Cambridge Audio 840A Class XD, White Paper

Page 1: ...ed amplifier was one part of our new 8 Series A number of ideas came to the fore including the implementation of full microprocessor control for all functions nameable inputs and AV mode custom install features such as RS232 and outputs for our own Incognito multi room system But above all the unit had to be a true audiophile product in that sound quality was paramount We decided the 840A should i...

Page 2: ...and tweaks developed as part of the 840A s extensive measurement and listening test programme For a more in depth technical description of Class XD please refer to our UK Patent application number GB 0505024 0 Background The state of the art The great divide in solid state amplifier technology has always been between the efficient but ultimately compromised Class B approach and the beautifully lin...

Page 3: ... both can be made to sound wonderful when well implemented What we were seeking with the development of Class XD was a way of incorporating a lot of the advantages and sound quality of Class A at a far lower price level than normal and without the inefficiency inherent in that method Well designed Class B amplifiers can in fact achieve extremely low distortion levels of 0 001 at 1 kHz Ref 2 The Cl...

Page 4: ... negative feedback factor in any linear amplifier falls with increasing frequency The 4 Ohm case shows lower overall gain due to the increased loading and a drop off of gain at each side due to falling transistor beta current gain this latter distortion mechanism is however relatively easy to deal with by the use of negative feedback and other methods and is not considered further The awkward gain...

Page 5: ... Class AB would seem to be a perfect compromise between Class A and Class B operation it does have some hidden issues It can be shown Ref 1 that if Class AB is used to trade off between efficiency and linearity its performance is certainly superior to B below the AB transition level operating as it does in this region in pure Class A This can have very low THD indeed at less than 0 0006 up to 10 k...

Page 6: ...his would abolish the AB gain changes that cause the extra distortion This was the purpose of our Class XD development program could it be done The Class XD crossover displacement principle We can see that in Class B it would be better if the crossover region were anywhere else rather than where it is If the crossover were displaced away from its zero crossing position then the amplifier output wo...

Page 7: ...d sinks it into the negative rail For convenience we have called the current injection subsystem The Displacer Similarly the upper transistor is The Source while the lower is The Sink The displacement current does not directly alter the voltage at the output the output stage inherently has low output impedance and this is further lowered by the use of global negative feedback What it does do is al...

Page 8: ...ticated implementations could even vary with the signal We now have before us the intriguing prospect of a power amplifier with three output devices which if nothing else is novel The operation of the output stage is inherently asymmetrical and in fact that is its raison d être but this should not cause undue alarm Circuit symmetry is often touted as being a pre requisite for either low distortion...

Page 9: ...left from 0V to 8V Realisation There are several ways in which a suitable displacement current can be drawn from the main amplifier output node Resistive crossover displacement The most straightforward way to implement crossover displacement would be to simply connect a suitable power resistor between the output rail and a supply rail as shown in Fig 5 ...

Page 10: ...most cases is unregulated and carrying substantial 100 Hz ripple A regulated supply to the resistor could be used but this would be very uneconomic and even less efficient due to the voltage drop in the regulator The resistive system is inefficient because the displacement of the crossover region occurs when the output is negative of ground but when the output is positive the resistor is still con...

Page 11: ... use constant current displacement and this is where we started as in Fig 6 A constant current source is connected between the output and negative rail Efficiency is better as no output power is wasted in the displacer due to the high dynamic impedance of the current source The output of the current source does not need to be controlled to very fine limits Long term variations in the current only ...

Page 12: ...p was to move from a constant current to a voltage controlled current source VCIS whose output is modulated by the signal to further improve efficiency The most straightforward way to do this is to make the displacement current proportional to the output voltage Thus if the displacement current is 1 Amp with the output at quiescent at 0V it is set to increase to 2 Amps with the output fully negati...

Page 13: ...th a real resistance giving 12 5 efficiency at full power moves to a constant current source with effective infinite impedance giving 25 and finally to a push pull controlled current source giving 50 In the last case the sink transistor acts in a sense as a negative resistance though it is more usefully regarded as a driven source VCIS than a pure negative resistance as the current does not depend...

Page 14: ...ent varies in the correct sense for this This is equivalent to a decrease in output stage loading this is the exact inverse of what occurs with resistive displacement which increases output loading Lighter loading is known to make the current crossover between the output devices more gradual and so reduces the size of the gain wobble that causes crossover distortion Ref 2 In addition the crossover...

Page 15: ...o adding relatively minimally to total amplifier cost Performance Measurements are presented here to demonstrate how the crossover displacement principle reduces distortion in reality Fig 9 THD vs frequency for a Class B amplifier at 30W 8 Ohm 604b Firstly as a reference the results obtainable from well implemented Class B amplifier Fig 9 shows THD vs frequency for a Class B amplifier giving 30W i...

Page 16: ...erent Classes of operation in fig 10 this shows distortion against amplitude at 10 kHz over the range 200mW 20W is plotted in this covers the power levels at which most listening is done 0 dB is 30W into 8 Ohms Trace B is the result for the Class B operation the THD percentage increases as the power is reduced partly because of the nature of crossover distortion and partly because the constant noi...

Page 17: ... similar THD amplitude plot in Fig 11 now compares Class B with Class AB as most amplifier designers use and constant current XD Here the transition point from Class A to Class B is at 8 dB and gm doubling begins in Class AB at 7 dB Note now the XD distortion is actually always slightly below that from Class B and better than AB at high levels but worse than AB at low levels So this kind of Class ...

Page 18: ...ted at 10 kHz power as before The transition points can also be seen to be not quite the same 8 dB for push pull versus 11 dB for constant current The salient point is that at 2 dB THD is very significantly lowered from 0 0036 to 0 0022 by the use of the push pull method which reduces the magnitude of the current changes in the output transistors of the main amplifier In fact THD is lower at all p...

Page 19: ...cantly greater THD above its transition point say at 4 dB than Class B constant current Class XD gives slightly less and push pull Class XD gives markedly less Fig 14 returns to the THD frequency format and shows that XD push pull gives lower THD over the range 1kHz 30kHz than optimal Class B and is in fact better at all points ...

Page 20: ...s of complete output stage circuits The effects of transistor non linearity and so on are taken into account The results are therefore as real as extensive calculations can make them For comparison the classical calculations for Class B give a full power efficiency of 78 but more detailed simulations show that it is only 73 when typical losses are included The output stages were simulated using 50...

Page 21: ...er distortion away from the central point where the amplifier output spends most of its time Below the transition point the amplifier actually runs in pure Class A with no crossover artifacts at all Above the transition point the amplifier moves into an optimized Class B with still lower distortion than is possible with Class AB Much lower heat than Class A although more than conventional Class AB...

Page 22: ...mplifiers Electronics World Mar 1994 Class A efficiency canonical sequence 6 Self D Audio Power Amplifier Design Handbook as above p267 Class A distortion data 7 Moore B J An Introduction to The Psychology of Hearing Academic Press 1982 pp48 50 ...

Page 23: ... Application Number 0505024 0 o This document is copyright Cambridge Audio Ltd o Reproduction in whole or in part can be authorized by prior written arrangement provided the source is acknowledged Class XD trademark acknowledged and Patent Pending details given o To obtain reproduction rights or for further information on the 840A or any other Cambridge Audio product please e mail press cambridge ...