About the article

Protection system for power amplifiers

An indispensable companion for the paX amplifier

Protection system for power amplifiers
In Part I and part II about the paX amplifier we developed a complete error-correction power amplifier. In this accompanying article the protection system is described, that is constructed on a separate small circuit board that can be mounted directly on the speaker binding posts. We will also discuss the Safe Operation Area for the output devices and how it is modeled in the protection system.
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Gerber file

CAM/CAD data for the PCB referred to in this article is available as a Gerber file. Elektor GREEN and GOLD members can exclusively download these files for free as part of their membership. Gerber files allow a PCB to be produced on an appropriate device available locally, or through an online PCB manufacturing service.

Elektor recommends its business partner Eurocircuits as the company of choice for its own prototypes and volume production in selected cases.

Component list


(0.25W metal film)

R1 = 10kΩ

R4,R9 = 12Ω

R5,R12 =  9kΩ1

R6,R13 =  6kΩ19

R7,R11 = 2kΩ0

R8,R10 =  536Ω

R14,R15 = 270Ω

R16,R22 = 3kΩ9

R17 = 36kΩ

R18 =  523Ω

R19 = 16kΩ

R20 = 56kΩ

R21 = 18kΩ

R24,R25 = 22kΩ

R23 = 3.9kΩ 1W


C1,C2 = 100nF ceramic

C3 = 330µF 6.3V

C4 = 22nF ceramic

C5 = 33µF 10V

C6 = 4.7 µF 25V


D1,D6-D11 = 1N4148

D2,D5 = zener diode 18V 0.5W

D3,D4 = zener diode 12V 0.5W

J2 = Dual color LED

Q1 = 2SC2910

Q2 = 2SA1208

Q3 = BC546B

Q4 = BC556B

U1 = MCT6 (Fairchild)

U2 = UPC1237 (NEC) or NTE7100 (NTE Electronics)


J6 = header 2 x 1 pin

J7 = 2 x 3 pin DIL flatcable header

RL1 = Amplimo LS relay

DIL socket 2 x 4

SIL socket 1 x 8 pin

PCB, order code 071086-1
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