|
|
<!doctype html public "-//w3c//dtd html 4.0 transitional//en">
|
|
|
<html lang="en">
|
|
|
<head>
|
|
|
<meta http-equiv=Content-Type content="text/html; charset=windows-1252">
|
|
|
|
|
|
<title>The Class-A Amplifier Site - Design Notes</title>
|
|
|
<link rel="StyleSheet" href="class-a.css" type="text/css" media="screen, print">
|
|
|
<link rel="shortcut icon" type="image/ico" href="favicon.ico">
|
|
|
</head>
|
|
|
|
|
|
<body background="slate.jpg" bgcolor="#E8E8E8" lang=EN-GB link=blue vlink=blue>
|
|
|
|
|
|
<div class=Section1>
|
|
|
|
|
|
<p align=right style='text-align:right'><b>The Class-A Amplifier Site</b></p>
|
|
|
|
|
|
<p align=right style='text-align:right'><span style='font-size:10.0pt'>This page was last updated on 27 November 2002</span></p>
|
|
|
|
|
|
<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back to Index ]</a></span></b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p align=center style='text-align:center'><b><span style='font-size:20.0pt'>Design Notes</span></b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p align=center style='text-align:center'><b>Design notes for the JLH Class-A amplifier.</b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The 1996 version using the specified transistors with +/-22V supply rails
|
|
|
and a quiescent current of 2A has an approximate rms power output, into a
|
|
|
resistive load, of 10W into 16ohm, 20W into 8ohm, 15W into 4ohm and 10W into
|
|
|
2ohm.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>For minimum distortion, Tr1 and Tr2 should be a matched pair. If this is not
|
|
|
possible, the device with the higher gain should be used in the Tr1 position.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>Low gain output devices such as the 2N3055 should only be used with a high
|
|
|
gain driver transistor, for example the 2N1711 or 2N3019 (or a suitable
|
|
|
alternative - perhaps a specially selected BD139).</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The output transistors should ideally have an f<span style='font-size:8.0pt'>T</span>
|
|
|
of 4MHz or more, though many amplifiers have been successfully used with 3MHz
|
|
|
devices (or even lower). For information, the 2N3055 datasheet from ON-Semi
|
|
|
quotes an f<span style='font-size:8.0pt'>T</span> of 2.5MHz and that from ST,
|
|
|
3MHz (though other 2N3055 manufacturers quote a figure of 0.8MHz). TIP power
|
|
|
devices are usually 3MHz. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>Because of the high dissipation in the output devices (about 45W each for
|
|
|
the 1996 version), I suggest that any proposed output transistor should have a
|
|
|
thermal resistance (junction to case) of less than 1<>C/W. Alternatively, a
|
|
|
parallel pair arrangement (with 0R1 emitter resistors) can be used. For
|
|
|
guidance on heatsink sizing and transistor mounting see the <a
|
|
|
href="http://www.sound.au.com/heatsinks.htm" target="_blank"><EFBFBD>Heatsinks<EFBFBD></a> article at the ESP
|
|
|
Audio Pages.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>To minimise quiescent current and dc offset drift due to temperature rise,
|
|
|
resistor R10 (0R33) should be a 7W or 10W type or 3 x 1R0 3W in parallel. The
|
|
|
resistor(s) should be stood-off the pcb to ensure adequate ventilation. For the
|
|
|
same reason, Tr5 should have an adequate heatsink. From experience, I would
|
|
|
suggest a minimum of 10 <20>C/W, though around 6<>C/W would probably be better. If
|
|
|
a BD140 is used instead of the MJE371, a larger heatsink will be required since
|
|
|
its thermal resistance (junction to case) is much higher than that for the
|
|
|
MJE371. When laying out the pcb, try to keep R10 and Tr5 away from the output
|
|
|
transistor heatsinks.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>I must stress that the circuit diagram (Figure 3.) in the original article
|
|
|
for the 1996 version contains an error. The negative end of the feedback
|
|
|
capacitor (C4) is shown connected to the <20>ve supply rail. This will result in
|
|
|
excessive hum due to the supply rail ripple voltage being injected into the
|
|
|
feedback path (Tr4 emitter). To prevent this problem, the negative end of C4
|
|
|
should be connected to the 0V (earth) point.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The value of the input capacitor (C4 or C1) can be usefully increased to
|
|
|
lower the low frequency <20>3dB point and improve the bass response of the
|
|
|
amplifier. I suggest a value of between 1uF and 2.2uF. A polypropylene
|
|
|
capacitor is preferred in this position.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>The value of the blocking capacitor in the feedback circuit (C3 or C4) can
|
|
|
be usefully increased to reduce the low frequency distortion of the amplifier.
