## Audio Measurement Pre-Amplifier – Part 5 – Updated Ver1.1

Over the past year and a half since posting the series of pages about the measurement pre-amplifier I’ve designed and built, I’ve received emails from multiple people who were interested in building the pre-amp. I’ve happily shared with them the remaining boards I’ve had from that first batch I ordered at the time. These few boards were all given out, and I’ve therefore ordered a few extra boards to be able to keep offering these boards to people who would like to build such an instrument for themselves. Unlike the first batch, this time I’ve printed boards that fixed the issues I’ve reported when building my own unit and were fixed by a “bodge” over the original board. These modifications to the board design (and the writing of this post) were all done well over a year a go when I’ve originally assembled my unit, but I didn’t see a need to post them until this point in time.

This post is meant to share the updated schematic, as well as to offer additional information that can be of help to people who would like to assemble such an instrument.  I didn’t make any functional changes to the pre-amp at this revision, therefore I will offer no additional measurements in this post.

## Audio Measurement Pre-Amplifier – Part 4 – Casing the Pre-Amplifier

This is part 4 in the series of posts discussing the (audio) measurement pre-amplifier project. In part 1 I’ve covered the motivation for this project along with the circuit schematic and detailed circuit description. In part 2, I have gone through the board layout consideration and showed the assembled boards.  In part 3, I have gone through measurement results of the assembled pre-amplifier board, as well as some circuit modifications to extend its performance. In this post, part 4, I will briefly show the assembled unit, along with slight discussion of external and power supply coupling into the signal.

## Quick and Simple Notch Filter for THD Measurements

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.

## β22 Balanced Stereo Amplifier Build

The β22  from AMB  is one of the most highly regarded DIY headphone amplifiers you can meet around the web.It gets plenty of excellent reviews from plenty of people who have built it. Over the years I’ve had the opportunity to listen to quite a few headphone amplifiers, including DIY builds, and I ran across a β22 more than once. I’ve even had an opportunity to repair one for a friend after it got damaged due to an accidental short on the output. The β22 always sounded good to me, although I must admit that its one of these amplifier that didn’t give me that “wow” factor on our first encounter. In my book that can actually be a very good thing, as many of the amplifiers (and  any other stereo component) that give a “wow” feeling at first, prove to be too fatiguing and unrealistic sounding in the long run. The β22 is one of these amplifiers that you appreciate more as you spend more time with it.

I’ve been thinking of building a β22 for a fairly long time, with the cost being one of the factors against it. Just like with any other DIY project, and I’ve seen quite a few, the builder has significant wiggle-room regarding quality and cost, as well as functionality. However, I wanted to build one that could serve multiple functions, perform well, and look good. I wanted something I could be proud of building and owning, and to be happy with it for years to come. Eventually, I’ve decided to pull the trigger on this build. In this post I’ll share the steps and some of the technical considerations that came into play during this build.

## Low THD Oscillator Power-Supply, and PCB’s as Case Panels

As you’d expect from someone who’s hobby’s include both audio/stereo and electronics, I try to measure and quantify things even when they are related to audio gear. While I will prefer to tune things by ear at the final step, measurement gives significant insight to some problems, in a very accurate way, in short amounts of time. One of the tools I would love to have for this is a very accurate audio analyzer, like one of the Audio-Precision offerings. What I’d like to have is the ability to measure parameters like harmonic distortion down to very low levels of distortion. However, most of these instruments are so expensive, even when bought used, that I gave up on finding a good one of these very long ago. Thankfully, nowadays, you can get very good results with much cheaper PC based gear. This post will describe one of the steps I’m taking to try and extend my ability to measure these parameters with my laptop, keeping in mind this is aimed at hobby use and must therefore be reasonably priced. I will start this post off with a bit of an introduction, but will dedicate most of it the the low THD oscillator and its power-supply.