tube
power amplifier operating fully class A up to full power. The design utilizes six 7581A
output tubes (an enhancedcharacteristic 6L6) in pushpull parallel.
The first surprise with this amp in measuring it came in
the opencircuit frequencyresponse 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 opencircuit value reveal an output impedance on the
8ohm output of about 3.4 ohms. The highfrequency 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.
Frequencyresponse deviation for the NHT dummyspeaker 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 4ohm loading on the 8ohm
output. Not shown is the 1kHz THD+N for a 16ohm load which indicate, with the data on the
8 and 4ohm loading, that the best performance is with the rated output tap load (i.e.
, 8 ohm on the 8ohm output and 4 ohm on the 4ohm 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 8ohm loading on the 8ohm 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: 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 10W 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 
8ohm 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 4ohm load
