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<title>The Class-A Amplifier Site - JLH Class-A Update</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 17 August 2003</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></p>
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<p align=right style='text-align:right'> </p>
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<p align=center style='text-align:center'><b><span style='font-size:20.0pt'>JLH
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Class-A Update</span></b></p>
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<p> </p>
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<p> </p>
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<p>I had originally intended that this page would be a step-by-step record of the
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modifications carried out during the past year by one constructor – Tim Andrew.
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However, recent ill health has meant that I have been unable to spend much time
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sitting at my pc so, rather than incur yet more delay in publishing the
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results, I have decided to write a short summary instead. I am very pleased
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that Tim has taken the time to supplement this with his own comments. At the
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end of the page is a brief update on the higher power ‘JLH for ESL’ circuit.</p>
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<p> </p>
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<p>Tim is a professional musician (a classical concert pianist) and so I trust
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his subjective judgement when it comes to assessing the accuracy and realism of
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sound reproduction. Before Tim first contacted me, he had built a kit version
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of the 1996 design, which he had subsequently upgraded with higher quality
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components. Though Tim was happy with the results, he was keen to see if
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further improvements could be made to the sound quality and I was pleased to be
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able to suggest various circuit modifications, the majority of which
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subsequently proved to be very worthwhile. Each of the modifications was
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carried out separately so that the results could be evaluated on an individual
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basis.</p>
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<p> </p>
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<p>Rather than show schematics for each stage, I will start off with the
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penultimate circuit and include some appropriate comments.</p>
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<p> </p>
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<p align=center style='text-align:center'><img border=1 width=691 height=420 src="jlhupdatefig1.gif"></p>
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<p> </p>
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<p align=center style='text-align:center'>Fig 1 – The Penultimate Circuit</p>
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<p align=center style='text-align:center'> </p>
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<p align=left style='text-align:left'><b>Transistor substitutions</b></p>
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<p align=center style='text-align:center'> </p>
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<p>One of the first modifications was to try alternative output transistors.
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The MJL3281A gave an audible indication of oscillation and was quickly
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rejected. The MJ21194 sounded significantly better than the 2N3055 but, in
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Tim’s layout, introduced a low-level hum. The MJ15003 gave a similar
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improvement to the MJ21194, but without the hum, and so was retained for future
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use. At a later stage, the BC212 and 2N1711 (Q4 and Q3) were replaced with the
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2SA970 and 2SC3421.</p>
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<p> </p>
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<p><b>Output dc offset control</b></p>
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<p> </p>
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<p>The standard dc offset control circuitry (7815 and associated components)
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was replaced with a two transistor constant current source (Q5/Q6). I had
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various reasons for suggesting this change. Firstly, three terminal regulators
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are not renown for their quietness and so it did not seem like a good idea to
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inject the noisy output from one directly into the feedback loop. Also, I had
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received reports that certain 7815s oscillated due to the low current
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conditions under which they were being operated. </p>
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<p> </p>
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<p>However, one of the main benefits of the ccs is that the output dc offset
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variation as the amp warms up is greatly reduced. This is because the
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temperature coefficient of the ccs acts in the opposite direction to that of
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the input transistor (Q4) and negates the effect of temperature changes in Q4
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(assuming that the temperature of Q5 follows that of Q4). This cancellation of
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temperature coefficient effects can be put to further good use as will be seen
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later.</p>
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<p> </p>
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<p><b>Quiescent current control</b></p>
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<p> </p>
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<p>I first suggested that Tim try the 1969 bootstrap Iq control circuit, partly
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because the simulated distortion figures were half those for the 1996 version
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but mainly because I wanted to know how the two methods of Iq control compared
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in the same amplifier. I had received reports that the 1969 circuit (modified
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to dual supply rails) sounded better than the 1996 version, but I could not be
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sure that there were no other variables involved. As it turned out, the
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bootstrap circuit was a retrograde step and Tim immediately reverted to the
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original 1996 arrangement.</p>
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<p> </p>
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<p>I still had some nagging doubts about the 1996 Iq control circuit and so I
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suggested introducing another constant current source (Q7/Q8). As with the
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bootstrap circuit, the simulated distortion figures were still half those for
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the 1996 version but with the added advantage that the distortion did not
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increase at low frequencies due to a reduction in capacitor effectiveness. A
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further advantage was an increase in amplifier efficiency (or maximum output).
