β22 Headphone Amplifier ~ Build Log ~

Work of art in the making.

Thanks for the kind words! And apologies folks for the delay posting.

Originally I planned around two months for the fabrication of both cases, including as much drilling as possible. My M³ took a month from conception to completion, it's surprising how much work there is!

The first thing I began work on was, of all things, the wood sides. I spotted some chopping boards while I was shopping, they were the right thickness so bought two. Cut them both length ways to give me four pieces, these were then cut to the length needed, the ends rounded off and then the width planed down. Each measured 12" x 4 1/2" -  the maximum height and depth dimensions of the enclosures.

Similarly to the previous build everything is held together by the side brackets - I then cut 4 pieces of 1/8" (3mm) thick aluminium taking away just over 1/8" off each edge to allow for the panels to sit flush with the wood sides, 1" angle was then bolted to the top, bottom and ends of each plate by countersunk m4 bolts.

After cutting the various outside panels at the same 8" width and their respective depth or height, taped the boxes together to test the fit.

Now it's time start measuring and drilling the umpteen various holes for the panel mounted components and air vents, etc.

- One of the top panels after drilling the 30mm air vents.

Most likely, when looking at the back of an audio component, the power inlet will be on the right and usually the audio inputs/outputs will be located on the left side of the unit. This is of course is simply to keep the AC wiring and the transformer away from the audio signal. I've stuck with this layout and placed the DC in the middle of the PSU and amp so even if both are laid down side by side (with PSU to the left) or on top of one another, non of the wiring should cross any other. That then means the input selector will have to be located on the side of incoming signal and the volume pot on the opposite side of the amp.

After all the holes have been drilled I can then remove the burr, clean up the panels and prepare for brushing and finishing them. I used the same basic method as before, of pulling 60 grit production paper wrapped round a block of wood repeatedly towards me, then finishing with a couple of coats of clear lacquer.

One thing I wanted to avoid was having visible bolt heads on the face plates, so these were bonded to the fixing brackets with Terostat 9220 automotive panel adhesive.

I've chosen to mount both controls at the rear of the unit to keep the signal paths short, even so It's recommend that the length of the wire after the pot must be kept as short as possible, as the high impedance of the cable will pick up noise. I won't know for sure until I've actually build the amp and got it working if or how much interference it will pick up, as my case is quite small I'll have limited room for routing the wiring so there is a possibility I will have to use shielded wire.

I ordered a couple of shaft extension kits that included the rod, mounting bracket, coupler and mounting bush, they were a little rough around the edges... but nothing that a file couldn't fix.

The input switch is made by Seiden, it's of very good quality. I've used a cheap ALPS pot just as a pattern, I haven't decided yet if i'll use another TKD pot or a stepped attenuator this time, I'm rather tempted to try one from Khozmo

I like the look and feel of the knobs from Audio Note, these are 30mm diameter chrome plated solid brass.

That pretty much covers the case work for now.

This is a great thread. I am looking forward to the next instalment.
Thanks.

My attempt to make a Cmoy was a complete failure, I am tempted to try again.

Cheers Crossie - glad you're enjoying it.

I had problems with my first Cmoy too, ended up having to sling it away. But I've made four of them now and they all work fine! So it's worth giving it another shot.

Thanks for sharing that Lonely Boy, I will have another go.

Just a quick update.

I've just put an order in for a Khozmo 50k stepped attenuator, this thing costs $179 - Can't wait to get my hands on it!

Also I'm just about to finalise my (hopefully) last order from Mouser for most of the remaining parts. The only thing I'm struggling to find is a suitable transformer. If everything goes to plan, I can start building it next weekend.

Fantastic! This is the amp I have built and use. I went for the 2 case, 1xo22 and 3 channels of B22 config, in Glaxay Mag. cases from modushop. It is great to see you doing the case work from scratch - I'm proficient in the workshop but never had the patience to design and build them myself.

If it helps, I used the following transformer from Conrad Electronics:

http://www.conrad.com/Toroidal-transformer-2X30V-80VA.htm?websale7=conrad-int&pi=518536&ci=SHOP_AREA_17430_2170280&Ctx={ver/7/ver}{st/3ec/st}{cmd/0/cmd}{m/websale/m}{s/conrad-int/s}{l/int/l}{sf/%3Cs1%3E518536%3C/s1%3E/sf}{p1/c35bce154ca6036449b8ced3737fd0fd/p1}{md5/5096f873325dd20e4bd9bc9087484d7d/md5}

Fits the bill just right and isn't too dear.

It was great fun building soldering the boards - the first one I built, though, I didn't fully appreciate the sheer number of components!

Brilliant - I've been looking everywhere but could only find 2x25v types, this is exactly what I've been after, thanks for the link.

Yep the casework is really time consuming - still haven't finished it even after nearly three months, also I realise now that they could be just a little bit bigger!

