Wednesday, May 2, 2012

JLH Class A Amplifier (1996)

Class-A Power

After  two  and  a  half  decades,  John Linsley-Hood’s  Class-A  power  amp  is still  rated among the  best.  Here,  John  explains  how  to bring  the  design up  to date,  adding enhancements such as dc-coupled output.

Electronics World, September 1996 



The  current  debate,  among  the  more  reactionary  of  the hi-fi  devotees,  about the  relative  merits  of thermionic valve operated audio amplifiers makes intriguing reading, if only because, in a sense, this is ‘where I came in’. I will explain.

I have had an interest in the reproduction of music, principally from gramophone records, for a very long time. I made my first, two-valve, battery-operated, audio amplifier as a twelve year old school boy, some time before the outbreak of the 1939-1945 war.

This  gave way  –  in  the  interests  of  economy,  –  to a series of mains powered amplifiers, which were usually combined with a radio receiver. Electricity from the mains was free, to me at least, whereas high- tension batteries had to be bought from my pocket money.

My early work culminated,  in 1951, with  the assembly of a  luxurious kit  for  the highly esteemed high- fidelity Williamson 15W  amplifier  design.  Although,  by  this  time, I  had  my  first  proper  job  –  in  the electronics  labs of  the Sellafield nuclear research establishment  in Cumberland – and cash was a bit more plentiful, I still wouldn’t  have built that  particular,  rather  expensive  version of  the hardware  if  I hadn’t heard  through  the  lab  grapevine  that  one of  the  research  chemists  had bought  himself  a Williamson kit, but, on receiving the parcel, lacked the courage to assemble its contents. Rumour had it that he was open to offers, and I was happy when he accepted mine.

This was an excellent amplifier, and was better, in my judgement, by a greater or lesser extent, than any of its  predecessors  of my  own design,  or,  indeed,  any  of  the other  valve amplifiers,  belonging  to my friends, with which I had had a chance to compare it. It gave me great pleasure until early 1968, when I replaced it with a solid-state equivalent.

What I replaced it by, and the circumstances of this replacement, were described in an article in Wireless World  in April 1969, entitled  ‘A simple class A amplifier’. This was a  long  time ago. In  the  light of  the current debate, it seems possible that both my listening trials at the time, and an up-dated version of my original class A design, may be of interest to you. By up-dated, I mean using more modern components and delivering a bit more power output.

The Williamson Amplifier

In  the  inter-war  years, with  the  improvement  in audio  quality  of  both  gramophone  records  and  radio broadcasts,  considerable attention  was  paid  to  improving  the  quality  of  ac  mains-powered audio amplifiers. A number of interesting designs were offered. These were mainly based on the use of push-pull output stage  layouts. Relative  to straight single ended circuits, push-pull stages would give greater output power for a given distortion level.

At that time, there were audiophiles who decried the use of push-pull output stage layouts. They claimed that the best audio quality was only obtainable from the much less efficient single ended arrangements, i.e. those in which the output valve had a simple resistor, choke or output transformer load. Interestingly, this  is a claim which was examined and dismissed by Williamson at the  time, but which has  recently been resurrected.

Using negative feedback

Almost all valve operated audio power amplifiers  require an output transformer  to match  the  relatively high output  impedance of  the  valve output  stage  to  the  low  impedance  load presented by  the loudspeaker.

In general, the transformer is the most difficult and expensive part of the system to design and construct. This is because of the following conflicting demands:

•  For a  low  leakage  reactance – combining both  leakage  inductance and  inter-winding capacitance – from  the primary  to  the  secondary  windings, to avoid  loss  or  impairment  of  high  frequency  signal components.

•   For  a  low  level  of  leakage  inductance  from  one half  of  the primary  to  the other, to  reduce  the discontinuities due to push-pull operation, and the odd-order harmonic distortion resulting from these.

•  For a high primary inductance, to give a good low-frequency response.

•  For a low winding resistance, to avoid power losses.

•  For a good quality grade of core  laminations  to ensure a  low  level of core-induced distortion, due  to magnetic hysteresis and similar effects.

