4 channel 125 watt RMS Sound System

Designer: jim22747

Project Time: 20+ hours
Project Complexity: Professional
Project Cost: $500-$1000

It uses MJ-802 and MJ-4502 complimentary power transistors in the output in a direct-coupled arrangement. The frequency response of this system is flat from around 10 Hz to over 30K Hz (plus or minus 1 dB), with around .05-.07% THD up to full power output. Noise is around 90dB down. I built an audio sweep-function generator back then for testing audio systems, and had this amplifier feeding 100 watt 8 ohm power resistors until the glaze on them started to melt, with no problems and no overheating. Cranked up to full power with no signal, you can’t tell that it is on. It has a separate 4-Channel Bipolar Power Supply (+ and – 62 Volts) with three very large custom-wound power transformers inside (made for me as prototypes by a previous employer), 3 high current bridge rectifiers, and many very large computer-grade electrolytic capacitors in the power supply. It also has a separate 4-Channel Pre-Amp with relay switching for 2 or 4 channel audio. I’ve never had any problems with this system, but the pre-amp has seen several changes and updates over the years. The most recent being updated to interface it with my computer. The computer is an AMD Athlon Nforce 2 ultra 400 XP3200+ machine with big hard drives and a Santa Cruz sound card, which I use as a ‘Jukebox’. All enclosures, Printed Circuit boards, etc. were custom built by me.

About The Designer
I am a retired engineer, and have been working with electronics and automation for over 40 years. I have been building speaker systems, amplifiers, and instruments since I was 15. My back ground is Communications Electronic Tech in the U.S. Navy in the mid 1960’s, CNC Technician, Senior CNC Technician, Technical Supervisor, Department Manager and Process Development Engineer for Fortune 500 companies, using Autodesk Inventor for the last 10 years before retiring.

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