Ti Kan's Millett Hybrid Headphone Amplifier

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See the companion STEPS v1.2 power supply

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.

With BUF634P

The following are test results of the Millett Hybrid amplifier with TI/Burr-Brown BUF634P output buffers. This is the original design configuration. The BUF634P's pin 1 bandwidth resistor was 221Ω. GE 12AE6A, Westinghouse 12FM6 or Raytheon 12FK6 tubes were tested in the amplifier, each at several bias points and load impedances to explore the effects of changing tube bias and load.

With OPA551PA

The following two results were taken with OPA551PA opamps installed in place of the BUF634P, and wire jumpers installed between pins 2 and 6 below the DIP-8 socket. This operates the OPA551PA as a closed-loop voltage follower. Note that the upper order harmonic distortion and intermodulation distortion components are reduced in magnitude, compared to the BUF634P results above with the same tubes and bias points. I used 12AE6A tubes biased to 13.5V for these tests because the object of the test here were the buffer, not the tubes; and these GE 12AE6As exhibits the lowest distortion amongst the different tubes in my collection.

With Discrete Diamond Buffers

The following results were taken with the discrete diamond buffer (a project by steinchen and n_maher) installed in place of the BUF634P. As with the OPA551PA tests above, I used the 12AE6A tubes biased to 13.5V for all these measurements.

This particular diamond buffer board was populated with 2N5087/5088 for the input and current mirror transistors, and MJE243/253 for the outputs. 2N5486 JFETs are used for the adjustable current source. These are all "standard configuration" parts as specified in the diamond buffer schematic diagram.

With the discrete diamond buffers, the Millett Hybrid exhibits less upper order harmonic distortion and intermodulation distortion than the BUF634P. The THD and IM percentage numbers don't tell the whole story. You must compare the graphs to see this. In fact, these measurements produced virtually identical results to those with the OPA551PAs (which has the benefit of being closed loop and thus very low distortion). The tubes remain the dominant source of distortion, so the differences in the output buffers is masked. What the results do tell us is that both the OPA551PA and the discrete diamond buffers outperform the BUF634P in this application.

I tested with the diamond buffers set to 15mA, 20mA, 25mA and 30mA bias (only the first three are shown below, the fourth is no different than the rest). The differences between these bias points were insignificant, thus I recommend setting to between 15-20mA for the lowest heat dissipation on the output transistors.

As expected, the frequency response, noise, and stereo crosstalk measurements were not affected by the choice of output buffers.

Other Test Results

These were measured using a Wavetek 188 4MHz sweep function generator and a Fluke 95 50MHz digital ScopeMeter.

Oscilloscope waveforms

The following shows the waveform response of the Millett Hybrid amplifier equipped with 12AE6A tubes, biased to 13.5V and BUF634 output buffers. 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 inverts absolute phase. There is significant slewing at the leading and falling edges of the 100KHz square wave, but no ringing or overshoot. The 100KHz sine, triangle, and Lissajous graphs also show some amount of phase delay between the input and output. The triangle wave peaks show visible rounding. Despite the -3dB frequency response of over 500KHz, this is obviously not a "fast" amplifier.

1KHz square wave

10KHz square wave

100KHz square wave

100KHz sine wave

100KHz triangle wave

100KHz Lissajous

Comments

As expected, this amplifier exhibits high total harmonic distortion. The intermodulation distortion is also high. The 12AE6A is the best performer in the bunch in terms of distortion, and the 12FM6 the worst. The 12AE6A pair I have is also better matched to each other than the other tube types tested. In all cases it was a bit surprising that the THD contains strong even and odd order harmonics. While the bias setting causes some change to the relative strengths of the harmonics, the effect is small as compared to the overall magnitude of distortion.

The voltage gain varies dramatically depending on the tube type used. The 12FK6 tube may not provide enough gain for some low-output source and high impedance headphone combinations. The maximum output voltage swing is dependent on the setting of the bias adjustment. For maximum swing the bias should be set to one half the supply voltage.

The clippihg behavior is classic tube-gear soft. When mildly overdriven this amplifier will not sound overly harsh. However if the bias is set far away from one-half the supply voltage, the clipping will be asymmetrical.

This amplifier has a low noise floor. When used with a ripple-free DC power supply, the spectrum is devoid of hum or noise spikes. The tubes are somewhat microphonic, so vibrations should be avoided.

The 22Ω output resistors cause the effective gain to drop by 4.7dB when 33Ω dummy loads are added (compared to 330Ω dummy loads). This also makes the RMAA noise floor appear to be 4.7dB lower because the input test signal level has to be increased to make up for the loss. The 33Ω load also causes the amplifier distortion to rise significantly. The stereo crosstalk also suffers with the low impedance load, primarily due to signal ground pollution as a result of increased output current return to ground. This makes sense because the difference in stereo crosstalk between 330Ω and 33Ω loads is about 20dB over most of the audio band, which a factor of 10.

The bass response is down 0.5dB at 20Hz when loaded with 33Ω dummy load, due to the high-pass filter characteristics of the 470µF output coupling capacitor. Otherwise, the frequency response is nice and flat across the audio band.

The high frequency response of this amplifier is quite extended, -3dB at 535KHz (I only tested this with the 12AE6A). This is in stark contrast to the rolled-off highs found on many tube amps.

The measurements show an amplifier's technical strengths and weaknesses, but it doesn't describe the sound. I am happy to report that despite the measurements, this neat little amplifier is fine-sounding, in a euphonic kind of way. It doesn't break new ground in dynamics, resolution or soundstage, nor was it ever intended to. This is a different kind of audio project, simple, safe and easy on the wallet. The sense of nostalgia when examining the vintage NOS tube boxes is a pleasure to behold. Moreover, no solid state device evokes the visual enjoyment that tubes provide, especially when displayed in their full glory with a clear-top case, and lit from below with blue light! :)



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