2A4WD-ZK-ST21Bluetooth Audio Power Amplifier

Product function (20-200HZ） Subwoofer expansion interface Product size 1 2 1 m m 1 0 0 m m 25mm 1 0 1 m m 1 8 m m 1 8 m m 1 9 m m 1 9 m m 92mm 7 7 . 5 m m φ2.2mm Connection method of speaker Divider Tweeter Bass or full frequency speaker Subwoofer or passive subwoofer To save costs, low-frequency or full frequency speakers can also be directly connected to the amplifier output, while tweeters can be connected in series with 2.2μF connect the capacitor with the subwoofer. 12-24V battery Switch power supply Adapter OR OR USB drive/ sound card input MIC extension interface Adding the front stage plate ZK-AMP of the microphone Add installation of pure rear stage power amplifier boards ZK-1602 and 3002 Power supply 12-30VDC Expansion port 1 Power supply 8-26VDC Connect the microphone socket board (Connected to microphone front stage board reverb input) (Connected to microphone front stage board reverb output) MIC front stage boardZK-AMP Connected to subwoofer speaker Power supply 18-50 VDC Power supply 12-30VDC Subwoofer small signal output (Connected to the input port of the pure rear power amplifier board) HiFi AMP 2.1 Channel BT Amplifier Board Model：ZK-ST21 Product parameters Product name: 2.1 channel BT amplifier board 100W+100W+200W Product model: ZK-ST21 Adapted power supply: The working voltage range is 12-30VDC, and it is recommended to use a DC power supply with a voltage of 24V-28V and a current of 10A or above. Maximum power: Left or right channel—— 105W@30V , 4Ω; 55W@30V , 8Ω. Subwoofer—— 208W@30V 2Ω; 105W@30V , 4Ω. In short, the higher the voltage, the smaller the resistance, and the greater the output power. Adapted speakers: Left and right channels, supporting 3-8 ohms and speakers up to 100W; The subwoofer supports 2-8 ohms and speakers up to 200W. BT chip: JieLi 5.1, transmission distance of 15 meters. Input method: BT+AUX+USB drive+sound card. Number of channels: 2.1, Left channel+right channel+subwoofer Power amplifier chip: Two TI original TPA3221 Protection mechanism: Input undervoltage, input anti reverse connection, overheating, overcurrent, short circuit protection. Product size: 121x100x25mm Product weight: Integrated without shell net weight: 192g; With packaging: 228g; Integrated with shell net weight: 261g; With packaging: 297g External lead type net weight: 254g; With packaging: 310g Same spacing as TB21 potentiometer Question and answer What kind of speaker does it come with? How much power is used? Recommend an amplifier? In fact, this is the same question because the three of them are related. I believe that after learning the following knowledge, no matter what kind of amplifier you make, you will calculate the compatibility relationship between the power supply, speaker, and amplifier yourself. 1. According to Ohm's law, power P=U * U/R, the higher the voltage, the smaller the horn resistance and the greater the power; So the final output power is not only related to the power amplifier, but also to the power supply voltage and the resistance of the speaker. According to the formula, if the power supply is 24V and the horn resistance is 4 ohms, then the peak power=24V * 24V/4 Ω=144W, and the effective value (rated power) of a 1kHz sine wave=24V * 0.707 * 24V * 0.707/4 Ω=72W; The peak power of an 8-ohm horn is 24V * 24V/8 Ω=72W, and the rated power is 24V * 0.707 * 24V * 0.707/8 Ω=36W; If the power supply is 12V and the 8 ohm horn is used, the peak power is 12V * 12V/8 Ω=18W, and the rated power is 12 * 0.707 * 12 * 0.707/8 Ω=9W. Our amplifier's nominal power is the rated power, and later on, it defaults to the rated power. 