This headphone amplifier circuit is different to conventional modern construction techniques in that it is air Wired,P2P (peer to peer) or free form wiring just like in the good old Valve days before the intervention of PCB's and the transistor.
Rather than a traditional enclosure, the hole circuit is encapsulated in polyester resin to enhance the internals.
If your reading this and thinking why do you need an amplifier for headphones then click here
Although allot of cMoy headphone amplifiers are designed to be portable this one is designed for the desktop although a battery pack could be made also.
This is a pretty long instructable so "make a brew" as we say in Yorkshire and get comfy .
On the upside there is plenty of pictures :)
Step 1: The Schematic
The component list is as follows
Power supply section
1x DC Power Jack
1x 5mm LED
R1LED : 1x 1k to 10k 0.6 watt metal film resistor (For the Power LED, Anywhere from 1k to 10k will be good it all depends on the input voltage and how bright you like your LED.)
CP1/2 : 2x 470uf 35 or 50v Power Capacitors
RP1/2 : 2x 4.7k 0.6 watt metal film resistors (For the power supply Voltage divider)
IC1 : 1x OPA2107 Dual Operational Amplifier
C1L/R : 2x Wima MKS 0.68uf 63v Capacitors (for the audio signal input)
C2/3 : 2x 0.1uf Polyester Box capacitors (To stabilise the OP-AMP)
R1LED : 1x 1k 0.6 watt metal film resistor (1/2 Watt)
R2L/R : 2x 100k 0.6 watt metal film resistors (1/2 Watt)
R3L/R : 2x 1k 0.6 watt metal film resistors (1/2 Watt)
R4L/R : 2x 10k 0.6 watt metal film resistors (1/2 Watt)
R5L/R : JUMPERED (optional,)
2x 3.5mm Stereo Jack Sockets
Downloads: EaglePCB .SCH Schematic and PDF below
Step 2: Making the Skeleton
This part is very fiddly !
It will test your bending and soldering skills
Everything has to be visually spot on because everything will be on show for all time when its cast in resin.
To create the power bus I used solid core 1.10mm wire taken from mains twin and earth cable used for the internal house wiring.
Only basic tools are required to construct the skeleton:
Solder (preferably thin gauge)
Flux Pen (optional)
Long nose pliers for bending
Step 3: External Power Supply
You will also need a power supply with a positive center pin this is denoted by the symbol with in the red circle on the picture.
If you detected any hum in you headphones when you test the circuit before the resin pour check all the circuit then try using a different model of power supply.
If the power supply you selected is a cheap wall-wart that contains a transformer (linear power supply) it will no doubt hum though the headphones
Step 4: Wiring the Power Jack
The Middle and side to ground (-Rail)
Step 5: Tip:Getting a nice bend
You can use different diameter screwdrivers for smaller or larger radius bends.
Step 6: Making the Skeleton 2
Here we can see the basic layout of the power supply section
It is a double-ended power supply that takes a single-ended input (12VDC) and splits it with a voltage divider.
The hoops on the right are for the op-amp circuit this requires +/GND/- instead of just +/GND.
What this basically means is the power input for the Burr Brown OPA2107 Operational amplifier or Op-Amp needs -Volts and +Volts the T shaped wire which runs down the middle is the ground or in this case a "virtual ground" produced by the voltage divider it never comes into direct contact with the main power ground incoming from the power jack.
The two 4.7k resistors near the back are the voltage dividers,the supply to the power jack in this case is 12VDC is then halved by the voltage divider producing -6v and +6v on both of the outer copper wires or you could call then buses.
The +V for the LED is fed straight out of the back of the power jack and uses the -6v copper wire for Ground through a 1k Resistor ,as this all comes before the voltage divider as far as the LED is concerned -6v is normal ground.
Now to start adding the other resistors as per the schematic.
Step 7: Making the Skeleton 3
Be careful to check the polarity of the capacitors before soldering
Step 8: Making the Skeleton 4
you can see the loops better in the second picture
In the 3rd picture the bottom 4 Leads can now be connected to the virtual ground (middle copper wire)
Step 9: Making the Skeleton 4
In the second picture the legs from the turquoise resistors are bent to form the output connection which will be hooked up to the headphone jack socket.
The 3rd & 4th picture shows hooking up the audio Input and headphone jacks.
I used enamelled wire from an old transformer to give a consistent look but it also has a good amount of insulation against shorts.
Step 11: Testing
Hopefully it tested ok and is sounding great!
Step 12: Pre Casting Sealing
Both of the audio Jack Socket sides were removed during the sealing process, the sides were then replaced after applying resin all around the edges.
Resin was also placed around all the connections pins around the bottom to ensure an airtight seal.
More resin was used around the bottom of the DC jack.I am hoping the extra resin will not show much in the finished casting.
