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Equipment Measurements

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.

Additional Data
  • Measurements were made with 120V AC line voltage and one channel driven (this is a mono amplifier).
  • Gain: 41.2x, 32.3dB
  • Output noise, 8-ohm load, unbalanced input, 1k-ohm 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.
Measurements Summary

Power output with 1kHz test signal

  • 16-ohm load at 1% THD: 1.4W
  • 16-ohm load at 10% THD: 32W

  • 8-ohm load at 1% THD: 5.5W
  • 8-ohm load at 10% THD: 44W

  • 4-ohm load at 1% THD: 5.5W
  • 4-ohm load at 10% THD: 38W


The DeHavilland Aries GM70 SET (single-ended-triode) amplifier measured uses a 125W plate-dissipation GM70 output tube. This tube has a larger plate structure than the more commonly used 211 or 845 triode output tubes.

Output-tube 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 high-frequency roll-off characteristic. The low-frequency response holds up down to 20Hz -- better than many of the low-/no-feedback SET designs. Output impedance is moderate, giving an approximate +1dB and -2dB frequency-response variation on the NHT dummy-speaker 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 8-ohm load. Distortion with a 4-ohm load is lower up to about 5W, and then gets greater than for the 8-ohm 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 4-ohm 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 4-ohm load as plotted in Chart 5, the signal harmonics decay uniformly into the noise, which is said to make for a musical-sounding amplifier. The amount of AC-line 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: 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

4-ohm output loading
Dark-blue 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 8-ohm load


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