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Equipment Measurements

March 2009

Raysonic M100 Mono Amplifier: 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 at 120V AC line voltage. Output taken from the 8-ohm output and loaded with 8 ohms unless otherwise noted.
  • This amplifier does not invert polarity.
  • AC line current draw at idle: 2.7A.
  • Input impedance @ 1kHz: 89.9k ohms.
  • Output impedance at 50Hz: 0.42 ohms.
  • Input sensitivity: 91.2mV.
  • Gain (8-ohm load): 31.0X, 29.8dB.
  • Output noise, 8-ohm load, 1k-ohm input termination:
    • Wideband: 0.30mV, -79.5dBW
    • A weighted: 0.049mV, -95.2dBW
Measurements Summary

Power output with 1kHz test signal

  • 8-ohm load at 1% THD: 99.2W
  • 8-ohm load at 10% THD: 118.5W

  • 4-ohm load at 1% THD: 47.0W
  • 4-ohm load at 10% THD: 83.0W

General

The Raysonic M100 is a beautiful-looking medium-power push-pull tube power amplifier utilizing four pairs of 5881 output tubes. Said to operate in class A, that didn’t turn out to be quite true, as the full-power AC-line draw was 326W and 257W at idle. True class-A operation would have a constant power draw as a function of power output up to clipping. In reality, the M100 is a class-AB design with very high idling current.

Starting to measure this unit, I quickly discovered that it was a design with too much negative feedback. The unit would go into subsonic oscillation with an open-circuit load. This would preclude using this amplifier with speakers that have what is called "LC tuning" or speakers that have a built-in bass amplifier. Both of these speaker types have a series capacitor as part of either its bass tuning or as part of the internal crossover to cut in the midrange above the active woofer. This would unload the amp at DC, thus causing low-frequency instability. Using this amp for these kinds of speakers would result in the amp using up its power in subsonic oscillations. To get the open-circuit frequency response, I had to place a suitable inductor across the amplifier output to give it a low-resistance DC path to ground yet be a high impedance to audio frequencies. This worked to a sufficiently low frequency to get the bulk of the curve where the lowering inductive reactance of the inductor as frequency went down started to load down the amp -- below 40Hz.

Chart 1 shows the frequency response of the amp with varying loads. The output impedance, as judged by the closeness of spacing between the curves of open-circuit, 8-ohm, and 4-ohm loading, is unusually low for a tube power amplifier and another clue to what would appear to be quite a bit more than the usual amount of negative feedback. Note also the peaking in the ultrasonic frequency range with any normal load. Surprising, the ultrasonic response is not peaking with the open-circuit load.

Chart 2 illustrates how total harmonic distortion plus noise vs. power varies for 1kHz and SMPTE IM test signals and amplifier output load. Distortion performance is best for a "matched load" or load value the same as the output terminal value. Power is reduced and distortion increases for the "half loading" of a 4-ohm load on the 8-ohm output.

Total harmonic distortion plus noise as a function of frequency at several different power levels is plotted in Chart 3. Amount of rise in distortion at high frequencies is quite pronounced but not unusual for many tube power amps. Very, very few power amplifiers of any type have a constant amount of distortion over the audio band at various output powers. Measurement at the rated 100W proved problematical, as the M100 really couldn’t produce the power above about 5kHz without excessive distortion.

Damping factor vs. frequency is shown in Chart 4. Here, we can see the unusually high damping factor over much of the audio range -- again a clue to the high amount of overall negative feedback used in the design.

A spectrum of the harmonic distortion and noise residue of a 10W 1kHz test signal is plotted in Chart 5. The principal signal harmonics are second and third with the remaining harmonics about 20dB or more below the level of the second and third harmonics. Amount of AC-line harmonics are reasonably low but with some intermodulation of the AC-line harmonics with the signal harmonics near the nulled-out 1kHz test signal.

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


8-ohm output loading
Cyan line: 100W
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 an 8-ohm load

 

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