Cambridge Audio Azur 840A White Paper
Cambridge audio azur 840a: supplementary guide
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White paper
February 2006
Cambridge Audio 840A Class XD™ integrated amplifier
Matthew Bramble – Technical Director, Cambridge Audio
Douglas Self – Electronic Engineer, Cambridge Audio
Cambridge Audio made a significant leap forward
with the introduction of the Azur range in 2003 by
pushing the boundaries of what was possible in
the budget audiophile segment of the market.
However, we were also keen to produce a high-end
product range which allowed us to flex our engineering muscles and show what we can do with
fewer constraints. A number of development programmes burgeoned while working on the original
TM
and V2 Azur ranges of which the 840A Class XD
integrated 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 in fact be a pre-amplifier and dual-mono power amplifier all in one
chassis. It should also have a balanced input for the matching CD player (of which more in a
separate white paper), two pairs of very high current output transistors per channel so it could drive
any speaker, a new acoustically damped casework and relay switching for input selection. A
relay/resistor ladder volume control scheme for excellent channel balance and low distortion was
also designed.
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Summary of Contents for Cambridge Audio Azur 840A
- Page 1 White paper February 2006 Cambridge Audio 840A Class XD™ integrated amplifier Matthew Bramble – Technical Director, Cambridge Audio Douglas Self – Electronic Engineer, Cambridge Audio Cambridge Audio made a significant leap forward with the introduction of the Azur range in 2003 by pushing the boundaries of what was possible in the budget audiophile segment of the market.
- Page 2 White paper Cambridge Audio prides itself on employing the finest staff and we are lucky enough to have Douglas Self, the renowned writer, designer and amplifier researcher working in our engineering team. When Doug joined us he already had the seed of an idea to develop a new amplifier topology and it wasn’t long before he’d pitched us his ‘brave new idea’...
- Page 3 White paper course avoids this small pitfall (because the transistors are always on) but at the expense of a lot of heat generation. Managing this heat and power dissipation inevitably means that Class A designs are much more expensive to implement and often of lower power output so as to minimise the heat as much as possible.
- Page 4 White paper loads are shown. You can see from the Y-axis that in the 8 Ohm case in particular, the gain variations are very small, with an inoffensive-looking gain ripple around the zero-crossing at 0V output. This gain ripple however does generate high-order harmonics that can be poorly linearised as the negative-feedback factor in any linear amplifier falls with increasing frequency.
- Page 5 White paper Fig 1: The incremental gain of a CFP output stage as output voltage changes. SPICE (Simulation Program with Integrated Circuit Emphasis) for 8 and 4 Ohm loads. There has always been a desire for a compromise between the efficiency of Class B and the linearity of Class A and the most obvious way to make one is to turn up the quiescent current of a Class B stage giving what is called Class AB operation.
- Page 6 White paper understood and often catches out the unwary. To demonstrate this, Fig 2 shows THD plotted against output level for Classes AB and B. Fig 2: THD vs. level for Class B and Class AB. (0 dB is 30W into 8 Ohms) So actually it would be much more desirable to have an amplifier that would give Class A performance up to the transition level, with Class B after that, rather than AB.
- Page 7 White paper This is the basic Class XD principle, and it’s a very simple one, develop a topology that displaces the crossover point to one side of zero crossing. The essence of the Crossover Displacement principle is the injection of an extra current, into the output point of a conventional Class B amplifier.
- Page 8 White paper The extra current therefore flows through Re1, and the extra voltage drop across it means the output voltage must go negative before the current through Re1 stops and that in Re2 starts. In other words, the crossover point when Q3 hands over to Q4 has been moved to a point negative of the 0V rail;...
- Page 9 White paper Fig 4: SPICE simulation of the output stage gain variation with and without a constant 1Amp of displacement current. The central peak is moved 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.
- Page 10 White paper Fig 5: The concept of resistive crossover displacement. In and the following cases, the crossover point is displaced positively by sinking a current into the negative rail. This method suffers from poor efficiency, as the resistance acts as another load on the amplifier output, effectively in parallel with the normal load.
- Page 11 White paper This method has the other drawback that the distortion performance of the basic amplifier will be worsened because of the heavier loading it sees, the resistor being connected to ground as far as AC signals are concerned. Fig 6: The concept of constant-current crossover displacement Constant-current displacement A much better solution is to use constant-current displacement, and this is where we started as in Fig 6.
- Page 12 White paper attenuated by the very low impedance of the basic power amplifier and its global negative feedback, so complex control circuitry is not required. The efficiency of this configuration is greater, because the output current of the displacer does not increase as the output moves more positive. The voltage across the current source increases and so its dissipation is still increased - but by a smaller amount.
- Page 13 White paper Fig 7: The concept of push-pull crossover displacement. The control circuitry implements a scaling factor of -a. The use of push-pull displacement is analogous to the use of push-pull current sources in Class A amplifiers, where there is a well-known canonical sequence of increasing efficiency, illustrated in Fig 8.
- Page 14 White paper also reduced at output powers less than the maximum. Similarly, there is a canonical sequence of efficiency in Crossover Displacers, though the differences are smaller. Fig 8: Left is a resistive Class A amplifier giving 12.5% efficiency, while centre shows constant- current Class A giving 25%.
- Page 15 White paper in reducing the inherently low output impedance of the output stage in the usual way, being unaffected by the addition of the displacer. So while the constant-current displacement method is simple and effective, the push-pull version of crossover displacement has many advantageous and was preferred for the best linearity and efficiency;...
- Page 16 White paper of the noise at 10 kHz, so this frequency has been chosen for the THD/amplitude tests below. This frequency provides a demanding test for an audio power amplifier. In all these tests the measurement bandwidth was 80 kHz. This filters out ultrasonic harmonics, but is essential to reduce the noise bandwidth;...
- Page 17 White paper level as the output begins to traverse the displaced crossover region. (In fact it slightly exceeds Class B in this case; this set of data was acquired before the prototype was fully optimised). Fig 11 THD vs. power out for Class B, Class AB, and Class XD with constant-current crossover displacement.
- Page 18 White paper Fig 12 showing crossover displacement against class B. Looking at push-pull crossover displacement, Fig 12 shows that push-pull crossover displacement (XD PP) gives much lower distortion than constant-current crossover displacement. (XD CONST) tested 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).
- Page 19 White paper Fig 13 adds a THD vs. level plot for Class AB to the Fig 15 diagram, making it very clear that Class AB gives significantly 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.
- Page 20 Efficiency The Crossover Displacement technique is highly effective but does increase the total power dissipated in the output stage. The dissipation in the Source transistor is increased by the displacement current flowing through it, while in the sink transistor it is unchanged. There is also the additional dissipation in the Displacer itself, which is mounted on the same heat sink as the main output devices.
- Page 21 Class A operation. This compromise gives genuine measureable and above all perceivable improvements in sound quality, and can give better sound quality than conventional Class AB. Class XD is patent pending Cambridge Audio technology and is unique to The advantages: •...
- Page 22 5] Self, D "Distortion in Power Amplifiers" 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. White paper...
- Page 23 Notices o Class XD is a trademark of Cambridge Audio Ltd / Audio Partnership Plc o The Class XD Crossover Displacement topology is Patent Pending Cambridge Audio Ltd / Audio Partnership Plc. UK Patent Application Number 0505024.0 o This document is copyright Cambridge Audio Ltd...