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449 lines
19 KiB
449 lines
19 KiB
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
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<meta http-equiv=Content-Type content="text/html; charset=windows-1252">
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<title>The Class-A Amplifier Site - A JLH Amp for the Quad ESL57</title>
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<link rel="StyleSheet" href="class-a.css" type="text/css" media="screen, print">
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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 page was last updated on 4 February 2002</span></p>
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<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back to Index ]</a></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:20.0pt'>A
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JLH Class-A for the Quad ESL57</span></b></p>
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<p align=center style='text-align:center'><b><span style='font-size:20.0pt'> </span></b></p>
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<p align=left style='text-align:left'>Credits: Original design - John Linsley Hood</p>
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<p align=left style='margin-left:50.2pt;text-align:left;text-indent:21.8pt'>Circuit modifications - Geoff Moss</p>
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<p align=left style='margin-left:50.2pt;text-align:left;text-indent:21.8pt'>Layout, pcbs and construction - Nick Gibbs</p>
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<p> </p>
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<p>This version of the JLH Class-A amplifier is the result of a series of
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emails and design discussions which culminated in the subsequent construction
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of two high current amplifiers specifically optimised to drive Quad ESL57
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electrostatic speakers.</p>
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<p> </p>
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<p>Some months ago, I received an email from Nick Gibbs regarding his 1969 JLH
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and a possible upgrade to a 1996 version with a higher quiescent current.
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Nick's 1969 JLH was about 16 years old and had been in almost daily use. It had
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a 27V supply rail and a quiescent current of 1.2A and Nick was using it to
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drive his Quad ESL57s, since neither his Quad 405 nor JLH MOSFET amps would do
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so without tripping the protection circuits. The little 10W JLH worked well
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with the ESL57s, albeit with some occasional clipping on louder passages, and
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Nick felt that a higher current version would best meet his needs.</p>
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<p> </p>
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<p>The ESL57 is a difficult load to drive in that it is capacitive and its
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impedance drops to a low of around 2ohm at 15kHz. A high current delivery is
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therefore required but, to offset this, the maximum voltage that should be
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applied to the ESL57 is only 33Vp-p. It seemed that a JLH Class-A with a
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reduced supply rail voltage and a higher quiescent current would be ideal.</p>
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<p> </p>
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<p>After we exchanged a number of (sometimes lengthy) emails, the final design
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evolved. It was a cross between the 1969 and 1996 versions (hopefully with the
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better parts of each <span style='font-family:Wingdings'>J</span>) operating
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off +/-20V supply rails and with a quiescent current of between 3.5A and 4A.
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The circuit is shown in Fig.1, but it should be noted that Nick used MJ802
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output transistors in place of the 2N3055s since he already had these devices
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available.</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=738 height=433 src="jlheslfig1.gif"></p>
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<p align=center style='text-align:center'>Fig. 1 - The Final Circuit.</p>
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<p align=center style='text-align:center'> </p>
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<p>As can be seen, the circuit is a mixture of the two original JLH versions,
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with modifications to enable an increase in quiescent current. Parallel pairs
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of output transistors have been used to keep the dissipation in each device at
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an acceptable level. The 0R1 emitter resistors are included to ensure equal
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cu+rrent sharing between each device. The quiescent current control is the
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standard 1969 bootstrap method whereby C4 maintains a constant voltage across
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RV2 and thus a constant dc current into the bases of the output transistors.</p>
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<p> </p>
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<p>The input stage of the 1996 version has been utilised, but for dc offset
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control the 7815 has been replaced with a constant current source to avoid the
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instability problems that have been encountered when the 7815 is operated at a
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low current. Several capacitor values have been increased to modify the low
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frequency ‑3dB point and to reduce low frequency distortion. High quality
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components have been used throughout.</p>
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<p> </p>
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<div class=MsoNormal style='margin:0cm;margin-bottom:.0001pt'>
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<hr size=3 width="100%" align=left>
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</div>
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<p> </p>
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<p>Addendum - 4 February 2002 </p>
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<p> </p>
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<p>Note, care must be taken to ensure that R5 and RV2 are adequately rated. The
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current through these components is slightly greater than the sum of the output
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transistor base currents. The output transistor base current is the output
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transistor quiescent collector current (Ic) divided by the current gain (Hfe)
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of the device. The current through R5 and RV2 is therefore approximately equal
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to 4 x Ic / Hfe and this should be calculated for the chosen output transistor
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quiescent current and output transistor type. It is recommended that output
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transistors with a gain of 100 or more at the working collector current are
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used in this design to reduce the power rating requirements for R5 and RV2.</p>
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<p> </p>
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<p>Whilst it should not be difficult to obtain fixed resistors with the
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required power rating, the preset potentiometer could be more of a problem
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since the more common ones are only rated at 0.5W or 1W, though higher rated
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devices are available. It must be remembered that the power rating of a preset,
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when connected as a rheostat, is proportional to the length of track in use.