|
|
|
Values between 470uF and 1000uF would be suitable. Rudy van Stratum has tried
|
|
|
values up to 1000uF and has found that 470uF sounded best in his modified (dual
|
|
|
rail) 1969 version.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>In theory, and in simulation, increasing the value of the bootstrap
|
|
|
capacitor (C1) in the 1969 version to between 470uF and 1000uF reduces the
|
|
|
frequency at which low frequency distortion starts to increase due to the
|
|
|
non-linearity of the current source that controls the output stage quiescent
|
|
|
current. Rudy has also tried values up to 1000uF in this position. Contrary to
|
|
|
expectations, 1000uF caused a <20>thickening<6E> in the bass and a loss of <20>air<69> and
|
|
|
<EFBFBD>finesse<EFBFBD> in the treble. He has now found that 470uF gives the best results.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>I previously suggested by-passing all electrolytic capacitors with a 100nF
|
|
|
polypropylene capacitor (in parallel with the electrolytic). This may not have
|
|
|
an audible effect, but it ensures a low esr at high frequencies. Rudy has reported
|
|
|
that, when he has tried paralleling capacitors in the past, the sound quality
|
|
|
has deteriorated in comparison to a single capacitor. His exact comments were: </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><EFBFBD>About 10 years ago this was standard practice for me, everywhere I used a
|
|
|
small film cap to better the high frequency behaviour. But I believe now that a
|
|
|
good design does not need such things, the best amplifiers I heard use no such
|
|
|
things. And on more than one occasion this bypassing produced sharp edges to
|
|
|
the sound. Somehow it seems that you can hear two different capacitors. Compare
|
|
|
it to the difference between a good broadband (full-range) speaker vis-a-vis a
|
|
|
two-way system: you always hear two units. Some of the natural integration is
|
|
|
gone.<2E></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>As other articles I<>ve read appear to come to the opposite conclusion, I<>ll
|
|
|
keep an open mind on this issue.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>I have received two reports recently regarding a problem with oscillation of
|
|
|
the 7815 voltage regulator in the 1996 design. There are several cures for this
|
|
|
problem. One would be to replace the 7815 (and C3, RV1 and R1) with an
|
|
|
adjustable constant current source (a decoupled resistance, an FET, an
|
|
|
LED/transistor or a two transistor circuit). The current source will need an
|
|
|
adjustable output of between 0.4 and 0.5mA. The second solution is to improve
|
|
|
the stability of the 7815 by increasing the output capacitor (C3) to between
|
|
|
22uF and 100uF. In addition, it could be worthwhile adding a resistor from the
|
|
|
output of the 7815 to earth to ensure a minimum output current. A value between
|
|
|
3k and 4k7 should be suitable. None of the 7815 data sheets that I have been
|
|
|
able to find has specified a minimum output current, but adjustable regulators
|
|
|
such as the LM317 call for a minimum current of around 3.5mA. From one
|
|
|
constructor<EFBFBD>s experience, the 78L15 seems to be more prone to oscillation than
|
|
|
the standard 7815 so, even though the current draw is less than 0.5mA (or 1mA
|
|
|
if one regulator is used to feed both channels as in the original diagram), I
|
|
|
suggest using the latter.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>When using the original (1969) bootstrap arrangement for quiescent current
|
|
|
control, care must be taken to ensure that R1/R2 (1969 article Fig. 3) or
|
|
|
R1/RV1 (1996 article Fig. 1) are adequately rated. The current through these
|
|
|
components is slightly greater than the sum of the output transistor base
|
|
|
currents. The output transistor base current is the output stage quiescent
|
|
|
current (Iq) divided by the current gain (Hfe) of the output devices. The
|
|
|
current through R1/R2 or R1/RV1 is therefore approximately equal to 2 x Iq /
|
|
|
