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.
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.