Specifications & benchmarksAll measurements were done with an M³ amplifier with the following configuration (except where noted):
The tests were conducted using the following equipment:
Here are the measured results:
RMAA test results
Several tests were performed, one with the M-Audio Firewire Audiophile mobile interface running in loopback mode to establish the baseline (lowest limit of measurement resolution), and anothers with the M³ amplifier added to the chain with 330&Omega, 33Ω or 8Ω dummy loads connected to the M³'s output.
The M³'s volume control knob is set to within 3dB of maximum for these tests. This is worthy of mention because many others test their amplifiers with the volume set lower, which would give artificially low noise and distortion results.
Click on the following links to see the results.
Comments on the RMAA test resultsEven though the M³'s measured results are excellent, it can be seen from the loopback graphs that the M³ added almost negligible distortion to the M-Audio Firewire Audiophile's baseline. Hence, the distortion measured is predominently the sound interface's performance, not that of the M³ amplifier.
The slight rolloff at the low end in the frequency response graph is due to the M-Audio's coupling capacitor. The M³'s actual frequency response is flat down to 0Hz. Likewise, the high-end rolloff in the graph is due to the M-Audio's anti-aliasing filters. M³'s actual response extends to 1.5MHz (-3dB) in the tested configuration.
The small spike in the M³'s noise floor near 7KHz is a measurement aberration due to interference.
The M³'s RMAA results remain essentially unchanged whether its outputs are loaded with 330Ω, 33Ω or 8Ω. The stereo crosstalk is the only item slightly affected at 8Ω load, but even with this severe load, the results are still better than most headphone amplifiers driving 330Ω loads. Many amplifiers' distortion performance deteriorate significantly with lower impedance loads, there is no such problem here.
These results amply illustrate the benefits of M³'s high-current class A MOSFET output stage, three-channel active ground topology and other design elements. See the Technical highlights section for details.
Oscilloscope waveformsThe following shows the waveform response of the M³ amplifier. In all graphs except the Lissajous waveform, the top trace is the input and the bottom is the output.
The square wave graphs show that there is minimum slewing and ringing at the leading and falling edges. The 100KHz sine, triangle, and Lissajous graphs also show very small amount of phase shift between the input and output. Within the 20Hz to 20KHz audio band, there is no measurable phase shift.
The output waveforms are also unaffected by the load. There is no discernable change in shape or amplitude, whether the amplifier output is unloaded, loaded with 330Ω or loaded with 33Ω.
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