Hfe.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>If the current/resistor values of Table 1 (1969 article) and output
|
|
|
transistors with a current gain of 100 or more are used, the resistor power
|
|
|
ratings shown in Fig 3 (1969 article) are adequate. If low gain (circa 50)
|
|
|
output devices are fitted, the resistor power ratings should be increased to
|
|
|
about double those shown in Fig 3. For other resistor values or quiescent
|
|
|
currents, the required power rating of R1/R2 or R1/RV1 should be calculated.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>Whilst it should not be difficult to obtain fixed resistors with the
|
|
|
required power rating, a preset potentiometer could be more of a problem since
|
|
|
the more common ones are only rated at 0.5W or 1W, though higher rated devices
|
|
|
are available. It must be remembered that the power rating of a preset, when
|
|
|
used as a rheostat, is proportional to the length of track in use. It is
|
|
|
therefore necessary to determine<6E> the power rating from the current flowing
|
|
|
through the preset and its total resistance value. It may be necessary to use a
|
|
|
fixed resistor in series with a lower value preset to form RV1 in order to keep
|
|
|
within the power limits of the preset. </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>For those who prefer the greater simplicity of the 1969 version, but wish to
|
|
|
avoid the output capacitor (C2), the circuit can be modified to operate off
|
|
|
dual supply rails. Figures 1 and 2 illustrate two methods of achieving this. It
|
|
|
must be stressed that Option 1 has yet to be verified in practice (so far as I
|
|
|
am aware), but Option 2 has been successfully implemented by at least one
|
|
|
constructor.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p align=center style='text-align:center'><img border=0 width=655 height=429
|
|
|
src="jlhnotesfig1.gif"></p>
|
|
|
|
|
|
<p><span style='font-size:6.0pt'> </span></p>
|
|
|
|
|
|
<p align=center style='text-align:center'>Figure 1. 1969 design with dual supply rails (Option 1)</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p align=center style='text-align:center'><img border=0 width=655 height=420 src="jlhnotesfig2.gif"></p>
|
|
|
|
|
|
<p><span style='font-size:6.0pt'> </span></p>
|
|
|
|
|
|
<p align=center style='text-align:center'>Figure 2. 1969 design with dual supply rails (Option 2)</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p>For more design information relevant to this type of amplifier, see <a
|
|
|
href="http://www.sound.au.com/project36.htm" target="_blank">Project 36</a> at the ESP Audio
|
|
|
Pages. The amplifier in this project is very similar to the JLH 1969 version
|
|
|
(the main difference being the addition of a transistor to the quiescent
|
|
|
current control circuitry) and Rod Elliott gives a good explanation of how he
|
|
|
determined that this is the optimum topology for a simple solid-state Class-A
|
|
|
amplifier.</p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><b><span style='color:blue'><a href="index-1.htm" title=index-1.htm>[ Back to Index ]</a></span></b></p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p> </p>
|
|
|
|
|
|
<p><b><span style='font-size:8.0pt'>HISTORY:</span></b><span style='font-size:
|
|
|
8.0pt'><EFBFBD><EFBFBD> Page created 01/05/2001 </span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>10/05/2001 Added link to Quiescent Current and
|
|
|
DC Offset page</span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>16/05/2001 Diagrams redrawn</span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>05/06/2001 Polarity of C3 in Figure 2
|
|
|
corrected</span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>05/08/2001 Capacitor notes revised and 7815
|
|
|
oscillation notes added</span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>31/01/2002 1969 bootstrap resistor power
|
|
|
rating notes added </span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'>27/11/2002 dc offset servo paragraphs removed</span></p>
|
|
|
|
|
|
<p><span style='font-size:8.0pt'> </span></p>
|
|
|
|
|
|
</div>
|
|
|
|
|
|
</body>
|
|
|
|
|
|
</html>
|
