ZK-ST21 TPA3221 Bluetooth 5.1 2.1 Channel Digital Amplifier Board

What Is This Product?

The ZK-ST21 is a high-power 2.1-channel Class-D digital amplifier board built around two Texas Instruments TPA3221 amplifier ICs. It provides separate amplified outputs for a left speaker, right speaker and passive subwoofer, making it suitable for powerful DIY 2.1-channel audio systems.

The board is rated as a 100W + 100W + 200W amplifier. More detailed specifications list approximately 105W per stereo channel at 30V/4Ω and up to 208W for the subwoofer at 30V/2Ω. At 30V/8Ω, each stereo channel is specified at approximately 55W, demonstrating why actual output depends heavily on supply voltage and speaker impedance.

Connectivity is one of the ZK-ST21's major strengths: it supports Bluetooth 5.1, 3.5mm AUX, USB flash-drive playback and USB sound-card operation. Independent controls are provided for master volume, stereo bass, stereo treble, subwoofer volume and subwoofer frequency.

What Can You Build / Do With It?

The ZK-ST21 can serve as the central amplifier for:

• High-power DIY 2.1 Bluetooth speaker systems
• Stereo bookshelf speakers with a passive subwoofer
• Home audio systems
• DIY home-theatre speaker projects
• Desktop Hi-Fi systems
• Multimedia speaker systems
• Active speaker cabinet projects
• Workshop audio systems
• Custom wooden speaker enclosures
• Audio electronics experiments

Its dedicated subwoofer amplifier means you do not need a separate subwoofer power-amplifier board for a conventional 2.1 setup.

Why This Product?

• Dual TPA3221 amplifier ICs
• True 2.1-channel amplified output
• Up to approximately 100W × 2 + 200W
• Wide 12V–30V DC operating range
• Bluetooth 5.1 wireless audio
• 3.5mm AUX input
• USB flash-drive playback
• USB sound-card functionality
• Separate subwoofer volume and frequency adjustment
• Independent stereo bass and treble controls
• Large heatsink for thermal management
• Multiple electrical protection mechanisms

The combination of four input methods and independent tone/subwoofer controls makes this a substantially more capable amplifier platform than a basic Bluetooth amplifier board.

Key Features

• 2.1-channel Class-D digital amplifier
• Two TI TPA3221 amplifier ICs
• 100W + 100W stereo output class
• 200W subwoofer output class
• Bluetooth 5.1
• Up to approximately 15m Bluetooth range in open conditions
• AUX audio input
• USB flash-drive playback
• USB sound-card functionality
• Master volume control
• Stereo bass control
• Stereo treble control
• Dedicated subwoofer volume
• Dedicated subwoofer frequency adjustment
• Large aluminium heatsink
• Reverse-polarity, undervoltage, overcurrent, overtemperature and short-circuit protection

Compatibility

Compatible With

Left & Right Channels

• 3Ω–8Ω passive speakers
• Up to approximately 100W per channel

Subwoofer Channel

• 2Ω–8Ω passive subwoofer
• Up to approximately 200W

Audio Sources

• Bluetooth-compatible smartphones
• Tablets
• Computers
• 3.5mm AUX audio sources
• USB flash drives
• Computers using supported USB sound-card functionality

Required

• DC power supply
• Left passive speaker
• Right passive speaker
• Passive subwoofer for 2.1 operation
• Appropriate speaker wiring

Recommended

For high-output applications:

24V–28V DC, 10A or higher

is the recommended power-supply range.

A lower-voltage supply can operate the board within its specified range, but the available maximum speaker output will be lower.

❌ Not Included / Required Separately

The current RoboGears listing includes:

1 × ZK-ST21 amplifier board only.

Therefore, the following are not included:

• DC power supply
• Speakers
• Subwoofer
• Speaker cables
• Audio cable
• Enclosure

Before Purchase

⚡ Power Supply Matters

Do not purchase this amplifier based only on the 100W + 100W + 200W headline.

To approach its higher output levels, the board requires an appropriately powerful DC supply and correctly matched speakers.

