M³ Headphone Amplifier ~ DIY Build Log ~

[lonely boy]

Thought this maybe of interest to some of you - I recently built an M3 headphone amplifier over the month of October last year - this is a log in a series of posts documenting the process. These posts will be quite long but I'll try and keep them as concise as possible. ~ Introduction ~ I'm relatively new to DIY electronics, my first attempt was around Christmas of 2009 when I built a Starving Student valve hybrid amp, until then I had no experience and very little knowledge about electronics in general. I have since built another Starving Student (a 12AU7 design) and a few CMoys. The M3, or M cubed as its pronounced, is a high-end three channel amplifier featuring a MOSFET output stage on each channel, hence its name. As this is a non-commercial design, you either have to build it yourself or commission someone to build it for you. The schematic is open source but the printed circuit board needed to build the amp is available from AMB, an organization that specialize in DIY audio and are also responsible together with members of the DIY community for the design.

Here's a link with more information - http://www.amb.org/audio/mmm/ ~ Design & Casework ~ The desire to build this amp was quite impulsive, the idea having been really at the back of my mind, I wanted to do something a bit further up the chain and had been considering a PIMETA as the next project. The idea for the case was something that I wanted to try for a while, so around the end of september while at work I spotted some 5mm thick aluminum lying around the guillotine, these were once used for temporary bus stop signs, they each measure around a foot square, not even knowing what the sizes should be I roughly estimated and cut 2 at 8" by 10" and 2 at 8" by 4", this is where it all came together and I decided to commit to this project! On returning home I checked the size of the PCB - 5" by 7" - so plenty of room, also I quickly made a 3D mock-up to give me an idea how it would look.

I wasn't originally going to put holes in the top and the volume was going to be centered with the headphone jack to the right, but form must follow function - from researching the amp six MOSFETs put out a bit of heat so I had to have a way for air to flow though the case, I also drilled half a dozen 1/2" holes in the base. The volume pot benefits from being mounted as close, if not, on the board itself otherwise there is the prospect of the wires picking up interference. As you can see there is space for two pots on the pcb, one volume and the other a variable bass boost, I decided not to go with this feature. After a lot of sanding - firstly the thick coating had to be removed, I could see what kind of dents and scratches were on the plates, then after filing the edges, carefully started smoothing them out with higher grade paper and always trying to rub along the "grain". I used a small metal block wrapped with production paper for the edges after filing to get the dimensions as close as possible. To hold the whole thing together I used galvanized steel brackets folded at each end as well as the bottom - this would allow the front, bottom and rear panels to be bolted (countersunk for the later two) and Instead of folding the top I decided on using two pieces of 1/8" thick 1" aluminium angle, that were tapped into so that they could be attached with machine screws, the wood sides also are mounted with screws from inside onto the brackets.

For the finish I decided on a brushed look and light coloured wood side panels to give the amp a fresh clean look. After the box was test assembled to make sure all was well, the aluminium panels were then prepared for brushing, this is something I'd never done before. A side note - I did the brushing/finishing and started assembling the pcb on the 29th October, so the case took around a month, on and off to make. After polishing the panels with 600 grade wet & dry, I then set up a work-mate type bench by wrapping masking tape backwards around one top so that the sticky side is facing outwards, this would hold the panels in place. I then wrapped some 80 grit production paper around a piece of wood wide enough so that I could hold it both sides, this then would allow me to roll my fingers round so that the top of my nails could press against the edge of the plates and keep the block straight as I pull it towards me. It took a bit of practice, but I gradually got the hang of it and the process didn't take as long as I thought it would. To finish I used some fine grade Scotch-brite to smooth out the surface a little, except for a couple of "witness marks" cause by slight sideways movements of my hands, they turned out pretty decent.

All that was left to do was to seal them with a coat of lacquer, the plates were thoroughly washed in washing-up liquid to remove dust, dried and then very carefully cleaned with Isopropyl alcohol to remove finger marks, etc. Then a couple of coats of Plasti-Kote Metal Sealer were applied to protect the surfaces from scratches and oxidization. That's it for now - next we'll start on the electronics!

 

[idc]

A build thread with lovely pictures, my favourite. Nice one Lonely Boy

 

[lonely boy]

Cheers idc - Glad you're enjoying it so far.

