USB-C Speaker & Discrete Loudness Meter

The amp board, revision 4, as ordered from JLCPCB

The Idea

Desktop speakers usually mean a wall adapter, an aux cable, and a premade amp module. I wanted to build the whole signal path myself and have it run off one cable: plug in USB-C and the board shows up as a sound card, negotiates its own power, and drives the speakers directly.

The other goal was the skill itself: I had never designed a PCB, and I picked this project specifically to learn. KiCad was the obvious tool, since it's free and does everything from schematic capture to the fab files JLCPCB builds from.

The Signal Chain

One connector carries everything. The data lines go to a PCM2704, which any computer sees as a class-compliant USB sound card, so no drivers are needed. On the power side, a CH221K requests 12V from a USB-C PD supply for the amplifier, and an AMS1117 steps that down to 3.3V for logic. The loudness meter taps the analog signal right before the speakers.

The Amp Board

The full schematic on one sheet: USB-C and PD on the left, PCM2704 in the middle, TDA7297 stages on the right

The amplifier is a TDA7297, a 15W×2 class-AB chip running directly off the 12V rail. Input protection is a polyfuse for overcurrent and an SMBJ15A TVS diode to clamp transients on VBUS. A crystal clocks the PCM2704, and the on-board volume buttons connect straight to it, since USB audio handles volume in the stream.

This is also the first schematic I ever drew. I did not know yet that net labels tie nets together, so every connection is a drawn wire. It works, and I left it unretouched. Schematic PDF →

The board went through four fabbed revisions. JLCPCB assembled the SMT parts, so each spin was cheap and fast enough to just order, test, and fix in the next version.

Revision 1

Revision 2

Revision 3

Revision 4, as ordered

V4 layout in KiCad: USB and clock on the left, power and amplification on the right

The layout follows the signal: the USB data pairs and crystal stay close to the PCM2704 on one side, and the 12V power path and speaker outputs stay on the other.

The Loudness Ladder

The second board is a stereo loudness meter: ten LEDs per channel that light in sequence as the music gets louder, with the top LED doubling as a clip indicator. Driver ICs exist that do exactly this, but I built it from discrete parts instead. Per channel, half an LM358 works as an envelope detector and gain stage, and each LED has its own MMBT3904 with a base divider that sets its turn-on threshold. Functionally this is a flash ADC: ten threshold stages compare the same input in parallel and output a thermometer code. The difference is that the code drives LEDs directly instead of being encoded into bits, and the thresholds are spaced by loudness rather than equal voltage steps.

One channel: envelope detector, ×2.1 op-amp bus, then ten common-emitter stages with individually sized base dividers

This is my second schematic, drawn after the amp's. Compared to the first one it uses net labels instead of drawn wires everywhere, and the signal path reads left to right in stages. The improvement between the two boards is what I picked the project for.

The interesting part is sizing those twenty resistor dividers. LEDs should light at equal loudness steps, and loudness is logarithmic, so each threshold sits a fixed number of dB below full scale, then gets snapped to real E24 resistor values. I derived the whole thing by hand and wrote it up: envelope detector ripple, the op-amp gain choice, the dB spacing, and the error introduced by E-series rounding. Full derivation PDF →

V2 layout: twenty near-identical stages, twenty different divider values

Visualizer V2: ten LEDs per channel, green through red on the assembled board

All SMT parts sit on the back, so the front face is just LEDs