As I’ve posted in the past, my audio measurement setup is built around an EMU 0404 USB sound card. Its a fairly old device, its driver is old too. On the other hand, you can get it for almost nothing on eBay, and it has excellent measurable performance for the price. It is good for 0.001% THD at 1KHz without any modifications. With some help, its front-end is good enough for even 0.0001% THD measurement, as I’ve showed in this post. However, as you increase the frequency, the distortion will grow, as you’d expect. Additionally, if you look inside the box, there are quite a few parts there that make you wonder “how good can it be if I put a few extra $ into it?”. That’s exactly what I wanted to find out. I didn’t want to spend much time, nor funds, as I was happy with the performance I was getting. This was mostly for fun, and the results are shown in this post.
One important tool that can help extend the capabilities of a distortion measurement setup is a notch filter. The logic behind it is fairly simple, if we are only interested in the distortion components, why should we even feed the fundamental frequency into the measurement setup? By eliminating it (or simply attenuating it sufficiently), we can reduce the harmonic distortion generated by the test equipment as a result of the large tone, effectively extending its capabilities for harmonic distortion measurement. There is obviously more than one way of doing it, and in this post I will only describe one way which was a good match for my needs.
I wanted to create a small box that would implement this function for my needs to allow me to extend further the THD measurement setup I have. In its simplest form, using the EMU 0404USB I’m able to measure THD of ~0.001% at 1KHz. By using an external low distortion 1KHz oscillator I was able to extend this down to ~0.0004%. However, I was looking for a way to get down to 0.0001% to allow measurement of high quality DAC’s. Since I know the external oscillator I use has sufficiently low distortion to support these figures, I needed a way to reduce the distortion caused by the input stage and ADC of the EMU. I have considered trying to hack the EMU and improve its input stage, but I expect the ADC will limit me before I can reach the target performance. Therefore I went with the option of removing the fundamental frequency from the signal before feeding it into the EMU, to reduce the distortion it generates.
This post discusses a topic I’ve shared quite a long time ago on a few other forums, I’ve decided to post it here on the blog just in case it will become unavailable on these forums at some point, as it is a fairly old post. I don’t have the original schematics anymore, so bare with the lower res images I’m copying over from my original post.
Many voltage regulators use the capacitance multiplier as a method of increasing the effective capacitance seen by a load. Some use it as a complete voltage “regulator” (although its more of a filter in that case than it is a regulator), while others use it as a low-pass-filter (LPF) for the error amplifier at the core of the regulator. The basic idea is to use a BJT transistor as a follower to amplify the capacitor current by ~hfe (small signal current gain) of the transistor, making the capacitor appear as if it was ~hfe larger in value. This simple structure is shown in Fig. 1.