Ti Kan's M³ Headphone Amplifier

July 28, 2005

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See the companion σ11 regulated power supply
and the official M³ headphone amplifier site.

Pure class A with a full dose of brawn and finesse

Configuration

This unit is configured as follows:

RMAA Test Results

RightMark Audio Analyzer software, running on a Toshiba 2.8GHz Celeron laptop computer via an M-Audio Firewire Audiophile mobile interface running in 32-bit, 96KHz mode.

This test provides data and graphs of frequency response, noise, dynamic range, total harmonic distortion, intermodulation distortion and stereo crosstalk performance.
Even 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.

Other Test Results

These were measured with a Wavetek 4MHz sweep function generator and a Fluke 95 50MHz digital ScopeMeter.

Oscilloscope waveforms

The 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. These were measured with a Wavetek 188 4MHz sweep function generator and a Protek 6510 100MHz oscilloscope.

The tests were done with the amplifier volume control set to maximum, and the output level is adjusted to slightly below the threshold of clipping using the function generator's amplitude control.

The graphs show that the amplifier preserves absolute phase. The square wave response exhibits 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.

1KHz square wave

10KHz square wave

100KHz square wave

100KHz sine wave

100KHz triangle wave

100KHz Lissajous


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