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.

As with many of my recent projects, I stuck to PCB’s for the front an rear panels of the pre-amplifier. The benefits are clear, its cheap, its very easy to design in the same software tools used for all of my circuit designs, and it offers electrical shielding due to the internal copper layers that are available to us. Unlike in my previous builds, this one is significantly larger, has very large holes, and even square cut-outs. Therefore, I wasn’t sure how well it will come out. To minimize the chance of an error I’ve printed the panels on a piece of paper and measured it in place before placing the orders. You don’t want to spend a few 10’s of $’s, and wait for a few weeks before you realize you’ve made a mistake 🙂 Continue reading “Audio Measurement Pre-Amplifier – Part 4 – Casing the Pre-Amplifier”

Audio Measurement Pre-Amplifier – Part 3 – Initial Setup and Measurement Results

This is part 3 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 this post, part 3, I will show some of the measurement results of the assembled boards. I will start with describing what it is I would like to measure, and how I plan on measuring it, including the limitations of the measurements I can make with the gear available to me. Then I will show the relevant result and discuss them.

The measurements I plan on performing can be split into 3 different groups. The first has to do with linearity of the pre-amplifier, to measure how much distortion it will have. Next are the noise measurements, as I want to verify the input referred voltage noise of the pre-amplifier to make sure it meets my target figures to allow measurement of low noise voltage regulators (and other devices). Finally are the “other” tests such as the accuracy of the True-RMS reading, the voltage limits of the output protection circuit, and so on.

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Audio Measurement Pre-Amplifier – Part 2 – Board Layout and Assembly

This is part 2 in the series of posts describing 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 this post, part 2, I will discuss the next steps related to the board design and assembly. This part won’t be as long and the first (I hope), but I would like to share some of the consideration I’ve made when laying out the board design.

The first step was deciding on a case size and layout for the front panel, as this will set some constraints on board dimensions and placement of connectors/switches/LED’s. I wanted to use a case that will be made of aluminium to use it as a shield, as at the highest gain setting the pre-amp has 60dB (X1000) of gain which makes it very sensitive to coupling from external signals. I also plan on placing the completed pre-amplifier on my work bench, so I wanted something that is relatively compact, but isn’t too cramped so that it isn’t comfortable to use. Something similar (or slightly smaller) than a bench DMM seemed like a good size for this as I would be able to stack it on top of my other instruments. The plan was to have all the relevant connectors and switches at the front, along with some LED’s for visual representation of the selected range, and a panel mounted voltmeter. Placing it all in a single row seemed impossible, or at least very uncomfortable to use. Therefore I’ve decide to split this into 2 different rows (heights). This put a constraint on the minimum height of the case, and meant I will have to split the design into 2 boards to support this since I don’t want to solder any wires. The schematics posted in part 1 of this series already represented this split board solution, with the second board used mostly for range selection.

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Audio Measurement Pre-Amplifier – Part 1 – Motivation and Circuit Design

As some my other posts show, I have been spending a significant amount of my spare time over the past few months on audio measurements related stuff. This included a low distortion oscillator, a notch filter to go with it, as well as modifying the EMU 0404 USB to extend its performance. One other item that has been in the works for quite a long time, is an audio measurement pre-amplifier. The motivation for this work is quite straight forward, I needed to find some way of turning the sound-card I’m using into a versatile measurement tool to do general audio measurements. The most significant limitation with sound-cards is their limited input voltage range, as most audio amplifiers put out voltages that are significantly higher than what you can safely feed into a sound-card. Indeed, this is what most people would use such instruments for. However, this is actually just a portion of what such a pre-amplifier could be used for.

This post will the first part of a series of posts that will describe my take on a measurement pre-amplifier. I will describe the motivation (requirements), the circuit design and implementation, measurement results, and more. I  will try to make this as informative as I can, and share some of the reasoning behind design decision. I think this can be of value for both people who would like to understand the circuit better, and people who would like to modify the circuit to better suit their needs.

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EMU 0404 USB – Op-Amps Replacement Worth the Effort?

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.

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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.

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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 well. 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.

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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.

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M³ Headphone Amplifier Build

This post will briefly describe the M³ amplifier I’ve built to drive my headphones. Over the years I’ve had an opportunity to listen to quite a few headphone amplifiers, some of which I really liked, and even built a few of. These included the Pimeta from Tangent, and a few of AMB’s designs, including the M³ I will describe in this post. The M³ is meant to be a DIY amplifier, with boards being sold by Ti on his website. The M³ is based on a 3-channel topology, in which the output ground is also created by an amplifier channel. There has been significant discussion about this topology over the web, with opinions going both ways. However, like with all other audio related things, I prefer to let my ears be the final judge, and in the case of the M³ I always liked what I’ve heard.

Some years ago a friend of mine asked me to build one of these for him, with the power-supply sitting in its own case(Fig. 1). When it was complete, I’ve had some time to use it before he picked it up, and I really liked what I’ve heard. It was driving my AKG K1000 headphones to sufficient volume without much distortion, and the overall sound signature was much better than I have heard with many other amplifiers. The conclusion from this experience was simple, I should build one of these for myself 🙂

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DIY Bookshelf Speaker Stands

This one is going to be the first, and one of the only posts I share, that isn’t electronics related. However, it is definitely audio related, as it is about a pair of stands I’ve built for bookshelf speakers. While I have been using mostly floor-standing speakers for some years now, every now and then I did use bookshelf speakers. This is especially true when talking about placing speakers next to my work bench, where space is at a premium, and large floor-standers aren’t really needed.

Therefore, in the spirit of DIY, I’ve decided its time to build a nice looking pair of stands that will be used for this purpose. I’ve looked at quite a few stands, but decided to take the shape from a pair of Sonus-Faber stands that I really liked. This was only fitting, as at the time I’ve had a pair of Concerto Home bookshelf speakers to match them. Since I have neither the tools, nor the experience, of working with steel, I’ve opted for a build made entirely out of MDF. In this post I’ll try to share some of the methods I’ve used in building these stands. I think they are quite straight forward and easy to follow, so that even a novice (like me) could use them quite easily.

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