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

This is part 3 in the series of posts discussing the 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 board.  In this post, part 3, I will show some of the measurement results of the assembled board. 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 parts. 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 is meets my target figures to allow measurement of low noise voltage regulators. 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 discussing the 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 topology 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 to 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. 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 mounter voltmeter. Placing it all in a single line seemed impossible, or at least very uncomfortable to use. Therefore I’ve decide to split this into 2 different heights. This put a constraint on the height of the case, and meant I will have to split the design into 2 boards to support this. 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 a 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. The biggest limitation with sound-cards is having a high voltage measurement capability, as most audio amplifiers put out voltages that are significantly higher than what you  can feed into a sound-card. Indeed, this is what most people use such instruments for. However, this is actually just a portion of what could be achieved with such a pre-amplifier.

This post will the first part of a series of posts that will describe my take on a measurement pre-amplifier. I will describe that 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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