Class D amplifier with a good performance and better sound quality. We can call it a Class D with Class A specs.
Story
I am a fan of audio technology as it is cleared from most of my articles. I always try some new ICs or audio related things here. And today is the time for class D amplifiers. I made a subwoofer speaker that can handle up to 200 W in stereo but I don’t have any good amplifier system that I can directly fit into my car. And that thing brings me towards TPA3221 IC. For the same price, anyone can get cheap Bluetooth speakers. But they distort at half volume, the bass rolls off a cliff. I want to drive my TPA3221 amplifier directly from the car battery in this case.
So that it can actually push real power into real speakers, with clean audio and proper engineering behind it. The goal was simple: a compact, high-efficiency stereo amplifier that can deliver 100 W per channel into 4-ohm speakers. Just a clean analog-in, speaker-out power amplifier that I could pair with any source. In this article, I will walk you through the full design.
Why TPA3221?
There are dozens of Class-D amplifier ICs out there. The TPA3116, TPA3255, MAX98400 but the TPA3221 sits in a sweet spot that is hard to beat for a DIY stereo amplifier. Lets see what made me pick it:
- 100 W stereo (2 × BTL) or 200 W mono (1 × PBTL) into 4-ohm loads.
- Closed-loop feedback architecture: It corrects for output filter and load variations in real time.
- 0.02% THD+N at 1 W into 4 ohms.
- >108 dB SNR which is better than most commercial amplifiers in this power class.
- >90% efficiency at full load.
- Wide supply range: 7 V to 30 V on PVDD.
- Built-in protections: undervoltage lockout, overcurrent and overtemperature.
- Selectable gains: 18, 24, 30, or 34 dB that can be set with a single resistor.
Circuit design and schematics
I designed the full schematic in EasyEDA and split it into four functional blocks: Audio Input, Power Filtering, Amplifier + Control, and Output Filter (Figure 1). Let me walk through each one.
 |
|
| Figure 1. | Circuit schematics. |
Audio input
The input stage is straightforward. I used single-ended inputs (IN1, IN2) through 1 µF AC coupling capacitors (C19, C20). These block any DC offset from the source and pass only the audio signal. The positive input pins (IN1_P, IN2_P) receive the signal. The negative inputs (IN1_M, IN2_M) are tied to AC ground through their own coupling caps. This sets up the single-ended input mode as described in the datasheet.
Power filtering
This is where I spent the most time. A Class-D amplifier switching at 600 kHz is essentially a high-frequency power converter, and the power supply decoupling has to be right or you will hear it. The main PVDD rail (7 to 30 V) gets filtered with two 470 µF electrolytic bulk caps (C14, C29) plus 100 nF and 1 µF ceramic caps placed close to the PVDD pins. For the 5 V rail, I made a deliberate choice. I bypassed the internal LDO and used an external 5 V supply. The TPA3221 has a built-in LDO, but for a high-power design, I prefer having a dedicated 5 V source. This keeps the analog supply clean and avoids loading the internal regulator.
Amplifier configuration
The TPA3221 is configured in stereo BTL mode (2 × BTL). Gain is set to 30 dB by the resistor divider on the GAIN/SLV pin. R5 (100k) pulls to +5 V and R4 (39k) pulls to GND. R3 (51k) on the FREQ_ADJ pin to GND sets the nominal PWM frequency to 600 kHz. The HEAD pin is left floating, which defaults to AD (amplitude-defined) modulation. AD mode gives lower idle current and better efficiency at low output levels. C1, C30, C31, C32 is 33 nF on each BST pin pair. These charge during the low-side on-time and provide the gate drive voltage for the high-side MOSFETs.
Reset and mute control
The RESET pin needs to be held HIGH during normal operation. When you first connect power, you pull it LOW momentarily to initialize the chip, then release it HIGH. I brought this out to a pin header (U4) for manual control. The CMUTE pin requires a LOW pulse to unmute the amplifier. When CMUTE is HIGH, the outputs are muted.
Output filter
Each half-bridge output gets an LC low-pass filter, a 10 µH inductor followed by filtering capacitors. This reconstructs the audio waveform from the PWM switching signal. The snubber networks are important and often overlooked in DIY designs. I placed a 2.2-ohm resistor in series with a 10 nF capacitor (R7-R10, C35-C38) on each output. These damp the ringing caused by the inductor-capacitor resonance and the parasitic inductance of the speaker wires. Additional 1 nF caps (C35-C38) sit across the differential outputs, and 1 µF caps (C6, C7, C41, C42) provide further filtering.
PCB design
This is a 4-layer board designed in EasyEDA shown in Figure 2. The dimensions came out to 71.25 mm × 62.1 mm which is compact for a 100 W stereo amplifier. I have used the top and bottom layer for signal and internal layer as references. Layer 2 for ground plane and layer 3 for power plane.
 |
|
| Figure 2. | A 4-layer board designed in EasyEDA. |
The TPA3221 sits at the center of the board. On the left side the audio input connectors (IN1, IN2 signal header) and power connectors (VDD, GND, +5 V) are placed. Right side is for speaker output connectors (Speaker 1 and Speaker 2, each with PLUS/MINUS pads). In the bottom left MUTE and RESET pin headers with a switch is placed for manual control. The two 470 µF electrolytic caps are placed one near the power input and one near the output stage. This distributes the bulk capacitance where it is needed most.
Thermal pad handling
Here in the IC thermal pad is on the top of the IC, not at the bottom. Most power ICs have a bottom-side exposed pad that you solder directly to the PCB and connect to a ground plane with thermal vias. With the TPA3221, the top-side thermal pad is designed to interface with a heatsink mounted on top of the chip. The datasheet recommends a junction-to-ambient thermal resistance of 44.8 degrees C/W without a heatsink and 5.5 degrees C/W with a fixed 85 degrees C heatsink.
Testing and results
 |
|
| Figure 3. | Powering the board. |
I powered the board with a 24 V bench supply and an external 5 V for the logic rail with another power supply to monitor the current in both (Figure 3). After the initial reset sequence (pull RESET low, then high) and unmuting via the CMUTE header, the amplifier came alive. I have tried music playback, frequency sweep, bass response, and real listening impressions with actual speakers (Figure 4).
 |
|
| Figure 4. | Real listening impressions with actual speakers. |
Audio quality is one of those things that does not translate well to text. You need to hear it. What I can say here at moderate listening levels, the amplifier is dead silent. No hiss, no hum, no audible switching noise. The closed-loop feedback does its job and the sound is noticeably cleaner than open-loop Class-D designs I have tried before.
Outro
The TPA3221 is a seriously capable Class-D amplifier IC. For a single chip with minimal external components, getting 100 W stereo with 0.02% THD+N and >108 dB SNR is impressive. All project files are available in the Downloads section.