2.In order to leave sufficient power margin and avoid clipping distortion, it is better to have an amplifier power of 1.5-2.5 times the speaker power. For example, if your single channel speaker is 100W, the corresponding amplifier board's single channel output power should be between 150W and 250W. Speakers can be connected in parallel or series. Two 8-ohm and 50W speakers connected in parallel are equivalent to a 100W4 ohm speaker. It is recommended to choose an amplifier with a typical load impedance value that is the same as the final impedance value of your speaker system. Our amplifiers generally support 4-8 ohm speakers, while a few high-power amplifiers support 2-8 ohm speakers. If the speaker resistance is too small, the amplifier chip is prone to overcurrent power protection or burning. 3.The choice of power supply is also very important. Some customers use a 5V2A mobile phone charger or a 12V1A2A adapter to drive a 50W100W amplifier, but the voltage and power are obviously not enough. Ultimately, the voltage is pulled down, and the sound becomes distorted and distorted when slightly louder, resulting in a cloudy sound or intermittent protection from the power supply causing sound lag. It doesn't matter if the power supply is too high, but if the power supply is not sufficient, the problem mentioned above will occur. So what size of power should be chosen? This issue involves two parameters: voltage and current. In theory, the power supply should be greater than the total power of the power amplifier. For example, if TB21 is a 50W+50W+100W power amplifier, you should choose a 24V8A or higher power supply. But there is also a situation where the speaker you choose is not that large. In order to facilitate power adaptation or reduce power costs, you can calculate the actual power required. For example, the subwoofer speaker you choose is 4 ohms 50W, and two stereo speakers are 8 ohms 30W. If you choose a 12V power supply, the maximum output rated power of the subwoofer is P=12 * 0.707 * 12 * 0.707/4 Ω=18W. Obviously, a 12V voltage cannot meet this requirement. Try a 24V power supply again. The subwoofer output P=72W, the stereo output P=36W+36W, and the total power is 144W. Therefore, a 24V6A power supply is sufficient. Some customers say that I have a 24V20A power supply with a power of 480W, but it's better to have an 8-ohm 100W …

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Shenzhen HTT Technology Co., Ltd. Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 1 of 3 Product Name: Bluetooth Audio Power Amplifier Model No.: ZK-ST21 EUT Constructional Details External Photo Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 2 of 3 Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 3 of 3 ------End------

Label and Location Size: L: 2.0cm*W: 1.0cm Label and Location FCC ID : 2A4WD-ZK-ST21

Shenzhen HTT Technology Co., Ltd. Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 1 of 2 Product Name: Bluetooth Audio Power Amplifier Model No.: ZK-ST21 EUT Constructional Details Internal Photo ANTENNA Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 2 of 2 ------End------

Shenzhen HTT Technology Co., Ltd. Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Page 1 of 8 RF Exposure MPE Report Reference No. ..................... : FCC ID............................................. : HTT202604437F02 2A4WD-ZK-ST21 Compiled by ( position+printed name+signature) . : File administrators Supervised by ( position+printed name+signature) . : Project Engineer Approved by ( position+printed name+signature) . : RF Manager Date of issue .................................... : Apr. 17, 2026 Testing Laboratory Name .............. : Shenzhen HTT Technology Co.,Ltd. Address............................................ : 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao’an District, Shenzhen, Guangdong, China Applicant’s name ............................ : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address............................................ : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Standard ........................................... : 47CFR §1.1310 47CFR §2.1091 KDB447498 D01 General RF Exposure Guidance v06 Shenzhen HTT Technology Co.,Ltd. All rights reserved. This publication may be reproduced in whole or in part for non-commercial purposes as long as the Shenzhen HTT Technology Co.,Ltd. is acknowledged as copyright owner and source of the material. Shenzhen HTT Technology Co.,Ltd. takes no responsibility for and will not assume liability for damages resulting from the reader's interpretation of the reproduced material due to its placement and context. Test item description ..................... : Bluetooth Audio Power Amplifier Manufacturer .................................... : Qingdao Wuzhi Intelligent Technology Co., Ltd. Trade Mark ........................................ WUZHI AUDIO Model/Type reference ...................... : ZK-ST21 Rating ............................................... : DC 24V Result ............................................... : PASS Report No.: HTT202604437F02 Page 2 of 8 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Page 2 of 8 T E S T R E P O R T Equipment under Test : Bluetooth Audio Power Amplifier Model /Type : ZK-ST21 Listed Models : ZK-ST21PLUS, ZK-MTC, ZK-502MT, ZK-PT1, ZK-TB21PRO, ZK-AS21P,ZK-1002Q, ZK-3002T, ZK-3002TPRO, ZK-1001UM, ZK-302T, ZK-DAM L1, ZK-DAM H1, ZK-DAM H2 Model difference : The PCB board, circuit, structure and internal of these models are the same, Only model number is different for these model. Applicant : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Manufacturer : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Test Result: PASS The test report merely corresponds to the test sample. It is not permitted to copy extracts of these test result without the written permission of the test laboratory. Report No.: HTT202604437F02 Page 3 of 8 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Page 3 of 8 Contents 1 T E S T S T A N D A R D S ................................................... 4 2 S U M M A R Y .............................................................. 5 2.1 General Remarks 5 2.2 Product Description 5 2.3 Special Accessories 5 2.4 Modifications 5 3 T E S T E N V I R O N M E N T 6 3.1 Address of the test laboratory 6 3.2 Test Facility 6 3.3 Statement of the measurement uncertainty 6 4 T E S T L I M I T 7 4.1 Requirement 7 4.2 MPE Calculation Method 7 4.3 Conducted Power Results 8 4.4 Manufacturing tolerance 8 4.5 Standalone MPE Result 8 4.6 Simultaneous Transmission for MPE Result 8 5 C O N C L U S I O N .......................................................... 8 Report No.: HTT202604437F02 Page 4 of 8 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Page 4 of 8 1 T E S T S T A N D A R D S The tests were performed according to following standards: ANSI C95.1–1999: IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields, 3 kHz to 300 GHz. FCC KDB 447498 D01 General RF Exposure Guidance v06: Mobile and Portable Device, RF Exposure, Equipment Authorization Procedures. FCC CFR 47 part1 1.1310: Radiofrequency radiation exposure limits. FCC CFR 47 part2 2.1091: Radiofrequency radiation exposure evaluation: mobile devices Report No.: HTT202604437F02 Page 5 of 8 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Page 5 of 8 2 S U M M A R Y 2.1 General Remarks Date of receipt of test sample : Apr. 10, 2026 Testing commenced on : Apr. 10, 2026 Testing concluded on : Apr. 17, 2026 2.2 Product Description Product description: Bluetooth Audio Power Amplifier Model/Type reference: ZK-ST21 Power supply: DC 24V Testing sample ID： HTT202604437-1# (Engineer sample) HTT202604437-2# (Normal sample) Hardware version: / Software version: / Bluetooth : Supported Type: Bluetooth BR/EDR Modulation: GFSK, π/4DQPSK Operation frequency: 2402MHz~2480MHz Channel number: 79 Channel separation: 1…

