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<title>The Class-A Amplifier Site - Updated Power Supply</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 1 May 2004</span></p>
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<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back
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to Index ]</a></span></b><b><span style='font-size:20.0pt'> </span></b></p>
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<p><b> </b></p>
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<p align=center style='text-align:center'><b><span style='font-size:20.0pt'>Updated
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Power Supply</span></b></p>
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<p> </p>
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<p align=center style='text-align:center'><b>A revised (but unpublished),
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regulated power supply for the 1996 JLH Class-A amplifier.</b></p>
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<p> </p>
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<p> </p>
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center'><img
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border=0 width=729 height=554 src="jlhnewpsfig1.gif"></p>
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<p> </p>
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<p> </p>
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<p><b>Notes</b></p>
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<p> </p>
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<p>The LM338T was specified by the originator of this diagram. Due to its poor
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junction to case thermal resistance (4degC/W) and its low maximum junction
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temperature (125degC), the size of the heatsink and the maximum volt drop
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across the device are extremely critical for satisfactory working. The volt
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drop should be limited to between 2.5V and 6V if at all possible (based on a
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heatsink of 1degC/W for each device and an ambient of 25degC). The lower limit
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is set by the drop out voltage of the regulator and the higher voltage will be
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determined by the transformer secondary voltage, the transformer regulation,
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the diode losses and the fluctuations in the mains supply voltage.<2E> </p>
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<p> </p>
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<p>The LM338K in a TO3 case (though much more expensive, at least here in the
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UK) has a greatly improved junction to case thermal resistance (1degC/W) and so
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will be more tolerant of heatsink size and will cater for a wider variation in
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volt drop across the device. I would therefore recommend that this version of
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the LM338 be used. </p>
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<p> </p>
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<p align=left style='text-align:left'>Bridge rectifiers BR1 and BR2 should have
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a minimum rating of 100V 25A (200V 35A preferred).</p>
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<p> </p>
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<p>C3 & C4 are best made from 3 x 10,000uF capacitors in parallel.</p>
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<p> </p>
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<p>C3 & C4 should be 50V minimum.</p>
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<p> </p>
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<p>C5 & C6 should be 25V minimum.</p>
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<p> </p>
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<p>C7 & C8 should be 35V minimum.</p>
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<p> </p>
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<p>VR1 & VR2 are adjusted, under load, to give +/-22V supply rails.</p>
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<p> </p>
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<p> </p>
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<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back
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to Index ]</a></span></b></p>
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<p> </p>
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<p><b><span style='font-size:8.0pt'>HISTORY:<3A><> </span></b><span
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style='font-size:8.0pt'>Page created 01/05/2001</span></p>
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<p><span style='font-size:8.0pt'>16/05/2001 Diagram redrawn and components
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renumbered </span></p>
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<p><span style='font-size:8.0pt'>14/08/2001 LM338 notes revised </span></p>
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<p><span style='font-size:8.0pt'>01/05/2004 Reference to separate regulators
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for each channel</span></p>
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<p><span style='font-size:8.0pt'><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD><EFBFBD> fed from a common
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rectifier/capacitor removed</span></p>
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<p><b><span style='font-size:8.0pt'> </span></b></p>
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</div>
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</body>
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