Headphone Amplifier Measurement Routine Development: THD+Noise
Preliminary Total Harmonic Distortion + Noise vs. Frequency
There's a long row to hoe here, so don't get too wrapped up with the measurements above. There are plenty of places for gremlins to hide in setting up these measurements for the first time. I've build one cable, but there are plenty more to go. Some of the measurements above have different cables with some adapters in the loop. Got to double check the system grounding. Bottom line: don't believe anything you see yet. That said, so far THD+Noise measurements have been pretty repeatable ... and I'm glad to see the system finally drawing lines about headphone amps.
Today's measurements were Total Harmonic Distortion + Noise vs. Frequency (THD+Noise vs. F). This is a measure of the linearity of the gain curve (or transfer function) of an amplifier. Imagine a big bass note going up and down at 40Hz with a little 10kHz high frequency note superimposed on it. As the bass note goes up and down, the high frequency note is sometimes being amplified on the positive part of the gain curve, and sometimes it's in the negative part of the gain curve. In a very linear amplifier, the high frequency note will amplified by exactly the same amount regardless. In single ended tube amplifiers, the gain is not linear, and the small high frequency signal will generally have less gain as the grid voltage goes positive.
Here's the simple version: if the gain isn't linear, the signal will get squished sometimes, usually near the top and/or bottom of the operating region. This squishing distorts the signal.
Here's the weird thing: when you put a pure sine wave of 1kHz, for example, in to an amp with some distortion, you'll get the 1kHz signal back out, but you'll might also get a little 2kHz, 3kHz, 4kHz etc. at the output as well. This is called harmonic distortion, and if you add up all the little distortion peaks you get the amount of Total Harmonic Distortion.
In the graphs above, the tester puts out a pure sine wave at 1Vrms. I adjust the volume of the amp so that it outputs a 1Vrms signal that gets sent back to the analyser. The measurement system then slowly sweeps the frequency of the audio signal from 10Hz to 22kHz in 100 steps. The signal out of the amp goes into the analyser where it first goes through a very deep notch filter that is stepped along with the frequency of the generator, which completely gets rid of the main driving tone. What's left is the various harmonics created by the non-linearities of the amplifier --- the harmonic distortion.
But! There's another thing there: the noise from the amplifier. So the meter in the analyser is reading both the total amount of harmonic distortion and the noise. That's why this measurement is called THD+Noise v Frequency.
Again, this is too early and there are too many places where gremlins can pop in to state the above graph is accurate, but let's just take it at face value for the moment and talk about a few of the graphs.
You can see the Burnson HA-160D has the highest amount of THD+Noise. This is most likely because this is the highest gain amplifier, and therefore has the most noise. The noise is a constant, so noise dominated measurements tend to be featureless and flat. You'll also notice a small dip around 120Hz. This likely occurs because the AC transformer is putting a little hum in the system. When the filter of the analyser gets swept past 120Hz, it gets rid of the 120Hz component of the signal. Since the dip is small, it's fair to say the contribution of the hum to the measurement is somewhat less than the broadband noise of the amplifier.
You can see that the Pico Slim has a large feature in the bass. This is a very small portable amplifier with a single ended supply and has some capacitive coupling. When low notes start getting filtered out by the capacitive coupling it distorts them, and results in a rise in harmonic distortion in the lows.
The TTVJ Slim is not as flat a line in general as the other amps. This means that there is some change in distortion with frequency. I'd bet a fine Belgian beer that this is evidence of Pete Millett's fancy fiddling. He tends to do some tricky stuff for really good reasons, even if it makes the numbers change a bit.
The HeadAmp Pico had the lowest THD+Noise of all the amps above 100Hz, but one channel is producing a rather noisy measurement. Here be them gremlins methinks. I've got to make some dedicated cables and retest --- getting the grounding absolutely nailed down is a huge deal in measurement systems.
The bottom trace is the measurement with the outputs of the Audio Precision tester just looped right back into the inputs of it's analyser. It is much lower in distortion than the amps measured, and tells me the system is not adding any artifacts to the other measurements.
Okie dokie, next is more cable making, and I'll probably try to make a THD+Noise vs. Power test. See you then!