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1 Appendix RF Test Data for BT Company: : Qingdao Wuzhi Intelligent Technology Co., Ltd. EUT : Bluetooth Audio Power Amplifier FCC ID: : 2A4WD-ZK-ST21 Model Number : ZK-ST21 Report No. : HTT202604437F01 Reviewed By : Heber He 2 1. Duty Cycle 1.1 Test Result 1.1.1 Ant1 Ant1 Mode TX Type Frequency (MHz) Packet Type T_on (ms) Period (ms) Duty Cycle (%) Duty Cycle Correction Factor (dB) Max. DC Variation (%) GFSK SISO 2402 DH5 2.880 3.749 76.82 1.15 0.01 2441 DH5 2.880 3.750 76.80 1.15 0.03 2480 DH5 2.880 3.751 76.78 1.15 0.03 Pi/4DQPSK SISO 2402 2DH5 2.885 3.749 76.95 1.14 0.04 2441 2DH5 2.887 3.751 76.97 1.14 0.03 2480 2DH5 2.886 3.750 76.96 1.14 0.03 3 1.2 Test Graph 1.2.1 Ant1 GFSK_DH5_LCH_2402MHz_Ant1_NTNV GFSK_DH5_MCH_2441MHz_Ant1_NTNV 4 GFSK_DH5_HCH_2480MHz_Ant1_NTNV Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV 5 Pi/4DQPSK_2DH5_MCH_2441MHz_Ant1_NTNV Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV 6 2. Bandwidth 2.1 Test Result 2.1.1 OBW Mode TX Type Frequency (MHz) Packet Type ANT 99% Occupied Bandwidth (MHz) Verdict Result Limit GFSK SISO 2402 DH5 1 0.967 / Pass 2441 DH5 1 0.980 / Pass 2480 DH5 1 0.986 / Pass Pi/4DQPSK SISO 2402 2DH5 1 1.244 / Pass 2441 2DH5 1 1.265 / Pass 2480 2DH5 1 1.251 / Pass 2.1.2 20dB BW Mode TX Type Frequency (MHz) Packet Type ANT 20dB Bandwidth (MHz) Verdict Result Limit GFSK SISO 2402 DH5 1 1.024 / Pass 2441 DH5 1 1.030 / Pass 2480 DH5 1 1.064 / Pass Pi/4DQPSK SISO 2402 2DH5 1 1.340 / Pass 2441 2DH5 1 1.354 / Pass 2480 2DH5 1 1.338 / Pass 7 2.2 Test Graph 2.2.1 OBW GFSK_DH5_LCH_2402MHz_Ant1_NTNV GFSK_DH5_MCH_2441MHz_Ant1_NTNV 8 GFSK_DH5_HCH_2480MHz_Ant1_NTNV Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV 9 Pi/4DQPSK_2DH5_MCH_2441MHz_Ant1_NTNV Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV 10 2.2.2 20dB BW GFSK_DH5_LCH_2402MHz_Ant1_NTNV GFSK_DH5_MCH_2441MHz_Ant1_NTNV 11 GFSK_DH5_HCH_2480MHz_Ant1_NTNV Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV 12 Pi/4DQPSK_2DH5_MCH_2441MHz_Ant1_NTNV Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV 13 3. Maximum Conducted Output Power 3.1 Test Result 3.1.1 Power Mode TX Type Frequency (MHz) Packet Type Maximum Peak Conducted Output Power (dBm) Verdict ANT1 Limit GFSK SISO 2402 DH5 -0.58 <=20.97 Pass 2441 DH5 0.13 <=20.97 Pass 2480 DH5 0.43 <=20.97 Pass Pi/4DQPSK SISO 2402 2DH5 -0.03 <=20.97 Pass 2441 2DH5 0.65 <=20.97 Pass 2480 2DH5 1.06 <=20.97 Pass 14 4. Carrier Frequency Separation 4.1 Test Result 4.1.1 Ant1 Ant1 Mode TX Type Frequency (MHz) Packet Type Channel Separation (MHz) 20dB Bandwidth (MHz) Limit (MHz) Verdict GFSK SISO HOPP DH5 1.024 1.064 >=0.709 Pass Pi/4DQPSK SISO HOPP 2DH5 0.979 1.354 >=0.903 Pass 15 4.2 Test Graph 4.2.1 Ant1 GFSK_DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV 16 5. Number of Hopping Frequencies 5.1 Test Result 5.1.1 HoppNum Mode TX Type Frequency (MHz) Packet Type Num of Hopping Frequencies Verdict ANT1 Limit GFSK SISO HOPP DH5 79 >=15 Pass Pi/4DQPSK SISO HOPP 2DH5 79 >=15 Pass 17 5.2 Test Graph 5.2.1 HoppNum GFSK_DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV 18 6. Time of Occupancy (Dwell Time) 6.1 Test Result 6.1.1 Ant1 Ant1 Mode TX Type Frequency (MHz) Packet Type Duration of Single Pulse (ms) Observation Period (s) Num of Pulse in Observation Period Dwell Time (ms) Limit (ms) Verdict GFSK SISO HOPP DH1 0.380 31.600 307 116.660 <=400 Pass DH3 1.636 31.600 159 260.124 <=400 Pass DH5 2.884 31.600 107 308.588 <=400 Pass Pi/4DQPSK SISO HOPP 2DH1 0.396 31.600 316 125.136 <=400 Pass 2DH3 1.648 31.600 162 266.976 <=400 Pass 2DH5 2.896 31.600 89 257.744 <=400 Pass 19 6.2 Test Graph 6.2.1 Ant1 GFSK_DH1_HOPP_Ant1_NTNV GFSK_DH1_HOPP_Ant1_NTNV 20 GFSK_DH3_HOPP_Ant1_NTNV GFSK_DH3_HOPP_Ant1_NTNV 21 GFSK_DH5_HOPP_Ant1_NTNV GFSK_DH5_HOPP_Ant1_NTNV 22 