Ti Kan's Dynahi Headphone Amplifier

February 19, 2006

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Overkill taken to the Nth degree

Dissipating some 70W as heat when most headphones require only milliwatts to achieve loud listening levels...

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.

The same measurement methodology is employed for all of my RMAA tests. Namely, the amplifier-under-test has its volume control set to maximum position, and the test levels are adjusted using the sound card software. The amplifier outputs are loaded with a custom switchable dummy load box.
RMAA test result comments

The Dynahi measured well, producing excellent THD and very good IMD results. The THD and IMD are largely unaffected by the load, recording virtually identical numbers and graphs for 330Ω and 33Ω loads, and deteriorating somewhat with 8Ω load. Thanks to the separate PSU chassis construction, the noise spectrum is devoid of any spikes that would indicate AC hum interference. The broadband noise floor is low, but not the lowest of all the amplifiers I've tested. The stereo crosstalk is excellent with 330Ω load, but impaired by some 8dB or more above 1KHz when loaded with 33Ω, and deteriorating an additional 10dB over much of the audible spectrum with 8Ω load. This is not surprising for a passive-ground topology. Nevertheless the 33Ω result is still very good thanks to careful headphone jack ground return wiring (See my discussion on this subject at the Headwize DIY Workshop).

Other Test Results

These were measured using a Wavetek 188 4MHz sweep function generator, a Fluke 95 50MHz digital ScopeMeter and a Protek 6510 100MHz oscilloscope. * Sustained maximum output into 8Ω is not recommended due to PSU regulator heat dissipation concerns.

Oscilloscope waveforms

The following shows the waveform response of the Dynahi amplifier. In all graphs except the Lissajous waveform, the top trace is the input and the bottom is the output. These were measured using 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. There is a very slight overshoot at the leading edge of the square wave response. The rising edge of the square wave has a peculiar uneven slope. When the output amplitude is reduced to one half of maximum, the slope shape straightens out, but there is pronounced ringing at the edge. These are illustrated in the expanded 100KHz graphs shown below. These characteristics are not unique to this build. Another Dynahi (built by dgardner) also exhibited similar behavior. The ringing and the rising frequency response above 500KHz suggests that the 33pF compensation capacitor is not sufficient. Increasing the value of the capacitor would stabilize the amplifier at the expense of slew rate and bandwidth.

There is minimal phase shift even at 100KHz, as shown in the Lissajous graph.

The output waveforms are somewhat affected by the load. The graphs below are taken with 330Ω loads connected to the output. There is a noticeable rounding of the square wave edge, and a slight reduction of voltage amplitude when a 33Ω load is switched in.

1KHz square wave

10KHz square wave

100KHz square wave

100KHz sine wave

100KHz triangle wave

100KHz Lissajous

100KHz square wave
(expanded, full output)

100KHz square wave
(expanded, half output)
 



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