June 2004
deHavilland
Aries GM70 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 and one
channel driven (this is a mono amplifier).
 Gain: 41.2x, 32.3dB
 Output noise, 8ohm load, unbalanced input, 1kohm input
termination: wideband 2.1mV, 62.6dBW; A weighted 0.35mV, 78.2dBW.
 AC line current draw at idle: 2.65A.
 Output impedance at 50Hz: 2.05 ohms.
 This amplifier inverts polarity.
Power output with 1kHz test signal
 16ohm load at 1% THD: 1.4W
 16ohm load at 10% THD: 32W
 8ohm load at 1% THD: 5.5W
 8ohm load at 10% THD: 44W
 4ohm load at 1% THD: 5.5W
 4ohm load at 10% THD: 38W
General
The DeHavilland Aries GM70 SET (singleendedtriode)
amplifier measured uses a 125W platedissipation GM70 output tube. This tube has a larger
plate structure than the more commonly used 211 or 845 triode output tubes.
Outputtube plate current was at the desired point as
received, so no adjustment was necessary. Frequency response, as seen in Chart 1, rolls
off at both ends of the audio range with a nicely controlled highfrequency rolloff
characteristic. The lowfrequency response holds up down to 20Hz  better than many of
the low/nofeedback SET designs. Output impedance is moderate, giving an approximate +1dB
and 2dB frequencyresponse variation on the NHT dummyspeaker load. Total harmonic
distortion plus noise for a test frequency of 1kHz and SMPTE IM distortion as a function
of power output and load is plotted in Chart 2. The chart shows that the amount of
distortion at the 1kHz test frequency is reasonably low at less than 2% up to 10W output
for the 8ohm load. Distortion with a 4ohm load is lower up to about 5W, and then gets
greater than for the 8ohm load. Any load between 4 and 8 ohms is a good match for the
amplifier. Total harmonic distortion plus noise as a function of frequency at several
power levels is plotted in Chart 3 for 4ohm loading. Amount of distortion over the main
midrange energy band is about 1% or less for power outputs of up to 5W. Admirable is the
relatively low amount of distortion increase at the higher frequencies. However,
distortion does rise considerably at the low end of the audio range as is typical for SE
tube designs. Damping factor versus frequency shown in Chart 4 is a reasonable 3.9 over
most of the frequency range. In the spectral plot of distortion and noise for a 10W 1kHz
signal into an 4ohm load as plotted in Chart 5, the signal harmonics decay uniformly into
the noise, which is said to make for a musicalsounding amplifier. The amount of ACline
hum components adding up to about 2mV would likely be audible on speakers with
efficiencies of 90dB or more.
Chart 1
 Frequency Response of Output Voltage as a Function of Output Loading 
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 
(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 
4ohm output loading
Darkblue line: 50W
Cyan line: 30W
Blue line: 15W
Magenta line: 5W
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 8ohm load