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The required power rating must therefore be calculated from the current flowing
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through the preset and the full preset resistance value. </p>
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<p> </p>
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<p>With high gain (>100) output transistors and a quiescent current of 3A, a
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1W device should be adequate for R5 and 2W for RV2, provided RV2 is no greater
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than 500ohm. If a larger value of RV2 is found to be necessary, it will be best
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to use a 2W fixed resistor in series with RV2 to avoid the need for a higher
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power rated preset. (Note, the original value shown in the Fig. 1 for RV2 was
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2kohm. This value has been changed due to the power rating considerations).</p>
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<p> </p>
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<p>If RV2 needs to be set to below about 300ohm due a particular combination of
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quiescent current and transistor gain, I suggest that R5 be reduced to between
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50 and 100ohm to avoid the need for increasing the size of the bootstrap
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capacitor C4.</p>
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<p> </p>
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<div class=MsoNormal style='margin:0cm;margin-bottom:.0001pt'>
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<hr size=3 width="100%" align=left>
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</div>
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<p> </p>
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<p>The power supply (one for each channel) is shown in Fig. 2. This is
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basically the standard LM338K circuit, included elsewhere on this site, with
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some capacitor variations/additions.</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=766 height=542 src="jlheslfig2.gif"></p>
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<p align=center style='text-align:center'>Fig. 2 - The Power Supply</p>
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<p align=center style='text-align:center'> </p>
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<p>Nick initially adjusted the quiescent current to 3.5A but found after
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listening tests that increasing this to 4A gave a noticeable improvement. Even
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at an Iq of 4A the amps run cool due to the substantial heatsinking (0.5degC/W
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for each output device and each LM338K). Variations in quiescent current and dc
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offset with temperature are minimal, with an Iq of 3.8A and a dc offset of less
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than 35mV at switch-on. </p>
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<p> </p>
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<p>As for the sound quality, Nick's initial comments are summarised below:</p>
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<p> </p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>I have just spent two hours listening ... I cannot
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believe the improvement over my old JLH. I have ended up with 20V rails and an
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Iq of 4A. You may well understand that I am feeling a little emotional at the
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moment so I will attempt to quantify the sound in a point form:-</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>Female vocal - incredible, makes the hairs on the back
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of my neck stand up.</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>Instruments and singers now appear as solid 3D
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objects, they have constant depth, if that makes sense?</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>No blurring of image or loss of depth on loud moments.</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>Acoustic guitar - real!</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>Bass - although the ESLs are 3dB down at 55Hz
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everything is so well defined, I would say at this point that my old JLH was
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brilliant here too.</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>I can hear more hiss from the source material,
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although I cannot as yet fault the top end reproduction, the ESLs are very
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revealing.</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>I think more than anything else it's the fact that the
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amps NEVER appear to get confused (?) (increasing the Iq from 3.5A to 4A
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prevented slight confusion/clipping (recovery) with heavy mid to upper
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band periods, at the levels I listen at). Constant image solidity,
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depth and remarkable detail at all times are what these amps are about,
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bloody brilliant!</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>I have broken into my Cambletown Whisky as celebration
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</span></p>
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<p style='margin-top:0cm;margin-right:42.55pt;margin-bottom:0cm;margin-left:
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42.55pt;margin-bottom:.0001pt'><span style='font-size:11.0pt'> </span></p>
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<p>And a few days later:</p>
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<p> </p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>The amplifiers just get better the more you listen,
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real instruments and human voice are truly superb and very involving, plus of
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course the sensational 3D solid imaging. I wish you could hear them. </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>I am slowly working my way through my CD collection
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with the new amps, and it just gets better!</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p>And Nick's most recent comments:</p>
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<p style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;margin-left:42.55pt;
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margin-bottom:.0001pt'> </p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>The combination of Marantz CD17 MkII + new amps +
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ESL57 is the first system that I have EVER heard that can do justice to the
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sound of a piano. I have been listening to a Deutsche Grammophon recording of