Pleasure

The sheer size of the B22 always amazes me - 2 cases of nearly the same size as my integrated speaker amplifier. Weighs more, too. Stark difference to the size of the headphone amplifier which fits in those tiny iPod Nanos!

I know this is an old thread but as this project is now officially completely finished (hurrah!) I thought that it was high time we had an update.

The amp has been functioning happily since July 2011, unfortunately a combination of problems with the build and life in general getting in the way contributed to me loosing interest in this project a number of times.

According to my notes, work started on the actual electronic bits around early May. This involved, firstly, filling the PCBs with a plethora of components. What should have been the easiest and therefore most boring part of the build turned out otherwise!

One factor contributing to the difficulty and time of a build is sourcing the parts and sufficent numbers thereof. As mentioned before, the b22 features a lot of components, most are easy to obtain and are reasonably priced, such as resistors and capacitors but we are dealing with a complex discrete design and that means finding an abundance of the various types of semiconductors.

Very basically three types of transistors are used over three stages.

Firstly we will find the J-FETs (Junction Field Effect Transistor) in the input stage, these have low operating noise but also are useful as they have a high input impedance and present a low output impedance to the next stage, so they can be used as a buffer as well as allowing direct coupling.

Secondly the voltage gain stage features BJTs (Bipolar Junction Transistor), these are the simplest (basically two diodes) and earliest type of transistor. This is where the incoming signal voltage is multiplied, therefore making it louder. In this build I've decided to set the gain at five times the input voltage.

The output stage uses MOSFETs (Metal Oxide Semiconductor FET) these are useful for high current applications and will provide ample power for low impedance headphones.

All three stages operate in class-A, the topology features cascoded transistors ( a way of stacking devices for better performance) and is directly coupled thoughout, so there are no capacitors directly in the signal path.

Time for some pretty pictures and my first bodge.

Test fitting of the three amp boards, it's a bit tight in there...

This is what happens when you don't think things through properly, the case is obviously too small, there isn't sufficent space for proper ventilation. This isn't a complete disaster as I drilled holes for mounting two boards in case I decided just to build a two channel version.

The problem stems from not having the PCBs at hand when designing and making the case, lesson learned, it seems that it's a good idea to have everything you will need beforehand so you know how exactly things will physically fit together.

The case needs to be much bigger than you think it needs to be for accessing the fiddly bits, even though I have small hands working in some parts of the case was a nightmare.

This is the intended method for mounting, the bolts are countersunk into the angle.

Time to actually stuff the boards!

I began by checking the parts I had against the parts list and ordering them appropriately, ie R1, R2, etc. All four of the boards, including the sigma, were done at the same time, so all resistors fitted, than diodes, etc.

As usual, the lowest in height components go in first.

The sigma PCB. The ground planes on all the boards are quite thick and require a little more heat and the use of flux for a proper joint.

Things are shaping up nicely! Actually no they aren't, this is where I made another bodge. Didn't know it at the time.

Almost complete, just the MOSFETs and heatsinks need to be fitted, and then the problems start!

I can't find the sodding backup disc with pictures and notes concerning the next part so I'm just going to mention an important factor when dealing with transistors in general.

As the gain of transistors varies not only across different brands and models but between the actual units themselves this particular amplifier will benefit from devices matched within certain parameters.

BJTs are easy to match as a lot of Digital Multimeters have the ability to do this. In this case a similar hFE, or forward current gain, is required. This is simply achieved by selecting the relevant setting and plugging the device into the correct connectors.

The other devices are a little bit more complicated. The JFETs (part numbers Q1 to Q4) need matching for idss within a tolerance of 10%. As four of these (two each of N-channel and P-channel type) are used per board, that means ordering plenty. The output MOSFETs will need matching for VGS within 0.3volts.

AMB sell matched sets but I wanted to do this myself so ordered untested items from B&D Enterprises.

I put together a quick circuit for testing each type of transistor, JFETs go on the left and MOSFETs on the right, a 12volt DC adaptor provides power and the values can be read with a DMM.

After all of the components have been soldered in place, it's time to wire up and test the sigma power supply. This went without a hitch and I got pretty close to +/- 30 volts.

Next come the amp boards. I really thought at the time that it wouldn't be long before there would be sweet sounds emanating from my beta. But during the setup procedure it became apparent that I had made a big gaff and fitted all of the matched JFETs the wrong way round!

It turns out that the silkscreen outline on the PCBs is for another as yet unavailable on the market transistor, the JFETs are mounted another way, obviously I had overlooked this minor detail. At this point I would implore any potential builder of this amplifier to make sure you fully understand what you are doing. Pulling stuff off of boards is no fun at all.

Anyway, after ordering copious amounts of ready matched sets from B&D the boards were ready for action.

Apart from a couple of cold joints the left and right channels worked perfectly. The ground channel was misbehaving and could have been damaged during the initial power up. It was then that I decided to go with a two channel configuration and put the defective board aside for later.

Wow! Is is a seriously good DIY! 

How much did it cost you?