Pi/4DQPSK_2DH1_HOPP_Ant1_NTNV Pi/4DQPSK_2DH1_HOPP_Ant1_NTNV 23 Pi/4DQPSK_2DH3_HOPP_Ant1_NTNV Pi/4DQPSK_2DH3_HOPP_Ant1_NTNV 24 Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV 25 7. Unwanted Emissions In Non-restricted Frequency Bands 7.1 Test Result 7.1.1 Ref Mode TX Type Frequency (MHz) Packet Type ANT Level of Reference (dBm) GFSK SISO 2402 DH5 1 -0.87 2441 DH5 1 -0.19 2480 DH5 1 0.02 HOPP DH5 1 0.63 0.63 Pi/4DQPSK SISO 2402 2DH5 1 -0.89 2441 2DH5 1 -0.20 2480 2DH5 1 0.00 HOPP 2DH5 1 0.46 0.46 7.1.2 CSE Mode TX Type Frequency (MHz) Packet Type ANT Level of Reference (dBm) Limit (dBm) Verdict GFSK SISO 2402 DH5 1 -0.87 -20.87 Pass 2441 DH5 1 -0.19 -20.19 Pass 2480 DH5 1 0.02 -19.98 Pass HOPP DH5 1 0.63 -19.37 Pass 0.63 -19.37 Pass Pi/4DQPSK SISO 2402 2DH5 1 -0.89 -20.89 Pass 2441 2DH5 1 -0.20 -20.20 Pass 2480 2DH5 1 0.00 -20.00 Pass HOPP 2DH5 1 0.46 -19.54 Pass 0.46 -19.54 Pass 26 7.2 Test Graph 7.2.1 Ref GFSK_DH5_LCH_2402MHz_Ant1_NTNV GFSK_DH5_MCH_2441MHz_Ant1_NTNV 27 GFSK_DH5_HCH_2480MHz_Ant1_NTNV GFSK_DH5_HOPP_Ant1_NTNV 28 GFSK_DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV 29 Pi/4DQPSK_2DH5_MCH_2441MHz_Ant1_NTNV Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV 30 Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV 31 7.2.2 CSE GFSK_DH5_LCH_2402MHz_Ant1_NTNV GFSK_DH5_LCH_2402MHz_Ant1_NTNV 32 GFSK_DH5_MCH_2441MHz_Ant1_NTNV GFSK_DH5_HCH_2480MHz_Ant1_NTNV 33 GFSK_DH5_HCH_2480MHz_Ant1_NTNV GFSK_DH5_HOPP_Ant1_NTNV 34 GFSK_DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV 35 Pi/4DQPSK_2DH5_LCH_2402MHz_Ant1_NTNV Pi/4DQPSK_2DH5_MCH_2441MHz_Ant1_NTNV 36 Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV Pi/4DQPSK_2DH5_HCH_2480MHz_Ant1_NTNV 37 Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV Pi/4DQPSK_2DH5_HOPP_Ant1_NTNV

Shenzhen HTT Technology Co., Ltd. Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China FCC PART 15 SUBPART C TEST REPORT FCC PART 15.247 Report Reference No. .................... : FCC ID. ........................................... : HTT202604437F01 2A4WD-ZK-ST21 Compiled by ( position+printed name+signature) . : File administrators Supervised by ( position+printed name+signature) . : Project Engineer Approved by ( position+printed name+signature) . : RF Manager Date of issue .................................... : Apr. 17, 2026 Testing Laboratory Name .............. : Shenzhen HTT Technology Co.,Ltd. Address ............................................ : 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao’an District, Shenzhen, Guangdong, China Applicant’s name ........................... : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address ............................................ : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Test specification ........................... : Standard ........................................... : FCC Part 15.247 Shenzhen HTT Technology Co.,Ltd. All rights reserved. This publication may be reproduced in whole or in part for non-commercial purposes as long as the Shenzhen HTT Technology Co.,Ltd. is acknowledged as copyright owner and source of the material. Shenzhen HTT Technology Co.,Ltd. takes no responsibility for and will not assume liability for damages resulting from the reader's interpretation of the reproduced material due to its placement and context. Test item description ..................... : Bluetooth Audio Power Amplifier Trade Mark ....................................... : WUZHI AUDIO Manufacturer .................................. : Qingdao Wuzhi Intelligent Technology Co., Ltd. Model/Type reference ...................... : ZK-ST21 Listed Models ................................... : ZK-ST21PLUS, ZK-MTC, ZK-502MT, ZK-PT1, ZK-TB21PRO, ZK-AS21P,ZK-1002Q, ZK-3002T, ZK-3002TPRO, ZK-1001UM, ZK-302T, ZK-DAM L1, ZK-DAM H1, ZK-DAM H2 Modulation ....................................... : GFSK, Π/4DQPSK