Despite the fact that the headphone amplifier really sounded very good (it’s incredibly fast making my headphones sound more like electrostatic headphones), I wasn’t really happy with the measurements I made with the QA401 Audio Analyser.
Now that I have a bit of holidays I decided to see what was wrong and what I could do to improve things.
The obvious place to start with (since it seems I have power related distortion) was adding a EMI power line filter. Although I had one laying around when I designed the PCB’s I decided not to use it because I preferred to keep the transformer as far away as possible from the rest and using a bulky EMI filter would have meant putting everything much closer together. It looks like I made a wrong decision here since there is a clear impact on the background noise. Especially in the higher frequencies as can seen from these 2 plots. The first one without and the second one with the EMI power line filter in place.
More power problems
Somehow I came up with the brilliant idea to perform a loop test on the QA401 (directly connecting OUT to IN). I got this surprising result:
This doesn’t look good at all. I expected a flat response with only 1 spike centred around 1 kHz.
After a lot of head scratching I figured out that the USB power the QA401 receives must be very bad (coming from a USB3 hub which also powers my Ducky keyboard with LED back lighting). Powering the QA401 directly from the PC produced a lot cleaner result.
Back to measuring the headphone amplifier
Now that we know we can trust the measurements, I inserted the headphone amplifier back in the loop. As you can see from this plot, the result is virtually identical to the loop test with the exception of a 6 dB gain provided by the amplifier.
The frequency response is ruler flat as well:
We clearly bump into the limitations of the QA401 audio analyser. And it will be hard to improve on this I think.
But we can conclude that the headphone amplifier not only sounds very good but measures equally good as well. Lesson learned as well is that you should not leave an EMI filter out of your power circuit. Even when they’re a bit bulky and take up a lot of place they certainly have a positive impact.