Ti Kan's M³ Headphone Amplifier

July 28, 2005

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See the companion STEPS v1.2 power supply
and the official M³ headphone amplifier site.

Pure class A with a full dose of brawn and finesse

The M³ is a high-end stereo headphone amplifier I co-developed with Morsel, as a community service project for the DIY audio hobbyist. It is capable of driving virtually any type of dynamic headphone and extract the optimum performance from the best of them.
This amplifier received many accolades and has proven itself not only in objective measured performance, but also in head-to-head listening comparisons with other top-tier commercial or DIY amplifiers.
I am now offering professionally-manufactured printed circuit boards and some related parts for sale at very reasonable prices.
For full details please visit the official M³ headphone amplifier site.
This unit was my developmental prototype. I use it daily and it serves as the demonstrator/showpiece in meets and gatherings.
A companion outboard STEPS v1.2 power supply, set to 27V, is used to power the amplifier. See my STEPS page for details.

Configuration

This unit is configured as follows:

AD8610 opamps in all three channels
Output power MOSFETs biased to 85mA per channel
Opamp class A biasing
Vishay-Dale RN55D mil-spec 1% metal film resistors
Wima MKP polypropylene capacitors
Vishay-Roederstein MKT metallized polyester capacitors
Nichicon HE low-ESR high reliability electrolytic capacitors
Alps RK27112A blue-velvet volume control and bass boost potentiometers
Voltage gain set to 5
22pF C1 compensation capacitors on the left and right channels, 33pF on the ground channel
Custom-made enclosure with aluminum base and acrylic front/rear panels and top cover

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.

M-Audio Firewire Audiophile loopback baseline

Comparison of M³ driving 330Ω, 33Ω and 8Ω loads

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.

Input impedance
47.6KΩ
Output impedance
less than 0.05Ω at 1KHz
Maximum output voltage
(prior to onset of clipping)
4.6Vrms (13Vp-p), 27VDC supply, AD8610 opamps
7.8Vrms (22Vp-p), 40VDC supply, OPA604 opamps
Maximum output power
(prior to onset of clipping)
0.6Wrms into 33Ω, 27VDC supply, AD8610 opamps
2Wrms into 8Ω, 27VDC supply, AD8610 opamps
1.8Wrms into 33Ω, 40VDC supply, OPA604 opamps
6Wrms into 8Ω, 40VDC supply, OPA604 opamps
Frequency Response
(at 1Vrms output, sine wave)
0Hz - 1.5MHz, +0, -3dB
Rise time
(10KHz square wave, at maximum output, 10% to 90%)
0.25µS
Slew rate
(10KHz square wave, at maximum output)
45V/µS

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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