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.
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.
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.
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 🙂
Some time ago I was playing around quite a bit with vintage audio amplifiers/receivers, and in many of them I was improving the power supply portion for the low current differential amplifier stages. This was always a simple and cheap task, that proved well worth the time when it came to sound. In a desire to “do this differently”, I didn’t want to use an IC for this, but rather wanted to go with a discrete yet simple design. The circuit I came up with was very well suited for such applications, and I therefore decided it would be a good idea to make an independent regulator PCB out of it for general use in audio stuff I build. At the time I also had limited experience with PCB design, so this seemed like a great project to start with. There’s no better way to learn than simply giving it a try.
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.
Like any vintage audio equipment user knows, one of the main issues of having such gear is the occasional problem that will need fixing. This project started because of one of these problem. As part of a discussion on another internet forum, one forum member (lets call him Tom :)) approached me and asked if I would be interested in helping him sort out a problem with a Sansui 9090DB receiver. This is a fairly old, but highly regarded piece of audio gear. The problem he described was the main amplifier board, named F2624, which is also shared with a few other Sansui models. These boards became unreliable over the years, with problematic traces, difficult to obtain parts, and so on. At the time I was just a M.Sc. student and had the free time to do things just for fun, so I jumped aboard. Following a short discussion we’ve agreed we will aim at creating a whole new board, that will be a plug-in replacement for the original F2624, but with modern parts.
The first step, was obviously getting a hold of the schematic. However, as it turns out, it doesn’t really exist anywhere that I know of. This is because the schematic that was published in the Sansui service manual was of an early version of the board, and had a couple of mistakes in it. So what do you do when you can’t get the original schematic? You draw one yourself. So that is exactly what I did, by getting an original Sansui board, and going over it. Again, as far as I know, this is the only complete schematic of this board version that is available online. Therefore, I would like to share it with others who might need it one day to repair such a board. We (myself, and Tom) have already published this on 2 internet forums to make it available to anyone who needs it. I will also include it as an attachment at the end of this post.
A DC protection circuit is typically included at the output of most audio amplifiers, and is meant to disconnect the loudspeakers if a significant DC component is present at the amplifiers output. This is important as excess DC current through the loudspeaker (or headphones) will generate significant heat and can damage it. Unlike commercial products, most DIY builds I’ve seen over the years, don’t include a DC protection at the output. This of course leaves the loudspeaker/headphones connected to it vulnerable in case of a problem in the amplifier. Therefore, when I was planning one of my previous amplifier builds, I’ve decided I should first design, a DC protection circuit to add the output of the amplifier. I’ve decided to slightly enhance the circuit to include a few extra features other than just DC protection, and make it as versatile as possible:
Supply voltage of +/-12V to +/-75V (or single 24V-150V supply)
Wide input swing of +/-55V
Support single-ended/balanced/active-ground amplifiers
2 channels input per board
Independent detection per channel for a robust design
Support 2 outputs (A/B/A+B) with relay switching
Visual notification(LED) of active output, and fault
Accelerated shut-down for reduced “popping” noise
40mA supply current with single relay energized
Up to 8A load current with default relay
The circuit can obviously be modified if needed for a simpler build (no output selection for instance), and can be extended as far as voltage range is concerned. For instance, I have since used the same board with a few less parts with a single 12V supply for the output of a small headphone amplifier.