“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 on 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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DIY Discrete and Simple Voltage Regulator

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.

Fig. 1. Toli’s Minimal Voltage Regulator Schematic

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Enhanced DC Protection Circuit

A DC protection circuit is typically included at the output of most audio amplifier, 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:

Main Features/Specifications:

  • 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
  • Adjustable sensitivity
  • 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
  • Delayed start-up
  • 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 (not 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.

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