For example:

30V + 4Ω → up to ~105W on an L/R channel

while:

30V + 8Ω → approximately 55W on an L/R channel.

Similarly, the subwoofer output is specified at approximately:

208W @ 30V/2Ω

versus:

105W @ 30V/4Ω.

This makes speaker impedance and power-supply selection important when designing the system.

Applications

• DIY 2.1 Bluetooth speakers
• High-power home audio systems
• Home-theatre projects
• Passive subwoofer systems
• Bookshelf speakers + subwoofer
• Desktop Hi-Fi systems
• Multimedia speakers
• Custom wooden speaker cabinets
• Workshop sound systems
• Audio prototyping
• Electronics projects
• Custom amplifier builds

📦 What's Included

This matches the package contents currently stated on the RoboGears product page.

❌ What's Not Included

• Power supply / SMPS
• Left & right speakers
• Subwoofer
• Speaker wires
• 3.5mm AUX cable
• USB cable
• Enclosure

The RoboGears listing explicitly states that the power supply, speakers, cables, enclosure and other accessories are not included.

Getting Started

Step 1 — Select the Speakers

Choose compatible passive speakers:

L/R: 3Ω–8Ω

Subwoofer: 2Ω–8Ω

Step 2 — Select the Power Supply

Use a DC supply within the 12V–30V operating range.

For higher-power applications, 24V–28V DC at 10A or higher is recommended.

Step 3 — Connect the Speakers

Connect:

Left Speaker → L Output

Right Speaker → R Output

Passive Subwoofer → SUB Output

Step 4 — Connect an Audio Source

Choose from:

Bluetooth / AUX / USB Flash Drive / USB Sound Card

Step 5 — Power On

Connect the correct DC supply and turn the master-volume/power control clockwise.

Step 6 — Test at Low Volume

Begin playback at low volume and verify that the left, right and subwoofer channels are operating correctly before increasing output.

Controls

The ZK-ST21 provides five primary adjustments:

MASTER VOLUME — Overall output level and power control

TREBLE — Adjusts high-frequency output of the stereo channels

BASS — Adjusts low-frequency output of the stereo channels

SUBWOOFER VOLUME — Controls the subwoofer output level

SUBWOOFER FREQUENCY — Adjusts the subwoofer's frequency/crossover behaviour

The documented control layout confirms these independent adjustments.

📚 Resources

Technical documentation is available for the ZK-ST21 covering:

• Product specifications
• Connection layout
• Speaker requirements
• Power-supply requirements
• Bluetooth operation
• AUX input
• USB playback
• USB sound-card operation
• Control functions
• Protection mechanisms
• Installation and safety information

🔧 Prototyping and Engineering Tips from RoboGears

1. Do not undersize the power supply. For high-output builds, use a quality 24V–28V supply capable of at least 10A.
2. Speaker impedance directly affects available output. A higher-impedance speaker generally results in lower maximum output from the same supply.
3. Do not connect a 2Ω load to the stereo L/R outputs. The documented L/R speaker range starts at 3Ω; the subwoofer output supports 2Ω–8Ω.
4. Use heavy enough power wiring. A high-power amplifier drawing several amps should not be powered through thin jumper wires.
5. Keep the heatsink ventilated. Do not completely seal the board inside a small enclosure without adequate airflow.
6. Test each channel at low volume first. Verify left, right and subwoofer connections before increasing the output.
7. Keep speaker-wire strands under control. A loose strand touching an adjacent terminal can create an output short.
8. Maintain correct polarity. Consistent speaker polarity is particularly important for proper bass response in a 2.1 system.
9. Keep low-level AUX wiring away from high-current power wiring where practical to reduce unwanted noise.
10. Measure the actual board before machining an enclosure. The documented size is approximately 121 × 100 × 25mm, but allow additional clearance for terminals, knobs, wiring and ventilation.

⚠️ Common Mistakes

1. Expecting 400W from a Small Power Supply

The amplifier cannot produce its highest advertised output if the power source cannot provide sufficient voltage and current.

