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<title>The Class-A Amplifier Site - JLH Class-AB Amplifier</title>
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<body background="slate.jpg" bgcolor="#E8E8E8" lang=EN-GB link=blue vlink=blue>
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<div class=Section1>
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<p align=right style='text-align:right'><b>The Class-A Amplifier Site</b></p>
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<p align=right style='text-align:right'><span style='font-size:10.0pt'>This
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page was last updated on 20 July 2001</span></p>
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<p><b><span style='color:blue'>[</span> <a href="index-2.htm"
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title=index-2.htm>Back to Index</a> <span style='color:blue'>]</span></b></p>
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<p> </p>
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<p> </p>
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<p align=center style='text-align:center'><b><span style='font-size:20.0pt'>15-20W
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Class AB Audio Amplifier</span></b></p>
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<p> </p>
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<p align=center style='text-align:center'><b><span style='font-size:16.0pt'>A
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design with class-A performance but reduced thermal dissipation</span></b></p>
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<p> </p>
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<p align=center style='text-align:center'><b><i>by J. L. Linsley Hood</i></b></p>
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<p class=MsoNormal align=center style='text-align:center'><span
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style='font-size:12.0pt'>(Wireless World, June/July 1970)</span></p>
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<p class=MsoNormal><span style='font-size:12.0pt'> </span></p>
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<p class=MsoNormal><span style='font-size:12.0pt'> </span></p>
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<p>I have included this article because, in a way, it is a follow-up to the original JLH Class-A design. The topology is very similar to the Class-A circuit with the exception that the output stage operates in push-pull, therefore the amplifier can continue to deliver the necessary power (within the limits of the power supply and output transistors) when the load requires currents above the Class-A bias level. This circuit may be of interest to those with a limited size of heatsink or who are looking for a simple Class-AB design that has proven subjective qualities.</p>
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<p> </p>
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<p>There are plenty of Class-AB designs on the Web that abound with differential input stages, constant current sources, current mirrors, cascoding, Darlington/compound pair output stages etc. etc., but I have seen very few, if any, that offer a simple, current feedback circuit of proven ability.</p>
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<p> </p>
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<p>The article gives an insight into JLH’s thought process when designing amplifiers and further confirms that his designs are based on both subjective listening and objective measurement (contrary to some of the suggestions that I have seen).</p>
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<p> </p>
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<p>The amplifier can be biased into full Class-A operation and, as with the 1969 Class-A amplifier, this circuit could be modified to operate from dual supply rails.</p>
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<p> </p>
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<p style='margin-left:55.0pt'><a name=index></a> </p>
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<p style='margin-left:55.0pt'><b>Part 1 -</b> <b><a href="jlhab1.htm" title="Class Distinction in Audio Amplifiers">Class Distinction in Audio Amplifiers</a></b></p>
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<p style='margin-left:55.0pt'> </p>
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<p style='margin-left:55.0pt'><b>Part 2 -</b> <b><a href="jlhab2.htm" title="15-20W Class AB Audio Amplifier">15-20W Class AB Audio Amplifier</a></b></p>
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<p style='margin-left:55.0pt'> </p>
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<p style='margin-left:55.0pt'><b>Part 3 -</b> <b><a href="jlhab3.htm" title="Letters to the Editor">Letters to the Editor</a></b></p>
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<p style='margin-left:71.5pt'> </p>
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<p> </p>
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<p> </p>
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<p><b><span style='color:blue'>[</span> <a href="index-2.htm" title=index-2.htm>Back to Index</a> <span style='color:blue'>]</span></b></p>
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<p> </p>
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<p><b><span style='font-size:8.0pt'>HISTORY:</span></b><span style='font-size: 8.0pt'> Page created 20/07/2001 </span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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