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.
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.
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.
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.
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 years 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.
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 a high-voltage PS that can reach as high as 400V or over. 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.
One tool I use quite often when testing amplifier is a dummy load. Because of that I have a large box of high power (50W) resistors, and a large heat-sink that is tapped for easy attachment of the resistors to it. I typically have 8 resistors of 2ohm each, connected according to the requirement of the measurement i’m doing at the moment. More often than not, they are wired as 2 independent 8ohm resistors to measure speaker amplifiers (see Fig. 1). However, when I need to measure headphone amplifiers, I typically only need lower power loads, and therefore use a couple of resistors from the spare parts box. This got frustrating over time, soldering the resistors to a TRS plug, then soldering/clipping on a couple of wires to the scope/other test instrument. Therefore I’ve decided to do a small side-project of building a simple dummy load box for headphone amplifiers testing.
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.