170W Audio Power Amplifier Circuit by NE5534

170W Audio Power Amplifier Circuit by NE5534


The circuit aims to produce a power amplifier with the use of an operational amplifier NE5534 at its input compared to classic designs which use differential amplifiers.

TerminologyNE5534 – a high performance low noise operational amplifier having a combined excellent AC and DC characteristics and features such as capability of external compensation, capability of offset nulling, wide range of voltage supply, low harmonic distortion, with output voltage swing at peak to peak, high slew rate, 100 dB common mode rejection ratio, high gain of DC Voltage, unity gain bandwidth at 10 MHz, and noise voltage of equivalent inputMJE350 – a plastic medium power PNP Silicon transistor designed for use in line-operated applications such as low power, line-operated series pass and switching regulators requiring PNP capability due to its plastic thermopad package, excellent DC current gain, and high collector-emitter sustaining voltageBD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximumBC556 – PNP general purpose transistors used for amplification and switching due to its low current and low voltage Specifications

Input sensitivity
Input impedance
Output Power  (0.1% THD)
Slew Rate
Signal to Noise ratio
IMD (50HZ : 1KHZ   4:1)
: 1 V(RMS)
: 17.8 KOhm
: 164 Watt -8 Ohms      275 Watt- 4 Ohms
: 20 V/μs
: >96 dB
: <0.004% (1KH<) in 1 Watt -8 Ohms and <0.05% (20HZ-20KH<)
: <0.003% in 1 Watt 8 Ohms  and <0.0035% in 100 Watt 8 Ohms
: >345 in 1 KH< and <275 in 20 KH<

Circuit Application

The circuit is being powered by two 15V power supply for each channel, that is regulated by the combination of resistors R7-9 and Zener diodes D1-2. The frequency bandwidth is limited to a specific value due to the presence of low pass filter from R2 & C2 and high pass filter from R3 & C1. The total feedback of the circuit is concentrated on the positive non-inverting input of the IC1 that functions as differential amplifier. The node of R4-5 is applied with the feedback voltage from the output through R6. The current regulated by R10 & R14 across Q1-2 is 10mA as they operate in class A while in class AB, the power transistors Q8-11 operate in the output with a regulated constant current of 100mA each power transistor, done by trimmer TR1 and Q3. This will match the voltage drop at 27mV, higher than the provided voltage of R25-28. 

A heatsink with sufficient dimension to provide cooling is placed above Q1-2-3-6-7 to maintain the possible thermic stability while the protection from reverse voltages from the load is performed by diodes D9-10.Additional fan can also be employed to suppress high temperature for efficient operation. The stability during high frequencies is sustained by the Zobel network consisting of R29 & C18. Due to capacitive load, the amplifier output is protected by a 1mm diameter 15-coil inductor L1 with R30 for square pulses passage. 

Each power amplifier that is created will correspond to a transformer to handle large capacitors and provide proportional power. The transformer input is connected to the AC line which can incorporate delay circuit for protection from excessive supply. The cables that would carry the current to the amplifier should be appropriate enough.

Part List

R5-8-10-14=560 ohm
R7-9=3.3Kohm 5W
R11-15=150 ohm
R16=680 ohm
R17=180 ohm
R18-19=10 ohm
R24=56 ohm
R25-26-27-28=0.27 Ohm/ 5W
R29=10 ohm/  5W
R30=2.2  ohm/5W
C1=2.2uF 100V MKT
C2=1nF 100V MKT
C4=33pF 160V  polystyrene
C5=47pF  160V polystyrene
C6-7=100uF  63V
C12=470pF  160V polystyrene
C13=680nF 100V MKT
C16-17=150nF 100V MKT
C18=33nF 250V MKT
C19-20=100uF 100V
L1= See text
F1-2=6.3AT FUSE
TR1=250 ohms  TRIM
D1-2=15V 1.5W Z


The specific wattage of this power amplifier circuit at 170W can be used in stereo midrange/high frequency bi-amping applications, low power distribution systems, and near field monitoring. It can also be used effectively in providing power to acoustic instruments and vocals setups in bars, restaurants, and small clubs. In home theaters and PCs, they can power subwoofers and similarly, car booster amplifiers. The demands of gigging musicians, DJs, and sound engineers are met by this design.

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