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 🙂
So I started working on that build. Unlike some other DIY builders, I much prefer a single case build. I think that despite the difficulty of minimizing coupling from the transformer, the final results looks and feels better. One way of minimizing coupling is using a shielded transformer, and I chose an Antek Inc AS-0515 for this. I’ve added a CA-050 steel case for it to minimize coupling further. All routing inside the case was made with a shielded wire for similar reasons.
For the power-supply of the amplifier I was aiming at a 30VDC supply, and have chosen to use one of my own PCB’s for the low-noise regulator, with a current limit of 1.5A just in case. Other fail-safes around the amplifier include an active-zener circuit to be used as a clamp in case the regulator output rises to over 35V, and a basic ground-loop-breaker. To protect the headphones in case of fault, I’ve added a DC-protection circuit to the output. Since this build was completed before I have designed the enhanced DC protection circuit, I have opted for one of AMB’s designs, the ε12.
For the amplifier board I have used the tallest heatsinks I could find so that I could operated with sufficient bias current in the output stage. Based on my previous experience with possible op-amps, the OPA627 were chosen. The 627 is compensated internally for stable operation with a gain of 1 (0dB), so it can also be used for the active-ground channel.
Once all boards were assembled and tested independently, it was time to start working on casing. I have ordered a moderately sized aluminium case for eBay, there are quite a few to choose from these days, so it was fairly reasonably priced. I have mounted the transformer and most of the circuitry to the bottom side of the case. However, to minimize coupling from the transformer/power portion further, I’ve added a vertical aluminum plate. Incidentally, it was also well placed to mount the ε12 to it. The ALPS RK27 potentiometer was also mounted on a small PCB, only to keep wiring organized, it will later be mounted to the front-panel.
Next step was to get the panels ready. In the spirit of DIY, I’ve decided to use my CNC machine for this. It was a small machine with limited capabilities that I have built years ago, but it was definitely up to the task. For the back panel all that was needed are some holes for the RCA connectors, along with a ‘L’/’R’ marking, and a larger square cut-out for the IEC module.
For the front panel there was a bit more work. Other than the project name, I wanted to add counter-sinking for both output jacks. A TRS (6.35mm) jack for single-ended headphone, and a 4-pin XLR one for balanced headphone. The M³ isn’t a balanced amplifier, but having a balanced connector will allow me to use such headphones without another external adapter.
I have considered filling the engraved portion with black paint, but was unsatisfied with the way it came out, so I’ve cleaned it up, and left it as in the picture above.
Final step was to power it up and start listening to some music with it. I have used it with quite a few headphones, and was always happy with the result. It has more than sufficient power to drive practically all headphones. The only exception might be the K1000’s where it is more than lou enough for my needs, but might be a tad limited for these who want to feel like they are in a rock concert – in which case I’m not sure the K1000’s are the right choice anyway.