May 2004
Audio Electronic
Supply SixPac 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, using the
8ohm output terminals, and with one channel driven (this is a mono amplifier).
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination, 0dB feedback: wideband 1.14mV, 67.9dBW; A weighted 0.23mV, 81.8dBW.
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination, 10dB feedback: wideband 3.78mV, 57.5dBW; A weighted 0.74mV, 71.6dBW.
 AC line current draw at idle: 1.72A.
 Output impedance at 50Hz (0dB feedback): 6.6 ohms.
 Output impedance at 50Hz (10dB feedback): 2.1 ohms.
 This amplifier does not invert polarity.
Power output with 1kHz test signal
0dB feedback
 8ohm load at 1% THD: 57W
 8ohm load at 10% THD: 70W
 4ohm load at 1% THD: 47W
 4ohm load at 10% THD: 68W
10dB feedback
 8ohm load at 1% THD: 58W
 8ohm load at 10% THD: 72W
 4ohm load at 1% THD: 53W
 4ohm load at 10% THD: 68W
General
The Audio Electronic Supply SixPac is a moderatepower tube
amp utilizing three pairs of triodeconnected EL34 output tubes. Four or 8ohm load
matching is provided for the output terminals with a twoposition toggle switch. A
frontpanel switch gives the option of 0 or 10dB of negative feedback.
The following measurements were made with the output load
selection switch set for 8 ohms. Relative behavior in the 4ohm position was about the
same for 4 and 2ohm loading. Distortion was measured with the 10dB of feedback applied.
In general, distortion without the feedback was about 23 times greater.
Chart 1a shows the frequency response of the amp with
varying loads and with the 10dB of feedback engaged. Output impedance as judged by the
spacing between the various loadings is typical of many tube power amplifiers. The effect
on delivered frequency response to a typical speaker load is also shown for the NHT dummy
load. Chart 1b is with no feedback. Here, the output impedance is much higher, and, in my
personal opinion, too high with the consequence that the delivered frequency response to
many speakers would be way off from flat. Chart 2 illustrates how total harmonic
distortion plus noise versus power varies for 1kHz and SMPTE IM test signals and amplifier
output load. In the case of this amplifier, the 4ohm loading produces about the same
power as with the more "matched" 8ohm load. Total harmonic distortion plus
noise as a function of frequency at several different power levels is plotted in Chart 3.
Damping factor versus frequency is shown in Chart 4 for both the 0 and 10dB of feedback. A
spectrum of the harmonic distortion and noise residue is plotted in Chart 5. As seems to
be the case with many amplifiers measured, this one has a rich series of AClinehum
harmonics with some sidebands of 120Hz about the nulled fundamental frequency. The
harmonics series of the signal test frequency is quite dense and extends over the rest of
the audio range.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
(Chart 1a: 10dB feedback)
Red line: open circuit
Magenta line: 8ohm load
Blue line: 4ohm load
Cyan line: NHT dummyspeaker load
(Chart 1b: 0dB feedback)
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 
(10dB feedback)
(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 
(10dB feedback)
8ohm output loading
Cyan line: 50W
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
Red line: 10dB feedback
Magenta line: 0dB feedback
Chart 5  Distortion and
Noise Spectrum 
(10dB feedback)
1kHz signal at 10W into an 8ohm load
