Clip-on Portable Sub-mini Tube DAC & Headphone Amplifier

Hello fellow makers,

In this Instructable I want to share with you how I created this portable USB digital to analogue converter and headphone amplifier using some new/old stock Soviet sub-miniature tubes.

The inspiration for this project came from one of my previous Instructables where I created a Mini Tube Headphone Amplifier/DAC using a 12AU7 tube, while that one works great I wanted something with a much lower current draw that I can clip onto my laptop bag and take with me to enjoy my music directly from my phone.

This initial design uses an NJM4556 op-amp for the output stage but I'm also working on a design with a pure class-A output stage.

Let's get started...

To make this Instructable you will need the following:

• Access to a 3D printer with PETG or ABS filament

Amazon - Ender 3 3D Printer

• 2X Soviet Sub-mini Tubes (1J18B, 1J24B, 1J29B or similar)

Amazon - 1J18B Tube

• USB Sound card

Amazon - USB Sound card

• 3.5mm Audio jack

Amazon - 3.5mm jack

• Single sided copper PCB

Amazon - PCB

-or-

• Prototyping PCB

Amazon - PCB kit

• Photoresist Film

Amazon - Photoresist Film

• Step up module

Amazon - Boost module

• 4556 or similar op-amp
• Wire
• Soldering Iron and solder

Resistors:

• 2x 1M
• 2x 47K
• 2X 1K
• 2x 22R
• 1x 680R

Amazon - SMD Components Kit

Capacitors:

• 2x 0.1uf
• 2x 220uf+

*As an Amazon Associate I receive a small percentage from sales made through provided links at no cost to you, this helps fund future projects.

Soviet Subminiature Tubes are constructed entirely differently from other subminiature tubes. The internal structure of conventional subminiature Tubes and is easily recognized as a miniaturized version of the classic receiving tube structure, with a filamentary or a unipotential cathode, one or more thin wire ladders as grids, perhaps beam forming plates and, lastly, a classic anode plate.

The Subminiature Tube design is hardly recognizable as a triode, tetrode or pentode. The cathode is still a conventional filament, but the remaining elements are all constructed from similar gauge metal rods.

Three mica disks; 2 at the ends and one in the center of the tube is used to hold the rods in place, with the central mica assuring low microphonics, and consistent element alignment.

The rods are arranged to control the electron path from the filament to the a diametrically opposed pair of flattened rods that serve as the anode. The anode connection is always via the top wire.

You can find the full article and more information on sub-miniature tubes over at Radiomuseum.org

To make a PCB with a cheap 0.5Watt laser module have a look at my Instructable - Create High Quality PCB's at Home With Cheap Laser Module

Because I'm currently busy rebuilding my laser I had to find another way to transfer my design to the PCB. After some reading up and experimenting I found that printing out the design using a normal inkjet printer and then spraying the paper with some WD-40/penetrating oil made it transparent enough to expose the photoresist film with satisfactory detail.

First we need to prep our copper board, take some fine steel wool and give the copper a good scrub, be thorough making sure you remove all of the oxidation.

After the steel wool you should be left with a shiny copper board, now we need to take some acetone or alcohol and with a clean cloth wipe the entire board clean making sure not to touch any part with your bare hands.

Next we need to cut a piece of the photoresist film that is just slightly larger than your PCB design.

- Always work with the photoresist film in a dark room or you will expose the film -

Now remove the protective plastic film from the back of the photoresist film and lay it on your PCB, we will now need to heat up the board to make the film adhere.

For this I use the heated bed of my 3d printer, simply heat it up to 80-100degrees and leave the board on it for a minute.

When the board is heated take a soft cloth and rub the film onto the board, making sure to remove any air bubbles.

We are now ready to expose the design.

Place the design sprayed with oil onto the PCB with film and lay a pane of glass on top of it, now place your PCB in direct sun for about 30 seconds to a minute.

Now we need to develop the film.

Add 1 teaspoon of washing soda to about 200ml of water, you want about a 3% solution. Adjust this according to your specific brand of photoresist film. It's best to first try with a spare piece of film, you want it to take about 3 minutes to dissolve. If your mixture is too strong you'll also disolve the developed film.

Now remove the top protective film and submerge your PCB with the film into the solution. You will see the undeveloped film starts to get opaque and dissolve into the water.

Agitate your solution until all the undeveloped film has dissolved and you are just left with the dark blue developed resist.