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The maximum output voltage swing with the ccs is greater than that for the standard
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1996 circuit and the maximum output current increases from around 1.35 to about
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1.5 times the quiescent current. </p>
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<p> </p>
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<p>When carrying out this modification, Tim reused the existing MJE371 for Q8.
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R10 has been retained to provide an easy means of measuring the quiescent
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current. To my relief, Tim found the second ccs to be worthwhile improvement.</p>
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<p><b> </b></p>
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<p><b>Power supply</b></p>
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<p> </p>
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<p>Whilst making the other alterations, Tim also took the opportunity to
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upgrade his power supply, initially by fitting larger bridge rectifiers and snubber
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capacitors and then by replacing the LM338s with ‘follower’ type discrete
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regulators, in line with my desire to remove unnecessary feedback loops from
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the overall circuit. The ‘follower’ regulators, basically a capacitance
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multiplier circuit with a fixed voltage reference (derived from a resistor fed
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by a ccs), gave a small improvement. A much greater improvement was obtained
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when separate regulators were provided for each amplifier, whilst retaining a
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common transformer, rectifier bridges and reservoir capacitors.</p>
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<p> </p>
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<p align=center style='text-align:center'><img border=1 width=690 height=420 src="jlhupdatefig2.gif"></p>
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<p> </p>
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<p align=center style='text-align:center'>Fig 2 – The Final Circuit</p>
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<p align=center style='text-align:center'> </p>
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<p><b>Removal of the feedback capacitor</b></p>
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<p> </p>
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<p>I had received emails from a couple of constructors reporting on the beneficial
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effects of removing the feedback capacitor (C4). I passed these comments on to
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Tim and he decided to try this modification for himself. </p>
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<p> </p>
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<p>This modification should be treated with caution. I would not recommend
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trying it unless the dc offset ccs (Q5/Q6) modification has been done first
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because otherwise the output dc offset variation during the warm-up period is
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likely to be in the order of several hundred millivolts. In Tim’s case, with
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the dc offset ccs fitted, the output dc offset variation with the feedback
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capacitor removed was only slightly higher than that which he had previously
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with the standard 1996 circuit.</p>
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<p> </p>
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<p>I believed that the offset variation could be reduced further by utilising
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the temperature coefficient of the Q5/Q6 ccs. I therefore suggested that R11 be
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made adjustable so that the temperature rise of Q5 could be varied. In this
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way, the output dc offset variation due to temperature changes in all stages of
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the amplifier could be compensated for, though this requires a lengthy, iterative
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process. With the amp at its normal operating temperature, the offset is
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adjusted to near zero using VR1. The offset when the amp is cold is then
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measured. VR3 is adjusted slightly, the amp is allowed to warm up and the
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offset is re-zeroed using VR1. The offset is then rechecked when the amp is
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cold and the process repeated until the minimum offset variation has been
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obtained. Tim has been able to achieve an output dc offset variation between
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switch-on and normal operating temperature of less than 50mV.</p>
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<p> </p>
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<div style='margin-left:14.2pt;margin-right:14.2pt'>
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<div class=MsoNormal>
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<hr size=3 width="100%" align=left>
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</div>
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</div>
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<p> </p>
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<p><b>15/03/2003</b><b> Addendum</b></p>
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<p> </p>