Step 13: Pre Casting Sealing 2
Step 14: Elevating the Circuit
Step 15: Label the Audio Sockets
After measuring the sockets they were made to and printed to scale in Adobe PhotoShop then printed on thin photo paper then using double sided tape stuck to the socket sides.
Step 16: Making the Mould
I realise that there are better ways to create a mould like using silicone but the aim is to get the sides as square and true as possible as this is a one off project card seemed ideal.
Next I designed the mould templates in EaglePCB then using double sided tape stuck the print out on to the card to be cut.
When it came time for assembly of the mould each corner was tacked in place with super glue till all parts of the mould were together as one at which point I ran more super glue around the entire length of each side.After this had completely dried a second run of glue was applied to ensure the joints were completely sealed.
Downloads: Layout DXF and PDF below
Step 17: A different type of "volume" (updated)
I could have measured the mould with a ruler but this was quicker and gave me an indication of approximate weight of resin needed to fill the volume of the mould, you also have to factor in the displacement of the item being encapsulated.
I estimated water would roughly be a similar density and weight to the resin.
Now you know the volume you need to follow the instructions for the resin you have purchased to find the correct ratio of resin to hardener.
I used Polycraft DSM Synolite Water Clear Casting Resin + MEKP Catalyst (1 to 2%), I believe it is a polyester resin the ratio of Catalyst to Resin was around 1%.
It was pretty hard to measure out the catalyst in such small amounts.
There are many varieties, which all require different ratios of resin to hardener.
So mixing it etc. is really down to the type you use.
Step 18: Mixing the Resin
With the resin mixed I had to ensure I poured it slowly and close to the mould so not to encourage air bubbles.
You can see in the below picture that's there is a dome of resin rising above the mould, this is to allow for shrinkage as the resin cures.
Once the resin is mixed you will not have long to work with it before the curing starts so have everything you need to hand.
Step 19: Curing the Chemical Reaction
A chemical reaction will start and the cast will generate allot of heat this is the curing process at work
I used a no contact thermometer to measure the temperature as it cured
8 minutes in and things are getting hot
At this point the surface starts to gel, it shows as dimpling of the surface.
I left the cast for 24hours to fully harden before starting the next stage.
Step 20: Breaking the Mould
I used Belt sander was squarely clamped well in a vice (please be careful when doing this!)
After some wet sanding with P600 then P1200 Grit paper I was left with the basic shape.
Step 21: Knocking the Edges off
I Knocked off the sharp edges which would be prone to chipping.
The bearing on the router bit follows the flat side cutting an even chamfer around all the edges.
Step 22: Final Polish
I found that T-CUT or Brasso made an excellent buffing polish it literally shined the surface up from a dull finish.
The precautions when sealing the sockets worked rather well and no resin entered the Jack socket cavities,there are a couple of tiny air bubbles but nothing that can really be seen.
The only way to totally eliminate air bubbles would have been to use a vacuum chamber or dome since .Having thought about this I think it may well have forced resin into the air cavities.
One tip if you had a vacuum chamber or dome would be to just vacuum the resin after mixing before the pour as the mixing process introduces some small air bubbles.
Step 23: Precautions
If your using a manufactured power supply such as a wall wart or power brick and the jack has a positive centre this is not really an issue.
In the event of catastrophic failure capacitors are built with a fail safe to release pressure. On the end of the capacitor the cap is scored thus weakening it. This in turn stops the capacitor building too much pressure .
As a safety precaution pilot holes could be drilled as near to the capacitor ends (not into!) as possible.This would act as a weak link or escape valve for any build up of pressure
A diode could also be used to prevent reverse polarity.
Step 24: Testing the Voltage Rails
Although the circuit will no longer be serviceable once cast it will give me an incite in to what may have gone wrong by checking the the virtual ground (the wire stands) against the negative and positive power jack connections.
Here you can see the 12vdc split -6/+6 voltages
Step 25: Running Temperature
Regarding concerns about heat dissipation.......
Here are the results at 12vdc (-6/+6) playing music at above normal levels for 60 minutes
The Meter on the right is measuring ambient temperature of 16c
The Infra red thermo-meter is measuring above the IC chip at 18c
Even when running at 18vdc the temperature only varied by 1c
I already knew the circuit would not produce any significant heat before I began .If this was a concern I would have embedded a small heat sink on the top of the IC revealing itself on the top surface of the casting.
Although there is no metal shielding as you would have in a conventional chassis/PCB the amplifier exhibits no unwanted noise or RF interference as you may associate with an open chassis design such as this it is dead silent even though it is next to my mobile phone and WiFi router.
Electronic engineers have been encapsulating or potting electronics in resin's for decades usually for vibration dampening or moisture control its just I decided to make it look presentable :)