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Equipment Measurements

January 2002

Redgum Audio RGM175 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.
  • The RGM175 comes with a non-detachable passive preamplifier section. Signal input was into an input of the passive preamp section with the volume control at maximum unless otherwise noted.
  • Power output plotted with one channel driven (one of two power amplifer sections were available for measurements. Since both power amplifier channels are separately powered from the AC line, performance would be slightly different with both channels powered and operating).
  • Gain: 41.9x, 32.4dB.
  • Output noise, 8-ohm load, 1k-ohm input termination, volume control set for 1W/8 ohms with 1kHz 50mV input signal: wideband 0.220mV, -82.2 dBW; A weighted 0.053mV, -94.5 dBW.
  • AC line current draw at idle: 0.2A.
  • Output impedance (measured by an injection of a constant 1A of current at 50Hz): 0.01 ohms.
  • This amplifier does not invert polarity.
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 160W
  • 8-ohm load at 10% THD: 200W

  • 4-ohm load at 1% THD: 205W
  • 4-ohm load at 10% THD: 250W


This amplifier design has a non-flat frequency response with some low- and high-frequency boost. The output impedance is very low and, consequently, no difference can be seen in Chart 1 in the audio band with an open circuit, 8-ohm, and 4-ohm output loading. This low output impedance translates into the high damping factor, shown in Chart 4. Output power at the onset of clipping is about 150 to 200W with 8- and 4-ohm loading. Due to the low output stage idling current and consequent AC power draw, there is some crossover-type distortion and non-related signal asymmetry as evidenced by the rich series of odd and even harmonics of the 10W/4-ohm 1kHz spectrum shown in Chart 5.

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

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
Cyan line: 1W
Blue line: 10W
Magenta line: 60W
Red line: 180W

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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