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<p>It has been brought to my attention (thanks Mietek and Rudy) that removing
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the feedback capacitor increases the hum level at the amplifier output, which
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is particularly noticeable with high sensitivity speakers and if a simple rectifier/capacitor
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power supply is used. I had not anticipated this, but some quick simulations
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soon indicated that removal of the feedback capacitor reduces the PSRR of the
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amp by a factor of about 3, causing any supply rail ripple to become more
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audible.</p>
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<p> </p>
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<p>Fortunately, the cure for this problem is relatively simple. The PSRR of the
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input stage ccs can be improved by the addition of a single capacitor,
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connected between the junction of VR3/R11 (Fig 2) and the +ve supply rail. Doug
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Self’s ‘Audio Power Amplifier Design Handbook’ indicates that this modification
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will improve the PSRR of the ccs by about 10dB. A capacitor value of 47uF will
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suffice, but higher values (within reason) can be used.</p>
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<p> </p>
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<p>The higher power (‘JLH for ESL’) circuit can be similarly modified by
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splitting R11 (Fig 3) into two 4k7 resistors in series and connecting the
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capacitor from the mid-point of these resistors to the +ve supply rail.</p>
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<p> </p>
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<p>This modification can also be carried out even if the feedback capacitor is
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not removed, and will give an improvement in PSRR with the corresponding
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reduction in hum.</p>
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<p> </p>
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<p align=center style='text-align:center'><img border=1 width=547 height=312 src="jlhupdatefig4.gif"></p>
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<p> </p>
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<div style='margin-left:14.2pt;margin-right:14.2pt'>
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<div class=MsoNormal>
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<hr size=3 width="100%" align=left>
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</div>
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</div>
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<p><a name=Addendum></a> </p>
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<p><b>17/08/2003</b><b> Addendum</b></p>
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<p> </p>
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<p>Several constructors have found that adding the 47uF capacitor to the input
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stage ccs after having removed the dc blocking capacitor from the feedback
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network has caused the ccs to become unstable. This has manifest itself by
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relatively large output dc offset variations when taking voltage readings
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around the input circuit or when a hand is moved near to the ccs components.</p>
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<p> </p>
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<p>In Tim’s case, a successful solution to this problem has been to replace Q5
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and Q6 with ‘slower’ transistors. The MPSA56 appears to work well in the ccs.
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Alternatively, the 47uF capacitor could be removed and the PSRR of the ccs
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improved by omitting VR3 and replacing R11 with a 1mA constant current diode
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(or an FET wired as a ccs to give a similar current).</p>
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<p> </p>
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<p>Adding base resistors (100R to 1k) to Q5 and Q6 and/or a 1k resistor between
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Q6c and Q4e should also help to improve stability.</p>
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<p> </p>
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<div class=MsoNormal><span style='font-size:12.0pt'>
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<hr size=3 width="100%" align=left>
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</span></div>
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<p> </p>
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<p><b>Tim’s comments on the modifications (Updated 17/08/2003)</b></p>
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<p> </p>
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<p>A few years ago I built the 1996 version JLH Class-A amplifier. Constructors
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of this amplifier have commented about its smooth sound, with many favourable
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comments and comparisons against valve designs and a few not so favourable
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comments with regard to its limited power output. In its standard 1996 form,
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which I built from a kit using cheap components, my first impressions of its
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sound were of smoothness coupled with a relaxed liquid musical flow which I
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found far preferable to anything else which I had previously heard. In the