Frequency ......................................... From 2402MHz to 2480MHz Ratings ............................................. : DC 24V Result ............................................... : PASS Report No.: HTT202604437F01 Page 2 of 34 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China T E S T R E P O R T Equipment under Test : Bluetooth Audio Power Amplifier Model /Type : ZK-ST21 Listed Models : ZK-ST21PLUS, ZK-MTC, ZK-502MT, ZK-PT1, ZK-TB21PRO, ZK-AS21P,ZK-1002Q, ZK-3002T, ZK-3002TPRO, ZK-1001UM, ZK-302T, ZK-DAM L1, ZK-DAM H1, ZK-DAM H2 Model Declaration The PCB board, circuit, structure and internal of these models are the same, Only model number is different for these model. Applicant : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Manufacturer : Qingdao Wuzhi Intelligent Technology Co., Ltd. Address : No. 438, Xiaoxinhe Road, Huangdao District, Qingdao City, Shandong Province, China Test Result: PASS The test report merely corresponds to the test sample. It is not permitted to copy extracts of these test result without the written permission of the test laboratory. Report No.: HTT202604437F01 Page 3 of 34 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park, Hangcheng Road,Nanchang Community, Xixiang Street, Bao'an District, Shenzhen, Guangdong, China Contents 1 TEST STANDARDS .......................................................................................................................................... 4 2 SUMMARY ........................................................................................................................................................ 5 2.1 General Remarks .................................................................................................................................... 5 2.2 Product Description ................................................................................................................................. 5 2.3 Equipment Under Test ............................................................................................................................ 5 2.4 Short description of the Equipment under Test (EUT) ............................................................................ 5 2.5 EUT configuration .................................................................................................................................... 6 2.6 EUT operation mode ............................................................................................................................... 6 2.7 Block Diagram of Test Setup .................................................................................................................. 6 2.8 Related Submittal(s) / Grant (s) .............................................................................................................. 6 2.9 Modifications ........................................................................................................................................... 6 3 TEST ENVIRONMENT ...................................................................................................................................... 7 3.1 Address of the test laboratory ...........................................................................................................…

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Shenzhen HTT Technology Co., Ltd. Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 1 of 2 Product Name: Bluetooth Audio Power Amplifier Model No.: ZK-ST21 Test Setup Photo Radiated Emission Report No.: HTT202604437 Shenzhen HTT Technology Co.,Ltd. Tel: 0755-23595200 Fax: 0755-23595201 1F, Building B, Huafeng International Robotics Industrial Park,Hangcheng Road, Nanchang Community, Xixiang Street, Bao'an District, Guangdong Province, Shenzhen, China Page 2 of 2 RF Test ------End------