Intrinsic signal distortion of a valve amplifier stage could range from 0.5 to 10%, depending on its circuit form and operating characteristics. It had been appreciated  for some  time  that such  intrinsic distortion could be  reduced  significantly  by  applying  local  negative  feedback.  Various  amplifier  designs incorporating local negative feedback had been proposed. However, this still left the output transformer – however well made – as a major source of transfer and frequency response non-linearities.

At this point, D. T. N. Williamson, who was working at the time as a development engineer for the valve section of  the GEC Research Laboratories, described a high-quality audio amplifier design, using  the recently developed GEC ‘kinkless tetrode’ output valve, namely the KT66. In this design, a single overall negative  feedback  loop embraced both  the  whole of  the amplifier  and  the  loudspeaker  output transformer.

With  the exception of  the output  valves,  which  were  triode  connected  KT66s,  Williamson’s  design employed  triode amplifier  valves  exclusively,  because  these had a  lower  intrinsic distortion  figure. He also made use of extensive  local negative  feedback, provided by un-bypassed cathode-bias  resistors. This had the additional benefit of eliminating the electrolytic bypass capacitors – a philosophy which is in accord with much of contemporary thinking.

Williamson also used non-polar rather than electrolytic high-tension reservoir and smoothing capacitors, in the interests of more consistent ac behaviour. Electrolytic capacitors were much worse at that time.

If overall negative  feedback was  to be applied without causing either high or  low-frequency  instability, careful  design  was  essential  –  both  in  the amplifier  stages  and  in  the output transformer.  These problems had frustrated earlier attempts to do this – but Williamson demonstrated that it could be done.

The performance given by his design,  if his detailed  specifications were  carried out to  the  letter, was superb. The performance criteria of better than 0.1% thd, at 15W output, from 20Hz to 20kHz, and a gain bandwidth from 10Hz to 100KHz +/- 1dB, are at least as good as those offered by many of today’s better commercial designs.

The series of articles written by Williamson, in Wireless World over the period 1947 – 1949 described the power amplifier and its ancillary units. This series had enormous impact on audio design thinking, and if I may quote the WW editor of the time, in his introduction to a reprint of all these articles.

“Introduced in 1947 as merely one of a series of amplifier designs, the ‘Williamson’ has for several years been  widely  accepted as  the  standard of  design and performance  wherever  amplifiers  and  sound reproduction are discussed. Descriptions of  it have been published  in all  the principal countries of  the world, and so there are reasonable grounds for assuming that its widespread reputation is based solely on its qualities”.

All in all, the Williamson was a hard act to follow.


Alternative hardware

The world had not stood still since 1951. My equipment had remained monophonic, while the rest of the Audio world was changing over to stereo.

My main interest was in music, not in circuitry, so I thought it would be prudent to ask my ears what they thought of the alternatives, before I started to replace my hardware.

To  this end, I built or borrowed six well  thought-of audio amplifiers, my own Williamson, a Quad 2, two dissimilar but recently published class AB transistor amplifiers, a commercial 30W solid-state unit, and a simple Class-A unit of my own design.

I included the Class-A design out of curiosity. If it turned out to be any good, it would be cheap and easy to build. It was not expected to offer any special merit in performance.

In the event, as I reported at the time, (WW April 1969, p.152), the six amplifiers divided quite clearly into two  separate  tonal  groups.  The  three  class AB  transistor  amplifiers  formed one  group, while  the  two valve amplifiers and the simple class A amplifier formed the other.

To be fair, the differences between any of these were not very great – but they were audible. Once they were noticed, they tended to become more apparent on protracted periods of listening. Certainly, for me – and  I was doing  these  tests  for my own benefit –  in  these comparative  trials, the  two best were  the Williamson and  the  class  A.  They were  virtually  indistinguishable. Of  these  two, the Williamson was vastly more massive and costly to construct.

The only remaining question was, if I replaced the 15W Williamson with the 10W Class-A design, would the output  be adequate? Connecting an oscilloscope across  the  loudspeaker  terminals showed  that I seldom needed more that 2-3W from the power amplifier – even under noisy conditions.