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context of my system with speaker efficiency somewhere around 87dB/W and <span
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style='color:black'>with volume set correctly such as is appropriate for the
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perspective as recorded, or in other words "at a realistic level", </span>its
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limited power output has never been a problem. The amplifier and its power
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supply have since been subject to extensive component substitutions and
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substantial circuit modifications.</p>
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<p> </p>
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<p>As this section is about my impressions of the modifications that have been
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made to the circuit, a brief word on what I consider to be an
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"improvement" might be in order. I want to hear, with ease, the
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ambient signature of the recording venue, with a distinct impression of the
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space between its walls. Also, I want to notice, for example, the sound of the
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felt hammer of a piano hit the string, followed not only by the sound of the
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string vibrating but also the more subtle reflected and attenuated sounds of
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the hammer and its mechanism as these reverberate between the walls of the
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recording venue. This is sometimes more noticeable in larger venues where the
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reflected sound arrives later, albeit weaker. Those delicate piano harmonics
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must be reproduced with the greatest accuracy, enabling subtle shadings of
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timbre to be noticed, again with ease. As a pianist, I want to hear the
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"pitch" of the note as it decays through to its quietest moment as
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acutely as possible, but I want no hint of hardness or roughness. With
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orchestral strings for example, where there are many instruments playing
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together, I don't want to hear one homogeneous group, and I want transparency,
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not brightness.</p>
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<p> </p>
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<p>Professionally, I have a very close affinity with the piano. A difficult
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instrument to reproduce, it is perhaps more revealing of faults in the
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reproduction chain than can be the case with other instruments although the
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human voice is also very useful, for obvious reasons. It is my view that any
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modification that produces a more realistic rendition of the complex sound of
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this instrument, and the very subtle structure of its over-tones, will also
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represent an improvement in the accuracy of the amplifier overall. This has
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been the case during all my listening trials. It is worth mentioning that any
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modification which leads to an apparent decrease, for example in the level of
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the treble, will not necessarily be deemed to be an improvement, even if the
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new treble level is a welcome one, unless it is accompanied by an improvement
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elsewhere, improved detail or portrayal of nuance for example. From this, you
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will gather that I am not in the habit of 'voicing' the system, adjusting one
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thing to correct for another, but that I prefer to address the transparency of
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the system as a whole, with the aim of neutrality. Only then will I look at
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altering the balance, perhaps with a slight adjustment to the treble. It is
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through this approach (transparency first, followed by tonal balance) that I am
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now able to enjoy the vast majority of recordings in my collection, previously
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I had found many of these to be deficient in one way or another. Almost without
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exception, each modification has improved "difficult" recordings,
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whilst further improving others, often revealing a warmth and atmosphere, the
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previous lack of which had been wrongly attributed to the recording.</p>
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<p> </p>
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<p>Though considerable time has been expended on both the amplifier and its
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power supply, I find it sobering to say the least that improvements made to
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power supply, specifically to the method of its delivery into various parts of
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the amplifier circuit have been so rewarding. The following is a list of the
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modifications that, with considerable help from Geoff, I have been able to