Shall try and post some more stuff tonight if possible.

 

[lonely boy]

So now that the case work is prepared, we can move on to the important bit!

Most of the key components were ordered in the first week of October, including the PCB.

AMB as well as sell the blank PCB, also offer some of the components for sale. I decided to order these parts at the same time, this includes the six MOSFETs, JFETs, etc. I also ordered the remaining components at the same time from Mouser and the OP-Amps from Digi-Key. This was everything to complete the construction of the board.

One of the fun things about DIY is that you have the option of choosing the type and quality of the components you want to use. But as I still consider myself a novice, I thought it would be best to build a standard version using the parts as recommended by AMB, only deviating by splashing out on a better quality pot.

So on the afternoon of the 29th of October, after sorting out the various parts and making sure they were the correct values the evening before, I was ready to go!!

First we begin with the blank board and start adding the components, this is the easiest but for the me probably least satisfying part...

The best way to "stuff" a board is to start with the lowest components in height, and gradually move up to the taller parts. This just makes it easier to keep them flattened down on the board when soldering.

- Resistors fitted first.

- Diodes, Op-amp sockets, small capacitors, etc next.

It's also worth taking your time and double checking the components to that you have the right values, observe correct polarity, etc.

- Lastly, the electrolyic capacitors.

Well that didn't take long!

I think this took me around four hours to get this far, including a couple of breaks. All we need to add to the board are the MOSFETs.

As mentioned before, the M3 is a three-channel amplifier, this may sound rather strange as headphones only have a left and right and don't need a centre channel. But in order to keep stereo crosstalk low between both channels some designs implement the use of an active ground, the third channel (a balanced version of an amp such as a á22 uses four channels - L +, L -, R +, R -).

The output stage of each channel uses two MOSFETs , each pair consists of an N-channel and P-Channel type these are used to provide current gain - of course we have to be careful not to mix them up!

The best way to fit MOSFETs is to attach them to the heat sinks first before soldering them to the board. I've applied Antec Formula 5 Thermal compound first then bolted them together but left them slightly loose, this makes it easier to align every thing up. The heat sinks are then screwed to the board and the legs of the MOSFET are soldered. Lastly the bolt is tightened, I did each one at a time starting from the middle, working my way out.

Hey Presto - The board is finished!

 

[scene]

Great thread! I love reading the "Here's one I made earlier threads", I can feel the need to get out the soldering iron.... All I have to do is wait for the kiddies to get a bit older.

 

[lonely boy]

Thanks scene, it's certainly a rewarding way to spend time.

 

[lonely boy]

~ Wiring everything up & final construction ~

(30th October)

Now it is time to wire up the panel components.

Firstly I decided to go with a Bulgin vandal proof switch for the power, this is a latching as opposed to a momentary type so it will stay on without the need for a relay circuit. This is simply wired to the D.C input socket and the board, the inbuilt LED indicator is wired to the appropriate pads on the PCB.

For the RCA input jacks I chose a nice pair by Cardas.

I was after the type that could be mounted internally so that the nut fits on the outside of the panel, I envisaged all the components (except the power switch) being mounted in this fashion to allow construction of the case to be easier.

For internal signal wire I used Kimber TCSS, it's a stranded copper wire with Teflon insulation, this is handy as it doesn't shrink when heated. I had a little problem as it was a little to thick to go in some of the holes on the board! I ended up using Molex connectors for the audio input and soldering a component lead as tightly as possible onto the ends of the output wires.

As good as the Alps Blue velvet pot is I wanted something just a little bit better, but didn't want to pay the earth for it. A view held by some is that the better quality the pot the more transparent and quiet it is, that it shouldn't add noise or distort the incoming signal. Many people use stepped attenuators in their builds as these are considered to be more transparent than potentiometers. Essentially this is a multi position rotary switch with an array of resistors of fixed value in which you control the volume by selecting the desired resistance. They do have a draw back though, that being that you can't select a value between steps.

After doing a bit of research, I chose a 50k TKD 2CP-2511, this was purchased from Parts Connexion in the USA for around $70, this is the single most expensive part of the build. It certainly feels well made and has a very smooth action.

This had to be "air wired" to the pads in the space for the Alps pot, as the connectors are incompatible.