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Liszt's Hungarian Rhapsody, a little hissy, but for the first time, the attack
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(?, I don't know how to describe this), the first instances of a piano note and
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all that goes with it to convince you that you are listening to a piano, is
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there. I have friends who play the piano and so I often listen to the real
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thing. I consider this ability of the new amps very important. I thought the
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ESLs would give me this with pretty much any amp, but it has taken the new JLH
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amps to actually do it. Additionally, the insight into Bizet's Carmen, again on
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Deutsche Grammophon, is exceptional. </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>Increasing the size of the electrolytics from 220uF as
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in my original JLH to 470uF has very noticeably extended the bass response.</span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'> </span></p>
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<p align=left style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;
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margin-left:42.55pt;margin-bottom:.0001pt;text-align:left'><span
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style='font-size:11.0pt'>The original JLH is a magnificent amplifier, but with
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the modifications it has become outstanding."</span></p>
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<p style='margin-top:0cm;margin-right:1.0cm;margin-bottom:0cm;margin-left:42.55pt;
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margin-bottom:.0001pt'> </p>
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<p><b>Circuit Boards</b></p>
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<p> </p>
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<p>Nick has kindly supplied me with a copy of his pcb layout for both the
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amplifier board and the regulator board in case they are of interest to other
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constructors. These are reproduced below at full size. It should be noted that
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the amplifier board is laid out for Caddock MP930 series power resistors, on
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heatsinks, for R5, R9, R10, R11 and R12 and also that Nick has used two
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resistors in series (100ohm and 50ohm) for R5 as these were more readily
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available (and cheaper). Component overlay diagrams have also been included
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after the pcb diagrams.</p>
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<p> </p>
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<p align=left style='text-align:left'>The actual board sizes are: Amplifier board 8.55 x 5.25 (217mm x 133mm)</p>
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<p align=left style='text-align:left'>Regulator board 3.3 x 2.9 (84mm x 74mm)</p>
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<p align=left style='text-align:left'> </p>
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<p> </p>
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<p align=center style='text-align:center'><img border=0 width=501 height=814
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src="jlheslampboard.gif"></p>
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<p align=center style='text-align:center'>Fig. 3 Amplifier pcb (viewed from copper side)</p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'><img border=0 width=314 height=276
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src="jlheslregboard.gif"></p>
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<p align=center style='text-align:center'>Fig. 4 Regulator pcb (viewed from copper side)</p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'><img border=0 width=501 height=814
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src="jlheslampoverlay.gif"></p>
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<p align=center style='text-align:center'>Fig. 5 Amplifier pcb overlay (viewed from component side)</p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'><img border=0 width=314 height=276
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src="jlheslregoverlay.gif"></p>
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<p align=center style='text-align:center'>Fig. 6 Regulator pcb overlay (viewed from component side)</p>
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<p align=center style='text-align:center'> </p>
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<p align=center style='text-align:center'> </p>
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<p><b>Finally</b>, for those of you interested in seeing the results of Nick's labours:</p>
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<p> </p>
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<p align=center style='text-align:center'><img border=0 width=642 height=422
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src="jlheslphoto1.jpg"></p>
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<p align=center style='text-align:center'>Photo. 1 The pcbs</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=642 height=610 src="jlheslphoto2.jpg"></p>
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center'>Photo. 2 Nearing completion, a plan view</p>
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center'> </p>
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<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center'> </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=642 height=478 src="jlheslphoto3.jpg"></p>
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<p align=center style='text-align:center'>Photo. 3 The finished amplifier with top cover removed</p>
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<p> </p>
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<p align=left style='text-align:left'> </p>
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<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back to Index ]</a></span></b></p>
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<p> </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:
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8.0pt'>Page created 04/11/2001 </span></p>
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<p><span style='font-size:8.0pt'>18/11/2001 Credits, pcb details and photos
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added</span></p>
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<p><span style='font-size:8.0pt'>19/11/2001 pcb details corrected</span></p>
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<p><span style='font-size:8.0pt'>21/11/2001 pcb overlays added</span></p>
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<p><span style='font-size:8.0pt'>31/01/2002 R5/RV2 power rating notes added</span></p>
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<p><span style='font-size:8.0pt'>04/02/2002 R5/RV2 notes revised and separated
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as an Addendum. RV2 value changed to 500R in Fig. 1 (was 2k)</span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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