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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Effective Use of the Capacitance Multiplier for Voltage Regulators

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.

Fig. 1. Simplified Regulator Schematic

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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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“Upgrading” a Pimeta v1 Headphone Amplifier

Like I’ve mentioned in the M³ post, there are a few headphone amplifiers that I was always fond of. They weren’t always expensive or exotic amplifiers, but they simply sounded right to my ears. One of these is the Pimeta from Tangent. I first came across the Pimeta quite a few years ago when one of my friends had a portable unit powered from a battery. A few tears later, one of the amplifiers I’ve built was a Pimeta, that ran from a 24V regulated PS. Over the years that amplified has been modified a few times to suit the needs of the time, including a reduction of gain and PS voltage to fit more sensitive headphones and a smaller case. Recently, after a few years of not using it, I’ve had a renewed need for, and decided it was as good of a reason as any to give it a little “upgrade”. This post is meant to share those modifications, as well as give the Pimeta some more attention, as I think its a great little amp that isn’t getting enough love on the forums.

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DIY Programmable Power-Supply for Vacuum Tubes – Part 1

Preface: up until now, all posts I’ve shared were completed in a single post. This was due to the fact I’ve waited until I was done with it and only then posted. This allowed me to assemble/verify (when needed), and was much more comprehensive for readers. However, lately I’m finding it more difficult to find the time to cross items off my “diy to-do” list. Quite a few items get stuck for long periods of time in the design stage, due to lack of time to move it forward and complete the board layout/assembly/testing. Therefore, I’ve decided to gradually post a few of these on the blog as parts of a project. This post will be the first of a few such projects that will be split into several parts. Hopefully, even sharing partial information such as schematics will prove useful to some readers. </end preface>

One of the items that was on my “wish-list” for quite some time is a programmable power-supply (PS) that will be fit for work with vacuum tubes. The main reason I need it is because I’m missing is a high-voltage PS that can reach as high as 400V or even higher. Therefore, this was the main objective of the design I will present in this post. However, seeing as most transformers that are intended for these uses include a low voltage secondary winding for the heaters, it makes sense to have another channel that can supply the heater rail too.

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Repair of a Dual 60V/12A Switching Programmable Power Supply

A few months ago I came across a faulty programmable power-supply (PS) with a 60V/12A maximum rating on each of its two channels. The exact model is DTPS6012 from Horizon, a company I’m familiar with as I’ve used and owned a few of their linear PS’s (such as the DHR40-1). The problem that was observed during initial check at the seller’s  location was that upon power up one of the channels behaved as expected, while the other wasn’t regulating the output voltage. The voltage just kept on rising until it was ~10% over the 60V rating, at which point the over-voltage-protection (OVP) kicked in and switched off the entire unit except for the front panel.  Because the unit was faulty the price was quite low, so I’ve decided to purchase it and try and fix it. At the very least this could be an opportunity to have a look inside and learn how these things were built back then.

I should note that such a high power rating PS is more than I will probably ever need for my projects. However, I have had some projects in the past where the 2x3A rating of my existing PS’s wasn’t enough, even when I’ve used two such units. Therefore, a more capable PS, even if its not as low noise and ripple, is always welcome. Additionally, as I’ve noted earlier, I have owned and used elsewhere other PS’s from Horizon. I was always happy with the build quality and performance, especially for the price these things could be had on the used market.

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DIY CNC Machine for Hobby Use

One of the coolest tools you can have as a DIY’er is without a doubt a CNC machine. Nowadays, you can even buy one for a relatively low sum in the form of a kit, straight from eBay/AliExpress. A few years ago, while I was still a student, I’ve decided to build one myself. I’ve decided against a kit for two main reasons. The first was the cost, at the time these kits weren’t as wide-spread and cheap as they are now, and I was concerned with cost. The second being the desire to do something of my own, and learn in the process. My aim was to build a machine that will be sufficient for my needs, which means engraving panels for my other projects, as well as some work with wood (MDF mostly).

Since this was meant to be a learning project, I didn’t jump straight into buying everything, but instead took it step by step. As a first step, I went to one of the local junk-yards and bought a couple of stepper motors, along with a disassembled industrial scanner. It was very cheap, and seemed like a solid base to modify for use as the X-axis of the machine. After taking it apart for some well needed cleaning, and putting it back together it actually looked in good condition. It uses a belt to drive the frame, coupled to a Lin Eng. stepper with a 90degree gear-box. The belt is reinforced with some steel wires, so it seemed like it will suffice for my limited needs.

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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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