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carry out on the 1996 version of the JLH. Also included are my opinions of the
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results of these. Each substitution has been carried out individually, this has
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enabled subsequent and hopefully accurate (but not always positive!)
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evaluation. !</p>
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<p> </p>
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<p><b>The Amplifier</b></p>
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<p> </p>
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<p><u>Input capacitor.</u> </p>
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<p>The cheap polycarbonate(?) 1uF input capacitor was replaced with a 470nF
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Mcap "Audiophile" polypropylene type. This led to an improvement in
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both bass firmness and in detail, treble sounded less bright. Later, I replaced
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the Mcap 470nF with Audio Note paper-in-oil 470nF. This sounds very different,
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smooth, warm and open with much more textural detail and firmness in the bass.
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There is some loss of focus when compared with the better plastic types and the
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positioning of instruments within the stage is not as precise as it could be,
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however none of the plastic types I have tried has approached the naturalness
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and openness of the paper-in-oil, particularly in the treble, and any
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shortcomings are easily forgiven in light of considerable improvements
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elsewhere. This simple modification has since proved to be one of the most
|
|
|
effective. I have also tried a polystyrene type (333nF) which sounds more
|
|
|
detailed and focussed than anything else tried previously, though there is a
|
|
|
tendency to sound a little "squeaky" on occasions (placing a small
|
|
|
paper-in-oil capacitor across it improves this considerably), nevertheless I
|
|
|
prefer this to most polypropylene types, many of which sound hard and slightly
|
|
|
blurred to me. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Resistors.</u> </p>
|
|
|
|
|
|
<p>All standard grade metal film resistors in both critical and semi-critical
|
|
|
parts of the circuit were replaced with tantalum film types. </p>
|
|
|
|
|
|
<p>Improved smoothness and texture, with a more fluid sound. A slight
|
|
|
"mumbling" quality has been removed. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Output transistors.</u> </p>
|
|
|
|
|
|
<p>The 2N3055s were replaced with MJ21194. In comparison with these the 2N3055s
|
|
|
sound grey and rather diffused with less sense of authority, less detail and a
|
|
|
more prominent treble quality. In contrast, the MJ21194s have a noticeably
|
|
|
firmer sound with more ambience in the treble and greater detail. More natural
|
|
|
generally. Reluctantly, they were removed from the circuit due to a faint hum
|
|
|
which was not present with the 2N3055s.</p>
|
|
|
|
|
|
<p>Wanting to try something else, and now with the strong impression that the
|
|
|
2N3055s were less than ideal, I tried some MJ15003s.</p>
|
|
|
|
|
|
<p>This time, a substantial improvement over the 2N3055s. The MJ15003's bass is
|
|
|
both tauter and more authoritative, with cleaner treble and greater textural
|
|
|
detail.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>DC offset control.</u> </p>
|
|
|
|
|
|
<p>Replace 7815 with constant current source.</p>
|
|
|
|
|
|
<p>Result...Cleaner, smoother and weightier, with what can only be described as
|
|
|
an organic flow. It was obviously all there before, but I suppose it was masked
|
|
|
somewhat by the noise of the regulator. The volume can be increased further
|
|
|
without sounding "loud". A substantial improvement in all respects.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Iq control circuit.</u></p>
|
|
|
|
|
|
<p>The Iq control circuit was replaced with a bootstrap circuit (using an Elna
|
|
|
"Silmic"). Less clarity was the result, with less tonal variety and
|
|
|
focus, sounding more shut-in. The bootstrap simply doesn't sound as detailed. I
|
|
|
assume this is due to the presence of the bootstrap capacitor connected to the
|
|
|
signal path. Perhaps a Black Gate might improve things, but I suspect not
|
|
|
enough to equal the MJE371 circuit which is more transparent, open, dynamic and
|
|
|
uncoloured, the female voice sounds less "female" with the bootstrap
|
|
|
circuit. It strengthens my theory that those who prefer the earlier version of
|
|
|
the JLH do so because of the absence of the 7815 in the earlier circuit. I
|
|
|
would go further and say that due to the absence of both a bootstrap capacitor,
|
|
|
and an output capacitor, and with the ccs in place of the 7815, they might well
|
|
|
prefer the 1996 version, all other things being equal. My original Iq control
|
|
|
circuit was very quickly re-instated!</p>
|
|
|
|
|
|
<p><u><span style='text-decoration:none'> </span></u></p>
|
|
|
|
|
|
<p>It was not long until the original Iq control circuit was removed again,
|
|
|
this time replaced with a constant current source and with better results this
|
|
|
time. The initial reaction is to think that the treble detail and
|
|
|
"air" have been diminished with a reduction of transparency. On prolonged
|
|
|
listening things are rather different. There is actually more detail coming
|
|
|
across, coupled with a growing sense of "rightness". Sounds are
|
|
|
presented in a more natural light, gone is the spotlight effect with its
|
|
|
admittedly pleasant but artificial treble detail. String harmonics are more
|
|
|
balanced and proportioned with a sense that they now belong to the fundamental,
|
|
|
part of the whole. The gaps between rapid piano notes are often missed by
|
|
|
amplifiers, the JLH reproduces these well and they are even clearer now than
|
|
|