I suppose that the final proof of my satisfaction with the class A transistor amplifier was that, a year or so later, I gave my old Williamson to a friend.

Valves versus transistors

Not all of the considerations of valves versus transistors relate solely to performance. It is worth bearing in mind  that  products  involving  obsolete  technology  will  be disproportionately  expensive,  difficult to obtain and possibly of inferior quality.

Valves can also vary in operating characteristics from sample to sample – especially where two valves of  the  same  type are obtained  from  different  sources.  Characteristics  that  can  vary  are  mutual conductance, gain, operating grid bias, anode current impedance, and even usable anode voltage.

By  comparison, the performance  characteristics  of,  say,  a  range of  2N3055 epitaxial  base output transistors are almost identical, whether made in the Philippines or in Toulouse.

Again, all valves deteriorate  in use, exhibiting a gradual loss of cathode emission over a  typical 3000 hour service  life. If a valve  is persistently over-driven, the heating of the anode may cause the metal to out-gas. This impairs the vacuum essential to proper operation, and shortens the valve’s life.

A  further consideration  is  that valves are high voltage devices, which can be dangerous. And  the need for high working voltages can  lead  to more rapid  failure of other components  in  the circuit – especially capacitors.

The class A design

My  original  design  is shown  in  Fig.  1.  This  is still  a  valid design,  except that the MJ480/481 output transistors are now obsolete. However, they can be replaced by  the more robust 2N3055. In  this case, the epitaxial-base version of this device should be chosen rather than the hometaxial, since the fT of the output transistors should be 4MHz or higher. As I commented, at the time, the design gave a somewhat lower distortion if the hFE of Tr1 was greater than that of Tr2. This caused the output circuit to act as an amplifier with an active collector load rather than an output emitter follower with an active emitter load.



A simple modification which  takes advantage of  this effect  is  the use of a Darlington  transistor such as an MJ3001  for Tr1. At 1kHZ, this  reduces  the distortion  level at  just below  the onset of clipping  from about  0.1%  down  to nearer  0.01%.  As  before, the  residual  distortion  is  almost  exclusively  second harmonic. Also, as before, it fades away into the general noise background of the measurement system as the output power is reduced.

While this transistor substitution seems to be a good thing, it was not a modification whose effect I was able to check,  in  listening  trials, against the Williamson. As a  result,  for  the sake of historical  fidelity, I would still recommend the use of epitaxial-base 3055s as Tr1 and Tr2.

I have checked all the other changes which I have proposed with the exception of the power increase.

Improving performance

With regard to the original 10W design, as published, I feel the following improvements will be beneficial:

•  Provide a more elegant means of controlling output transistor operating current by including a variable resistor in the base of Tr2.

•   Arrange  the  circuit  so  that  it would operate between  symmetrical  power  supply  lines,  allowing  the amplifier to be directly coupled to the loudspeaker.

•  Increase output power from 10 to 15 watts per channel

•  Up-grade the smoothed but not regulated power supply arrangement.

In my postscript to  this design, which WW published  in December 1970, I suggested both alternative transistor types and an improved method of adjustment and control of the output transistor current flow, Fig. 2.

 
Although,  in  theory, this  layout  should  give a  superior  performance,  when  I  changed  my  prototype amplifier to this arrangement, I found little change in measured thd and I couldn’t hear any difference in sound quality.

Although directly  coupling  the amplifier  to  the  loudspeaker will  not  have much effect  on  thd,  it  is still beneficial  since  it  eliminates  the output  coupling  capacitor.  The most  obvious way  of  doing  this  is  to rearrange the input layout, around Tr4, so that it becomes the input half of a ‘long-tailed’ pair.

I am reluctant to do this because this would alter the overall gain/phase characteristics of the amplifier. It would also  require additional  high-frequency  stabilisation  circuitry,  with all its  incipient  problems  of  Transient intermodulation or slew-rate limiting.

Fortunately, the need  to  remove  the dc offset at the output can be achieved without altering  the good phase margins of the design, by simply injecting an appropriate amount of current into the base circuit of
Tr4.

