Easy Tube Headphone Amplifier/DAC
by JGJMatt in Circuits > Electronics
12010 Views, 137 Favorites, 0 Comments
Easy Tube Headphone Amplifier/DAC
Hello fellow makers,
This is a project I've been planning for a long time but never got around to.
I wanted the well known CMOY type headphone amplifier / pre-amp with a starved cathode tube input and then it had to run on a USB's 5V.
When I was busy designing it something else hit me as well....the input
There are a bunch of really cheap USB DAC's available on Amazon that don't sound bad at all when used connected to an amp but they fall short on driving headphones and often have very little bass response. So I decided why not use one of these as my input!
To see how everything turned out and to learn how to make one for yourself join me on my journey...
What You Will Need:
To make this Instructable you will need the following:
- Access to a 3D printer with PETG or ABS filament
- 12AU7 Tube
- 9-pin Tube socket
- USB Sound card
- 3.5mm Audio jack
- 50K Potentiometer
- Prototyping PCB
- MT3608 Step up
- Potentiometer knob of your choice
- 4556 or similar op-amp
- 5mm LED
- Wire
- Soldering Iron and solder
Resistors:
- 2x 1M
- 2x 47K
- 2X 1K
- 2x 22R
- 1x 680R
Capacitors:
- 2x 0.1uf
- 2x 220uf+
Design and Print:
I used Fusion 360 to create the enclosure, I wanted it as small as possible but also wanted it to include some protection for the tube when I throw it into my laptop bag.
The enclosure consists of three printed parts, the main enclosure, the top tube cage and the back lid. It would be possible to print the main enclosure and tube cage as one piece if you use supports.
I would also recommend printing with PETG or ABS as the tube heaters gives off quite a bit of heat.
My settings are.
Enclosure:
Material: Sunlu Black PETG
Speed: 40mm/s
Temp: 250 deg C
Nozzle: 0.4mm
Files: I have included the .stl, .3mf files and also the fusion 360 files so that you can edit it to suit your needs.
You can also find it here - TINKERCAD
Prime and Paint:
After your enclosure has finished printing it's time to put in some elbow grease...
First I like to take a coarse grit sanding paper like 180 to 220 grit and with a sanding block (this amazing little sanding block that I used can be found on Thingiverse) and get rid of most of the imperfections.
PLA and PETG can be a little difficult to sand so I sometimes use very coarse 80 grit paper to get rid of imperfections and then just go over it again with 220 grit.
After you've removed most of the blemishes with the 220 grit paper you want to spray your parts with some plastic primer, do this in thin coats and wait for each coat to dry completely.
TIP: Spray a very light coat of matt black paint over your primer before sanding, this will highlight any imperfections as you are busy sanding.
Wait for your primer to dry completely and then it's time to get sanding again...
Sand the primer with water and some 400 grit paper to get rid of as many imperfections as possible, you will also want to use a sanding block for most of the primer sanding otherwise you wont get nice clean edges.
When you are happy with the look of your primed and sanded parts you can go ahead and give them a few coats in the colour of your choice, I chose a classic satin black for mine.
Spray your parts by following the instructions included by the paint manufacturer.
Information on the Circuit:
This is a grounded cathode voltage amp followed by an opamp buffer.
The tube stage:
The triode is fixed-biased by the grid-charge current that flows through the 1meg resistor.
The hot cathode always emitts some electrons that have enough energy to fly up to the grid.
The grid resistor closes the current path between grid and cathode so that the grid discharges across the resistor and following Ohm´s law builds up a negative voltage between grid and cathode.
This way of biasing works for many small signal tubes like 12AU7, 6DJ8, ECC88, 6922 but also for 12AT7, 12AX7 etc.
Output capacitor (C2):
This capacitor is very important to prevent feeding DC current to your headphones.
Warning: Never go without output cap, otherwise your headphones (and probably your ears) will be ruined.
You may use an electrolytic capacitor of 220uF for headphones above 32 Ohms or 470uf if your headphones are below 32 Ohms.
Some people put a small film cap in parallel with the electrolytic to have better high frequency response.