Rinse with clean water.

Etch your PCB:

- WEAR ALL NECESSARY SAFETY EQUIPMENT! -

In a well ventilated room or outside mix two parts hydrogen peroxide and one part of hydrochloric acid together in a separate container. Ferric Chloride can also be used instead of this solution if you have some available. (this is your etching solution so handle with care)

Tip: The hydrogen peroxide + hydrochloric acid is a safer alternative to Ferric Chloride and is great to use when making your own PCB's

Carefully submerge the PCB into the etching solution, the copper that needs to be etched has to be completely submerged and facing upwards.

Agitate the mixture regularly.

Leave it in the solution until it has dissolved all of the exposed copper leaving behind only the copper under the resist.

When done rinse thoroughly in clean water.

I used Fusion 360 to design the case, it features a hoop around the tubes for protection and a carabiner clip to attach it to your jeans or bag to take your tunes with you.

The enclosure consists of two printed parts, the main case and the back lid. I designed it with no crazy overhangs so its a very simple print with no supports needed saving you a ton of filament.

I would also recommend printing with PETG, ASA or ABS for the added durability..

My settings are.

Material: Black ASA

Speed: 60mm/s

Temp: 265 deg C

Nozzle: 0.6mm

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

Now that your case has finished printing it's time to smooth out any imperfections and give it a light coat of paint...

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 surface imperfections, try and get rid of most of those pesky layer lines.

After you've removed most of the blemishes with the 220 grit paper you want to spray your case with some plastic primer, do this in thin coats and wait for each coat to dry completely. As I was happy with the smoothness of my case after the 220 grit paper I went ahead an sanded it smooth with some 400 grit, skipping the primer.

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.

Now we need to shape the "spring" of our carabiner.

You will need a piece of 2.5-3mm spring steel rod to make the clip out of, I used a piece leftover from an old car antenna but you can also buy some at Amazon - Stainless steel rod

Starting with a 140mm piece of rod bend it at 10mm -> 50mm -> 35mm -> 45mm -> 10mm as shown in the picture.

If your spring is slightly to long or to short to sit in the slot of the carabiner you can use a heatgun to carefully heat up the back of the clip until you can either bend it up or down.

Next you want to pull the two ends of the bent rod away from each other like in the first image, this will create the spring action when placed into the clip.

Now you can place the spring into the two corresponding holes in the case and test it out. If the clip stays open instead of closing you need to take it out and just bend the two ends to the opposite direction.

First we will want to solder in all the smd components as it is easiest when there is nothing else on the board.

Start by soldering in the 6 smd resistors ( 47k, 1k and ~22E ), then the capacitor above the boost module ( I used a 390nf ).

I then soldered in 4 header pins where the boost module will be and then solder in the module.

We can now start adding all of the through hole components, you will notice that I didn't add the input capacitor and resistor on the PCB and it's because they are already on the DAC module.

Now for the tubes, I bent the legs into place roughly how they will be connected onto the PCB making sure that they are not touching each other. Solder them into place, note that the center input leg is not connected to the PCB but directly to the output from the DAC. Solder two nicely coulered wires at the 12V pins going to the top connection of the tubes onto the PCB.

I added some hotmelt glue around the pins of the tubes to dampen vibrations and to add some support.

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 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 PCB.

Make sure to feed the USB wire through the case before soldering it onto the DAC.

Finally solder the three wires going to the headphone jack.

Circuit information:

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).

Now for the final assembly...

Slide the PCB into the case making sure not to put too much stress on the tubes, the PCB should sit on a small shoulder on the sides of the case. I used some of the hotmelt glue to keep it in place.

Next I slipped some silicone o-rings over the two tubes to give it a nice finish and provide some extra vibration dampening but it is not necessary if you don't have any.

Then we need to solder our top connection of the tube, make a hole in the case where you would like the wires to exit from this can be like mine from the top, it can be from the sides or even from between the two tubes. I put two pieces of heatshrink over the two wires to finish of the connection when done then soldered the wires onto the ends of the tubes.

Now push the headphone jack through the hole and tighten it with the provided nut.

Arrange all of the wires and DAC module in the case and then we can close it up.

Depending on the tolerances of your printer the lid should push in snugly and be held in place by the layer lines but I added a tiny bead of B-6000 glue to make sure it stays in place.

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.

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Happy making!

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