This is going to be the last part of DIY USB-C adapter and micro amplifier post series. Today, I will show, how the amplifier was fully assembled to a fully finished device.
Table of contents
- Previous parts for this topic
- Device’s construction
- Whole assembly and final thoughts
- Links and part list
Previous parts for this topic
- DIY portable headphone amplifier based on an INA1620 chip and TPS65135 (Part 1)
- DIY Micro PCB Headphone Amplifier (Part 2)
- Micro USB-C to 3.5mm Adapter and Headphone Amplifier (Part 3)
- DIY Micro USB-C to 3.5mm Adapter and Headphone Amplifier (Part 4) (This page)
Device’s construction
Whole device consists of two main parts. On the first cable end it has an USB-C DAC which takes digital audio signal and converts it into analog signal. On the other end there is the amplifier PCB which contains two main circuits: power delivery and amplification.
For the DAC to Amplifier interconnection, a simple 4 wire USB cable was used:
The ALC4042 USB-C DAC
The USB-C DAC (Links in the end of this post) uses ALC4042 IC to convert digital to analog audio. Its main parameters are 384 kHz sampling rate and 32 bit word length. Other parameters such as signal to noise ratio, THD, etc. are not known to me as I haven’t searched enough or did not take the measurements. Either way, it would be a bit naïve to expect extraordinary quality of such tiny, mobile oriented, device.
I have also tried a newer version ALC5686 DAC, but it had a bit of noise at output when idle (no music is playing) while ALC4042 was totally quiet. Although ALC5686 noise goes away when you play music (so it is not such a big deal), I eventually decided to stick with the older ALC4042 IC.
I have chosen to use USB connection instead of standard 3.5 mm jack just because USB can deliver power to the connected device, which is needed for the amplifier. There is only one tiny problem with USB-C DAC boards. Usually, they have GND, L, R and MIC pads on the board. In my case, I also needed +5V, which was taken from a random empty pad on the board:
After soldering the DAC board and a peace of wire together, the view was as follow:
As you can see in the photo above, the red wire was used for +5V, black – GND, white – Left channel, Green – Right channel. Additionally, I have used a peace of heat shrinkable tubing to cover the wires:
Finally, I have used some epoxy to glue the DAC’s housing pieces together.
The INA1620 amplifier board
The power delivery circuit is based on Texas Instruments TPS65135 IC. It changes voltage from single +5V which is taken from USB DAC’s PCB and converted into dual +-5V. The great thing of this IC is that it requires only one inductor and few resistors and capacitors, so whole circuit does not take a lot of space.
The analog signal amplification is based on INA1620 IC which is also made by Texas Instruments. It has gain set to 2. From testing in the previous post, this value was selected as it is the best compromise – it is enough to drive 300 Ohm headphones, but it is also not inadequately loud with 32 Ohm headphones. In other words, it is useable with both, high and low impedance headphones.
And here is the whole schematic:
The output from INA1620 is directly connected to 3.5 mm headphone jack. The jack itself is a simple connector which, by design, should be put on the end of an audio cable.
It has 4 pins: GND, L, R and MIC. The microphone pin was not used in this case.
The amplifier’s PCB would not fit inside this connector, so I have also 3D printed a longer end-cap:
After having this part printed and checked if it fits whole assembly correctly, it was time to solder the amplifier’s end:
Finally, this end with the end-cap and its original cover view:
With the longer end-cap modification, the audio jack became about 1 cm longer than its original design.
Whole assembly and final thoughts
So the final product looks like it is show below:
Visually it looks similar to a regular USB-C to 3.5mm audio adapter, except that the 3.5 mm jack is a bit longer.
In the end, it plays music much launder than from a standard 3.5mm phone jack and a bit louder than DAC-only solution (because the gain is only 2). The device is capable of driving high impedance (tested with 300 Ohm Sennheiser HD650). At high sound volume with low impedance (32 Ohm) headphones it distorts sound badly, but the solution to this problem is just to lower the volume :).
It is worth mentioning, that I firstly designed the amp and then bough the DAC. So, after trying out the DAC I have found out that it would be probably just enough of volume from the DACs output without additional amplification for driving 300 Ohm headphones. Which means that the cheaper and probably much simpler solution was just to buy a regular USB-C to 3.5mm DAC/adapter (like one of these on Aliexpress).
To sum up, although I could have got away with a simple USB DAC solution which is already available in the market, I am have with the idea and design of my solution. Power circuit and amplifier fits into a tiny board which is cool on itself (at least to me).
Links and part list
All hardware files (schematics, KiCAD project, Gerber, part list, etc.) can be found at my GitHub: DIY Micro Amp Git
All 3D files (used for printing) for the amplifier’s parts can also be found at Thingiverse: End Cap And Soldering Holder.
The 3.5 mm audio jacks were bought from Aliexpress (Affiliate)
The ALC4042 DAC was bought from Aliexpress (Affiliate) . As an alternative to it, ALC5686 DAC (Affiliate) can be used.