Now that everything is connected, it's simply a case of fixing the board (mounted on stand offs) into the chassis, I'm using M3 metal threads to hold it all together.

The front and back panels are then bolted through the side brackets. I've grounded the pot and RCA connectors to the case, the headphone jack, power switch and D.C connector (more about this later) are all isolated from the case. This should help the keep the amp quiet.

One last step before we can switch on and set up the amp, this involves the op-amps that are responsible for voltage gain part of the amp. A popular choice for the Mü are the Burr-Brown OPA 637, used for the left and right channels and the OPA 627 for the ground channel. As I wanted to build a standard configuration I opted for the Analogue Devices AD8610s, one draw back is that they are surface mounted but the Burr-Browns are more expensive at around £17 each.

In order to use these they have to be soldered to a SOIC-8 to DIP8 Adaptor, I'm using some manufactured by Aries, Browndog are another make available.

So the fun begins!

This is was my first attempt at surface mounted soldering. I'm using Chip Quik 0.20mm dia 63/37 solder for this task.

First step is to identify which way this thing sits, you'll need good light and eyesight for this - pin 1 is indicated by a very small dot.

Firstly I apply some solder to the outer side of each pad on the adaptor, then using a pair of angled tweezers, place the op-amp in position and heat up the solder so that it runs round each foot.

Done! It wasn't as difficult as I thought it would be.

Next - switch on and set-up!

 

[PJPro]

Excellent thread.

What are you going to use for the power supply? Sigma 11?

Didn't know there was a latching AV switch. For my dual mono power amps (uncompleted), I used the E24.

 

[lonely boy]

I'm using a Mascot 24v regulated power supply at the moment but I've been thinking of having a shot at the sigma sometime in the future.

It took a bit of digging to find that switch.

 

[idc]

I think that it is a good idea to splash out on the volume control, too many hifi volume controls are poor IMO especially at fine tuning.

 

[SpiceWeasel]

Looks like a nice little project there lonely boy, really like your brushed steel and wood DIY case work. I bet your case would look lovely with a couple of glowing tubes sticking out of the top

So how does it sound? What phones are you using?

 

[lonely boy]

Without much further ado...

Before we switch on, we must fit the opamps and do some checks and set-up the amp.

The M3 is a Class-A amp, this means that in very simple terms the amp is always passing a current even if it is amplifying a signal or not, this helps with linearity and lowers distortion. The amp has to be biased into Class-A, this is done by adjusting the appropriate trimpots in each channel on the board for the MOSFETs (BIASL,R,G) and opamps (BIASOPL,R,G)

Firstly the ground channel opamp is fitted and the power is turned on...

It's alive!

- Note, I didn't have an isolated jack with me, so temporarily wrapped insulation tape round the plug and jack whilst setting up and the next day mounted it on a piece of plastic bolted to the inside of the back panel.

Using a trimmer tool and multi-meter I set the bias to the opamp at around 0.4v to 0.5v and around 80mv to the MOSFET for the ground channel. I also checked for DC offset, which was pretty much zero. After switching off the amp and installing the left and right channel opamps, I repeated the steps for each channel. This procedure took me just over an hour, I kept the amp on for a while longer and ran through the tests and checked it again, then plugged in a source and an old pair of headphones...

Yes! I can hear music - through both channels with no funny noises!!

Some pictures of the finished amp-

So that's it then - thanks everyone for looking!

 

[lonely boy]

SpiceWeasel:
Looks like a nice little project there lonely boy, really like your brushed steel and wood DIY case work. I bet your case would look lovely with a couple of glowing tubes sticking out of the top

So how does it sound? What phones are you using?

Thanks SpiceWeasel, It sounds pretty good. I mostly use it with both the Sennheiser HD600 and HD650 also the Grado HF2.

I can safely say that I prefer it to my HP8 and Headmaster, even though they are good amps, this one strikes the right balance between being a bit too forward and lively (HP8) and slightly grey sounding and a bit too laid-back (Headmaster).

It also seems to have unearthed a more musical and sweeter sound from my Esoteric player, which before could sound a little sharp and strident. This is easily the best dynamic headphone amp I've heard and I wouldn't hesitate to recommend building one.