100W amplifier circuit

Posted on Feb 6, 2014

This is a 100 watt basic power amp that was designed to be (relatively) easy to build at a reasonable cost. It has better performance (read: musical quality) than the standard STK module amps that are used in practically every mass market stereo receiver manufactured today. When I originally built this thing, it was because I needed a 100 WPC amp

100W amplifier circuit
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and didn`t want to spend any money. So I designed around parts I had in the shop. The design is pretty much a standard one, and I`m sure there are commercial units out there that are similar. To my knowlwdge, it is not an exact copy of any commercial unit, nor am I aware of any patents on the topology. To experienced builders: I realize that many improvements and refinements can be made, but the idea was to keep it simple, and should be do-able by anyone who can make a circuit board and has the patience not to do a sloppy job. The input stage is an LF351 op amp which provides most of the open loop gain as well as stabilizes the quiescent dc voltage. This feeds a level shift stage which references the voltage swing to the (-) rail. The transconductance stage is a darlington, to improve high-frerqency linearity. The 2SC2344 by itself has a rather large collector-base capacitance which is voltage dependent. The MPSA42 presents this with a low-z and has a C(ob) of only a few pf that is effectively swamped by the 33pF pole-splitting cap. The stage is supplied by the 2SA1011 active load (current source) which is about 20 ma. The current to the stage is limited by the 2N3094 to about 70 ma under worst case. The output is a full complementary darlington with paralleled outputs. Although you could "get away with" only one if only 8 ohm easy-to-drive loads are used, this is not recommended. The use of parallel devices increases the...

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