before. Familiar recordings of woodwind and brass instruments sound remarkably
|
|
|
smooth and natural. Differences in scale between smaller chamber music
|
|
|
recordings and larger scale works are now more clearly conveyed. It is
|
|
|
interesting to compare the sound of the Iq ccs circuit with that of the
|
|
|
bootstrap which shared many of the attributes of the ccs but had a lumpy and
|
|
|
coloured, slightly congested characteristic which I found unpleasant. Returning
|
|
|
to the standard 1996 Iq circuit the next day was quite a relief, this time I
|
|
|
have no plans return. I would miss the qualities that the Iq ccs circuit has
|
|
|
brought to the amplifier. Final thought........Recommended for those who want
|
|
|
to sit down for an evening of good music and a fine wine. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Feedback capacitor.</u></p>
|
|
|
|
|
|
<p>The 470uF Oscon (previously a very similar sounding 220uF Silmic) feedback
|
|
|
capacitor was replaced with link (needing a small change in value to the DC
|
|
|
offset ccs preset). The result of this change was a more open and natural
|
|
|
treble with an increased sense of fluidity, depth and ease. Hot/cold offset
|
|
|
variation are much greater without the feedback capacitor, in my circuit a
|
|
|
variation of 150mV was observed (with the feedback capacitor it was around
|
|
|
65mV), this was reduced by controlling the current through the ccs in an effort
|
|
|
to adjust the temperature compensation, but on a recent re-build of the circuit
|
|
|
this arrangement proved ineffective and was subsequently removed. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Driver transistor (2N1711).</u> </p>
|
|
|
|
|
|
<p>This was replaced with a 2SC3421. As with the other transistor substitutions
|
|
|
I have made in the JLH, the actual pitch of a note is more easily heard with
|
|
|
the 2SC3421s. The same characteristics introduced by the Iq ccs circuit are
|
|
|
still there but each single note now conveys more "meaning", more
|
|
|
clearly defined in time. Timing, of course, is a musician’s greatest asset! The
|
|
|
Iq ccs circuit introduced a smoother, rounder sound with a somewhat darker hue,
|
|
|
the extra transparency and openness brought about by the 2SC3421s has lifted
|
|
|
that slight darkness away whilst apparently retaining the smoothness and
|
|
|
naturalness of the Iq ccs. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Input transistor.</u> </p>
|
|
|
|
|
|
<p>The BC212 was replaced with 2SA970 with similar improvements to those
|
|
|
noticed with the 2SC3421.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><b>The Power supply.</b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Rectifier diodes. </u></p>
|
|
|
|
|
|
<p>Having tried snubber capacitors across the original "standard"
|
|
|
diodes with no noticeable improvement, the originals (and snubbers) were
|
|
|
replaced with schottky types. This seemed to be beneficial with more smoothness
|
|
|
and an improved "woody" quality with woodwind. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Regulators.</u> </p>
|
|
|
|
|
|
<p>The LM338K regulator circuit was replaced with a capacitance multiplier. The
|
|
|
bass now conveys more authority and the amplifier sounds a little warmer, also
|
|
|
with more detail. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Dual regulators.</u></p>
|
|
|
|
|
|
<p>The single capacitance multiplier was replaced with a new (adapted) dual
|
|
|
version allowing separate regulation for each channel. This warrants a detailed
|
|
|
write-up so I shall list my observations in the order in which I noticed them
|
|
|
and in descending order of their magnitude.</p>
|
|
|
|
|
|
<p><br>
|
|
|
It is only now that I have heard the new dual power supply, that I can identify
|
|
|
the sonic effects of the single supply. For the first, and most important
|
|
|
observation, I shall use a single piano note as an illustration. With the
|
|
|
single supply, when the note is struck there is an initial transient 'bump' as
|
|
|
the hammer hits the string, followed by the decay, which starts after the
|
|
|
initial 'bump' has subsided. With the dual supply, this initial transient is
|
|
|
less 'loud' (better controlled?) and it carries more weight and meaning, this
|
|
|
is followed by the decay which not only conveys better pitch, leading to more
|
|
|
emotion and tunefulness, but the decay starts sooner, its first moments not
|
|
|
masked by the apparently exaggerated impact of the hammer blow introduced by
|
|
|
the single supply. Also, due to the increased definition, the note seems to
|
|
|
decay more slowly, incidentally this is one of the more significant differences
|
|
|
between a small grand piano, and a large 'concert' grand where, due to the increased
|
|
|
string length of the larger instrument, its sustaining power is much greater. A
|
|
|
single note can therefore be followed more easily from start to finish. The
|
|
|
tonal signature and real colour of all instruments are now better conveyed.</p>
|
|
|
|
|
|
<p><br>
|
|
|
There is also a significant improvement in the quality of the treble where
|
|
|
there is greater transparency. For most of the time, it is less obvious than
|
|
|
before, and smoother, but little details previously almost un-noticed are
|
|
|
conveyed more clearly and with improved texture. This treble improvement was
|
|
|
unexpected and is a constant pleasure! </p>
|
|
|
|
|
|
<p><br>
|
|
|
The third improvement I have noticed is an improvement in the positioning of
|
|
|
individual instruments. The perceived stage width is not obviously any wider
|
|
|
than before, although I couldn't fault it before, on a good recording the stage
|
|
|
width was almost limitless, on a bad recording it had definite limits. This
|
|
|
hasn't changed, what has improved is the positioning of instruments within the
|
|
|
limitations of the stage width imposed by the recording, with instruments on
|
|
|
the edge of the stage more clearly conveyed in space with a better
|
|
|
"floating" feel to the acoustic coupled with a more acute sense of
|
|
|
the venue.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Filter capacitors.</u></p>
|
|
|
|
|
|
<p>Having previously bypassed the standard grade electrolytics with Elna
|
|
|
"Silmic" 100uF with little, if any improvement, this time the
|
|
|
original capacitors (30,000uF per rail) were replaced entirely with
|
|
|
"Silmics" (18,000uF per rail). A superb improvement in definition.