Important: Check the polarity of the cap. The + side of the cap goes toward the opamp output!
The bleeder resistor (R3):
This resistor goes to ground and will allow the cap to charge at power on, this helps to minimise the POP when you connect the headphones.:
You should connect / disconnect the headphones always if the amp is up and running.
The bleeder resistor should be about 10 times higher than the value of the headphone impedance plus the output resistor value.
The output resistor (R4):
The output resistor should be between 22 and 150 Ohm depending on which final gain the amp should have as the output resistor forms a a voltage divider together with the headphone impedance.
As a rule of thumb choose the low value for high impedance headphones (>=250 Ohm) and the high value for low impedance headphones (32 Ohm).
The Electronics:
Now to start populating our PCB!
I started with a 40mm wide prototyping PCB that I cut down to ~34mm long so that it fits into the slot of the enclosure.
Then you can populate your PCB according to the schematic, it was a tight squeeze but everything fits.
Next I removed the plastic housing of the USB sound card ( a slight tap with a hammer on the side of the case should break any glue used to keep it in place ) and desoldered the headphone and microphone jacks.
You can now solder on two power wires to the USB pins as pictured (*more on this below) and also three wires from the headphone jack. The three headphone wires will go to the input on the PCB and the two power wires will be soldered on the input of your boost modules. From the boost module I soldered two positive and two negative wires, one pair going to the tube heaters and the other pair to the PCB.
On the tube socket I started by adding a blue 5mm led to the center that I connected to the V+ and ground of the heaters via a 680R resistor.
Next solder wires to the relevant pins and solder these wires to their points on the PCB as per schematic.
*Note that during testing everything worked as planed but when I tried to use it with my old laptop at my workshop it couldn't supply enough current to turn it on. To solve this problem I added an additional power input socket on the back of my prototype that I can plug in a 5V 1amp power supply. The current design should work fine on USB C devices.
Assembly:
Now to assemble all of the electronics inside the enclosure...
First we need to attach the tube socket to the inside, at first I wanted to add screw holes to the enclosure but seeing as most of the sockets available on Amazon are round PCB mount types I decided to just keep it "glue into place". If you want to add the holes it is already sketched in the fusion 360 file so you can just cut them and change the inside of the tube cage a bit.
I use my trusty 2-part CA glue & activator for all my gluing needs! (trust me the bonding power of this glue is crazy)
With the tube socket in place we can move onto the main PCB, with everything soldered correctly it should just slide snugly into place in the guides printed inside the enclosure. Unfortunately I measured wrong and my holes were 1mm too high that's why in the pictures my PCB isn't inside these guides but I did fix the 3D files that are provided here.
Now just tighten the potentiometer to the front of the enclosure with the provided nut.
We can now glue the USB DAC into the back lid, in the picture you can also see the additional plug I had to add for the 1A 5V supply.
We can now go ahead and close the enclosure up using 4 self tapping 3mm screws.
Next we insert our tube, line up the pins of the 12au7 to the socket and then with a rolling motion insert the tube.
Tube Rolling:
So what is tube rolling you ask?
Well tube rolling is the process of trying out a number of tubes in an amplifier and selecting the one that sounds best to you. This can be very helpful in optimizing the tone of the amplifier to your personal taste.
I would suggest you get a few differant tubes to try out, new old stock Russian tubes from eBay, new Chinese tubes from Amazon, pick some up at antique stores on your next road trip or get some of grandpa's old stock from the basement!
But before placing a new tube into your amp just make sure that it says 12AU7 on it or an equivalent.
You can use sites like Radiomuseum to look up any tubes you find.
Troubleshoot:
Troubleshooting your build:
My amplifier won't turn on.
- Check that all of your connections are properly soldered
- Double check your PCB against schematic
- Power is not sufficient, try another USB port or one of these DUAL POWER USB CABLE
One side is dead or crackling.
- Make sure that the tube is properly seated in the socket.
- Try another tube
Enjoy!
Now you can sit back, put on your headphones and enjoy some sweet tunes on your new creation.
I hope you guys enjoyed this Instructable and if you have any questions please feel free to leave me a comment bellow.