2. Connecting the Wrong Speaker Impedance

Use:

L/R → 3Ω–8Ω

SUB → 2Ω–8Ω

3. Using an Underrated Power Supply

A weak supply can cause reduced output, distortion or unstable operation at high volume.

4. Shorting Speaker Outputs

Never allow positive and negative speaker terminals or exposed wire strands to touch.

5. Poor Cooling

High-power continuous operation requires adequate ventilation around the heatsink.

6. Changing Connections While Powered

Disconnect the power before modifying speaker, power or other wiring.

Troubleshooting

Problem: Board does not power on

Possible Cause: Incorrect supply voltage, polarity or wiring.

Solution: Verify that the supply is within DC 12V–30V and check the power connections.

Problem: Sound distorts at high volume

Possible Cause: Insufficient power supply capability, speaker mismatch or clipping.

Solution: Reduce volume and verify the power supply and speaker configuration.

Problem: Subwoofer output is too low

Possible Cause: Subwoofer volume/frequency adjustment or unsuitable speaker configuration.

Solution: Check the dedicated subwoofer controls and verify the subwoofer impedance.

Problem: Only one stereo channel works

Possible Cause: Speaker wiring, speaker fault or input issue.

Solution: Check/swap the speakers and wiring at low volume to isolate the issue.

Problem: Bluetooth connection is unstable

Possible Cause: Excessive distance or wireless obstruction/interference.

Solution: Move the source device closer. The specified normal range is up to approximately 15m under suitable conditions.

FAQ

1. Which amplifier IC does the ZK-ST21 use?

It uses two Texas Instruments TPA3221 Class-D amplifier ICs.

2. Is it really a 2.1-channel amplifier?

Yes. It provides separate amplified outputs for left, right and subwoofer channels.

3. What is the output power?

The product is commonly rated 100W + 100W + 200W. More specifically, documented maximum figures include approximately 105W per L/R channel at 30V/4Ω and 208W for the subwoofer at 30V/2Ω.

4. What power supply should I use?

The operating range is DC 12V–30V. For higher-output operation, 24V–28V at 10A or higher is recommended.

5. Can I use 8Ω speakers?

Yes. The documented L/R speaker range is 3Ω–8Ω, while the subwoofer range is 2Ω–8Ω. Note that maximum output is lower into an 8Ω load than into a suitable lower-impedance load.

6. Does it support USB?

Yes. It supports USB flash-drive playback as well as USB sound-card functionality.

7. Does it have AUX input?

Yes. A 3.5mm Line-In/AUX input is provided.

8. Is the power supply included?

No. The current RoboGears listing includes 1 × ZK-ST21 amplifier board, while the power supply, speakers, cables and enclosure are not included.

🚚 Shipping & Delivery

• Dispatch: Within 24 hours*
• PAN India delivery
• Tracking information provided after shipment
• Delivery time varies by destination and courier

*Subject to order processing and stock availability. The current RoboGears listing states that the product ships within 24 hours and supports Pan India delivery.

⚠️ Important Notes & Safety

• Use only DC 12V–30V power.
• Never exceed the specified maximum input voltage.
• Disconnect power before changing wiring.
• Observe correct DC polarity.
• Do not short speaker terminals.
• Keep the amplifier away from moisture.
• Provide adequate ventilation around the heatsink.
• Match speaker impedance to the appropriate output channel.
• Do not touch or modify the board while powered.
• Start initial testing at low volume.
• For high-output applications, use appropriately rated wiring, speakers and power supply.

The ZK-ST21 documentation specifically warns against incorrect supply voltage, short circuits and inadequate ventilation.

Disclaimer

The 100W × 2 + 200W rating represents the amplifier's high-power capability under appropriate operating conditions and should not be interpreted as guaranteed continuous output under every configuration. Actual output depends on supply voltage, available current, speaker impedance, thermal conditions and audio signal characteristics. For example, documented L/R output changes from approximately 105W at 30V/4Ω to 55W at 30V/8Ω. Always select the power supply and speakers according to the intended operating conditions.