|
|
|
The scale of which came as quite a surprise.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><u>Conclusion.</u> </p>
|
|
|
|
|
|
<p>I consider the JLH in its present form, to be a very special amplifier. Its
|
|
|
ability to portray the acute sense of emotion and excitement contained in a
|
|
|
fine performance, through its accuracy and with such grace, coupled with its
|
|
|
ability to scale music's dynamic heights so convincingly, is rare. My most
|
|
|
sincere thanks to Geoff who, through spending so much time helping others like
|
|
|
me, has so far not had time to carry out these modifications for himself *.</p>
|
|
|
|
|
|
<p class=MsoNormal> </p>
|
|
|
|
|
|
<p><span style='font-size:10.0pt'>*<i> Unfortunately not the only reason -
|
|
|
Geoff</i></span></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<div style='margin-left:14.2pt;margin-right:14.2pt'>
|
|
|
|
|
|
<div class=MsoNormal>
|
|
|
|
|
|
<hr size=3 width="100%" align=left>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
<p><b> </b></p>
|
|
|
|
|
|
<p><b>Higher power circuit</b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The ‘JLH for ESL’ circuit, which can be used with conventional speakers as
|
|
|
well as electrostatics, already has a ccs for dc offset adjustment but it would
|
|
|
benefit from the other modifications outlined above. In particular, the use of a
|
|
|
ccs for quiescent current adjustment obviates the need for a high power preset,
|
|
|
which can sometimes be hard to find. </p>
|
|
|
|
|
|
<p align=center style='text-align:center'> </p>
|
|
|
|
|
|
<p align=center style='margin:0cm;margin-bottom:.0001pt;text-align:center'><img border=1 width=730 height=468 src="jlhupdatefig3.gif"></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p align=center style='text-align:center'>Fig 3 – The Higher Power Circuit</p>
|
|
|
|
|
|
<p align=center style='text-align:center'> </p>
|
|
|
|
|
|
<p>When used with conventional speakers, this circuit can deliver over 40W
|
|
|
provided the supply rail voltage and quiescent current are selected to suit a
|
|
|
specific load impedance. The supply rail voltage needs to be a couple of volts
|
|
|
higher than the peak output voltage swing and the total quiescent current
|
|
|
should be about 0.7 times the maximum output current. The power dissipated in
|
|
|
each output transistor (supply rail voltage times half the quiescent current)
|
|
|
should be limited to about 40 to 45W, assuming decent sized heatsinks are used
|
|
|
(0.6 to 0.8degC/W per transistor).</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The peak load voltage and current can be calculated from required power and
|
|
|
the speaker’s impedance in the normal way using:</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>Vpk = sqrt(2*Pwr*Rload) and Ipk = sqrt(2*Pwr/Rload)</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>To allow for speaker impedance variations, I would suggest that current is
|
|
|
calculated using ¾ of the speaker’s nominal impedance and voltage using 1½
|
|
|
times the nominal value. Of course, you are free to make your own assumptions
|
|
|
about speaker impedance variations and to calculate the required supply rail
|
|
|
voltage and quiescent current accordingly. From feedback I have received,
|
|
|
higher quiescent currents tend to sound better so you may wish to bias the
|
|
|
compromise between voltage and current accordingly (whilst keeping the power
|
|
|
dissipation in the output transistors at a safe level).</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The following table indicates the maximum power output into 8, 6 and 4ohm
|
|
|
loads for some standard transformer secondary voltages, assuming a resistive
|
|
|
load and without any allowance for the impedance variations mentioned above.
