ESP32 music player - part 2
I used the player for a few hours now, but I noticed a big flaw in my design. Well design is a little far-fetched. I just put a few parts together and called it a music player. :D
The first iteration
In the first iteration I tried my headphones directly on the PCM5102 and they sounded amazing and loud enough for my taste. But something felt off using headphones directly on a DAC preamp.
I did a little research and found out, that the PCM5102 is designed for low-current and high-impedance loads, like that of amplifiers. Not the low impedance and current-hungry nature of headphones. Talking 16-80 ohms headphones.
I figured, that it also could damage the DAC or my headphones in the long run.
The second iteration
I almost immediately went down to work and cracked the glued and beautifully painted housing open again. This hurt real bad…
I had a MAX4410 laying around from another project.
New goals
The goals for the second iteration were:
- rework the complete layout of the components
- remove the micro-usb port for charging and use the usb-c of the esp to both charge and program the player
- integrate the MAX4410 without making the overall footprint bigger
- use the switched pot of the max as the power button
Work…
So I removed everything. The switch, the buttons, all the wires, the modules. I started over again. Moved all the components to new places and I stuffed a lot more in there. A new step down module to power the esp, the dac, the sd card reader and the amp. Was a little worried that this would be a little much for the small esp ldo. I also added a mode button, so 3 buttons in the new version. And I added a neopixel to show the modes with different colors.
The headphone jacks of the dac and amp got removed to reduce their size and I soldered one of them directly to the pcb.
The traces of the volume pot on the amp were cut, and I placed it between the charger module’s 5V and the buck converter. So it can switch the whole player.
It was a success and I’m pleased that the whole thing really didn’t become much bigger.
a lot of new cables…
the finished thing
More flaws
I found out that the MAX4410 sounds like a good idea on paper, but it’s not really made for full-blown line-level signals like the PCM5102 puts out.
It ends up being way too loud, with barely any control over the volume. I had to set the gain to 0.05 on the software side, which is a very bad idea, because this throws most of the PCM5102’s dynamic range out of the window…
A fix for now
As the assembly and the components do not survive yet another disassembly, I will drop a simple resistor voltage divider between the DAC and the amp to reduce the input signal. That brings the volume down to something usable without losing the dynamics of the PCM5102.
For the next step
A better fit is something like the TPA6132. It handles proper DAC output levels, drives headphones cleanly, and just plays nicer overall. This chip comes on a small module, simple and plain. Without audio jack and volume pot.
So I ordered a bunch of new stuff for the third iteration.
What comes next?
The next iteration will be smaller and again a little better. I still want to use a 18650 battery with around 3500mAh, because they are safe and I like their heft and overall look. Also their easy way of being replaced.
I will not use a full ESP32 dev board, but a esp32 cam module. These modules come with a sd card reader on board and yet are smaller than the full dev boards. They have a separate programmer board, which is really nice.
I’m still going to use a PCM5102 as dac, I like them for their low noise level and high dynamic range.
A TPA6132 is going to be used as the headphone amp, with a stereo volume pot in between the dac and the amp.
I also found nice charging boards for the 18650 batteries with usb-c ports and I will combine them with fixed 5V to 3.3V step down converters. I played with the idea of skipping the boost and buck steps and using a buck directly on the 3.7V battery, but most of them - especially the small ones - need at least a 1V difference to work. So from 3.7V to 3.3V is not going to work for now. I’m sure there is a way, but I need a little more research for that.