Output power and dissipation

In essence,  all  that  is  required  to  increase  the power  output  from  the amplifier  is  to  increase  the  rail voltages and  the standing current through  the output devices. Restrictions are  that power consumption must remain within  the confines of what the mains  transformer and rectifier can deliver. Also, the heat-sinks must be able to dissipate the extra heat and the output transistors must be adequately rated.

For a 15W (sinusoidal) output into an 8Ω load, an 11VRMS drive voltage is required. This, in turn means a 31VP-P voltage developed across the load, and an output current into the load of 2AP. Since the circuit is a single-ended configuration, in which the collector current will not increase on demand, this means that the output transistor operating current must be at least 2A to allow this.

With the circuit shown, using the improved current control layout – which is rather less efficient than the boot-strapped load for Tr3 which I originally proposed – the rail voltage needed is +/- 22V.

This will lead  to a dissipation,  in each output transistor,  of  44W.  Prudence  suggests  that  a heatsink having a rating of no more than 0.6°C/W, should be used for each output pair.

Most 2N3055s have a Vce of 60V, a maximum collector current of 15A, and a maximum dissipation, on a suitable heatsink, of 115W. However, RCA’s 3055, and its complementary MJ2955, are rated at 150W.

Working conditions  for  the output transistors are entirely within  the devices safe operating area, so no specific overload protection circuitry  is needed. Even so, the  inclusion of a 3A  fuse  in  the  loudspeaker output line would seem prudent.

DC offset cancellation

Figure 3 shows the full circuit for one channel of the 15W Class-A audio amplifier. I have inserted a 15V three-terminal regulator ic into the positive rail to prevent any unwanted signal or hum intrusion into the emitter of Tr4.


It  is easy  to set the dc offset to within +/-50mV. The offset does not change greatly with time, although this assumes that Tr5 is not allowed to warm up too much. This is because the base-emitter potential of this transistor controls the operating current, which in turn, affects the output dc offset.

Small-signal bandwidth

In the original circuit the small-signal bandwidth was 10Hz–250kHz, +/-3dB, which was needlessly wide. Because of this, I have added an input high-frequency roll-off network, R3/C2, to the input circuit to limit the top end response to some 50kHz. This assumes an input source impedance of 10kΩ or less.

As it stands, the low-frequency –3dB point is about 7Hz. It can be lowered even further, if necessary, by making C1 larger – say to 1µF.

Supplying power

As  was shown  in  the 1970 postscript,  it  is  possible  to operate  this  amplifier  from  a  simple rectifier/reservoir capacitor  layout. Fig. 4  is an example. The only penalty  is a small 100Hz background hum,  probably  about  3mV  in amplitude.  However, I  feel  that,  if  you are  seeking  the best,  a proper regulated power supply is preferable, Fig. 5.




The  circuit  shown  for  the  current  booster  pass  transistors,  Tr1/Tr2,  is  one  suggested by  National Semiconductor. It takes advantage of  the  internal current  limiting circuitry of  the 7815/7915 devices  to limit the short-circuit current of these ICs to 1.2A. By choosing the correct ratios of R5:R7 and R8:R10, the short-circuit current drawn from Tr1 and Tr2 will also be limited.

For a  satisfactory  ripple  free dc supply of +/-22V, the on-load voltage supplied  to  the  regulator circuit should be +/-27V.

Performance

I prefer measurements made with appropriate instruments to judgements based on listening tests.

Measured distortion is less than 0.1% near the onset of clipping. It fades away into the background noise level of the measuring system as output power level is reduced. For me, the  fact that the distortion  given by this circuit  is  almost  pure  second harmonic  is  more persuasive of its performance than any ‘golden eared’ judgement of tonal purity.

If  you  then add  the observation  that the  circuit  remains stable on a  square-wave drive  into  typical reactive  loads, I  am  not  surprised  that  its  performance  was capable of  equalling  the Williamson on listening tests. No significant overshoot is observed on the square-wave, and stability is achieved without  the need for internal high-frequency compensation arrangements.

So, as a final thought, if any of you want to find out how a top quality valve amplifier like the Williamson sounds, you can find out at a tenth of the cost of building one by making up this Class-A design. It has the additional advantage of incorporating readily available and modern components.

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