|
|
|
The supply rail voltages assume a regulated supply, with the consequential volt
|
|
|
drop, and the quiescent current has been calculated from either the maximum
|
|
|
current into 4ohm or, in the case of the 25 and 30Vrms secondary, the
|
|
|
transistor power dissipation limit. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<table class=MsoNormalTable border=1 cellspacing=0 cellpadding=0
|
|
|
style='margin-left:14.2pt;border-collapse:collapse;border:none'>
|
|
|
<tr style='height:19.85pt'>
|
|
|
<td width=189 valign=top style='width:4.0cm;border:solid windowtext 1.0pt;
|
|
|
padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Secondary</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Voltage (Vrms)</p>
|
|
|
</td>
|
|
|
<td width=142 valign=top style='width:3.0cm;border:solid windowtext 1.0pt;
|
|
|
border-left:none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Supply Rail</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Voltage (V)</p>
|
|
|
</td>
|
|
|
<td width=142 valign=top style='width:3.0cm;border:solid windowtext 1.0pt;
|
|
|
border-left:none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Quiescent</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Current (A)</p>
|
|
|
</td>
|
|
|
<td width=142 valign=top style='width:3.0cm;border:solid windowtext 1.0pt;
|
|
|
border-left:none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Power</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>8ohm (W)</p>
|
|
|
</td>
|
|
|
<td width=142 valign=top style='width:3.0cm;border:solid windowtext 1.0pt;
|
|
|
border-left:none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Power</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>6ohm (W)</p>
|
|
|
</td>
|
|
|
<td width=142 valign=top style='width:3.0cm;border:solid windowtext 1.0pt;
|
|
|
border-left:none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>Power</p>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>4ohm (W)</p>
|
|
|
</td>
|
|
|
</tr>
|
|
|
<tr style='height:19.85pt'>
|
|
|
<td width=189 style='width:4.0cm;border:solid windowtext 1.0pt;border-top:
|
|
|
none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>18</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>18</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>2.8</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>16</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>21</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>32</p>
|
|
|
</td>
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</tr>
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<tr style='height:19.85pt'>
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<td width=189 style='width:4.0cm;border:solid windowtext 1.0pt;border-top:
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none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>22</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>23</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>3.7</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>28</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>37</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>56</p>
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</td>
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</tr>
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<tr style='height:19.85pt'>
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<td width=189 style='width:4.0cm;border:solid windowtext 1.0pt;border-top:
|
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none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>25</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>28</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>3.2</p>
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</td>
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<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>42</p>
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</td>
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|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
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|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
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height:19.85pt'>
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<p align=center style='margin-left:0cm;text-align:center'>56</p>
|
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</td>
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|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
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|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
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height:19.85pt'>
|
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<p align=center style='margin-left:0cm;text-align:center'>42</p>
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</td>
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</tr>
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|
<tr style='height:19.85pt'>
|
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|
<td width=189 style='width:4.0cm;border:solid windowtext 1.0pt;border-top:
|
|
|
none;padding:0cm 5.4pt 0cm 5.4pt;height:19.85pt'>
|
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|
<p align=center style='margin-left:0cm;text-align:center'>30</p>
|
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</td>
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|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
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|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
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|
height:19.85pt'>
|
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|
<p align=center style='margin-left:0cm;text-align:center'>33</p>
|
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|
</td>
|
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|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
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|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
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|
height:19.85pt'>
|
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|
<p align=center style='margin-left:0cm;text-align:center'>2.7</p>
|
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|
</td>
|
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|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
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|
height:19.85pt'>
|
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|
<p align=center style='margin-left:0cm;text-align:center'>60</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
|
|
<p align=center style='margin-left:0cm;text-align:center'>45</p>
|
|
|
</td>
|
|
|
<td width=142 style='width:3.0cm;border-top:none;border-left:none;border-bottom:
|
|
|
solid windowtext 1.0pt;border-right:solid windowtext 1.0pt;padding:0cm 5.4pt 0cm 5.4pt;
|
|
|
height:19.85pt'>
|
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|
<p align=center style='margin-left:0cm;text-align:center'>30</p>
|
|
|
</td>
|
|
|
</tr>
|
|
|
</table>
|
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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> </p>
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|
<p><b><span style='font-size:8.0pt'>HISTORY:</span></b><span style='font-size:
|
|
|
8.0pt'> Page created 27/11/2002 </span></p>
|
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<p><span style='font-size:8.0pt'>28/11/2002 Original table replaced with one
|
|
|
based on transformer secondary voltages</span></p>
|
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|
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|
<p><span style='font-size:8.0pt'>15/03/2003 Note regarding ccs PSRR improvement
|
|
|
added</span></p>
|
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|
<p><span style='font-size:8.0pt'>17/08/2003 Note regarding ccs instability
|
|
|
added</span></p>
|
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|
<p><span style='font-size:8.0pt'> Tim’s comments updated</span></p>
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<p><span style='font-size:8.0pt'> </span></p>
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<p> </p>
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</div>
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</body>
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</html>
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