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Home>
Audio-design
Design Center > How
To Article

Distortion in power amplifiers, Part VIII: Class A amplifiers

Douglas Self -February 20, 2008

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And finally
The
techniques in this series have a relevance beyond power amplifiers. Applications
obviously include discrete op-amp based preamplifiers11 and extend to
any amplifier offering static or dynamic precision. My philosophy

Table 3:(All for 8 Ω loads and 80 kHz bandwidth. Single
pole compensation unless otherwise stated.)

Figure 10: Distortion performance for CFP output stage with
2-pole compensation. The THD drops to 0.0012% at 20 kHz, but the extra VAS
loading has compromised the positive-going slew capability, The 2-pole trace
is shown moving off the graph at 50 kHz.

is that all distortion is bad, and high order distortion is worse ...
n2/4 worse, according to many authorities.12 Digital audio
routinely delivers the signal with less than 0.002% THD, and I can earnestly
vouch for the fact that analogue console designers work hard to keep the
distortion in long and complex signal paths down to similar levels. I think it
an insult to allow the very last piece of electronics in the chain to make
nonsense of these efforts.

I do not believe that an amplifier yielding 0.001% THD is going to sound much
better than another generating 0.002%. However, if there is ever a doubt as to
what level of distortion is perceptible, then using the techniques I have
presented, it should be possible to reduce the THD below the level at which
there can be any rational argument.

I am painfully aware of the school of thought that regards low distortion as
inherently immoral, but this is to confuse electronics with religion. The
implication is that very low THD can only be obtained by huge global NFB factors
which in turn require heavy dominant pole compensation that severely degrades
slew rate. The obvious flaw in this argument is that, once the compensation is
applied, the amplifier no longer has a large global NFB factor: Its distortion
performance presumably reverts to mediocrity, further burdened with a slew rate
of 4 V per fortnight.

To me low distortion has its own aesthetic appeal. All of the linearity
enhancing strategies examined in this series are of minimal incremental cost to
existing designs with the possible exception of using class A. There seems to be
no reason to not use them.

References
1. Moore, B.J. - An Introduction To The Psychology of
Hearing, Academic Press, 1982, pp 48"50.

2. Tanaka, S. - 'A new biasing circuit for class B operation', JAES
January/February 1981, p 27.

3. Fuller, S. - Private communication.

4. Nelson Pass - 'Build a class A amplifier', Audio, February 1977, p
28 (constant current).

5. Linsley Hood, J. - 'Simple class A Amplifier', Wireless World,
April 1969, p 148.

6. Self, D. - 'High-performance preamplifier', Wireless World,
February 1979, p 41.

7. Nelson-Jones, L. - 'Ultra-low distortion class A amplifier', Wireless
World, March 1970, p 98.

8. Giffard, T. - 'Class A power amplifier', Elektor, November 1991, p
37.

9. Linsley-Hood, J. - 'High-quality headphone amp', HiFi News &
RR, January 1979, p 81.

10. Nelson Pass. - 'The pass/A40 power amplifier', The Audio Amateur,
#4, 1978, p 4 (Push-pull).

11. Self, D. - 'Advanced Preamplifier Design', Wireless World,
November 1976, p 41.

12. Moir, J. - 'Just detectable distortion levels', Wireless World,
February 1981, p 34.

Printed with permission from Newnes Publishers, a division of Elsevier.
Copyright 2006. "Self
on Audio," by Douglas Self. For more information about this book, please
visit http://www.books.elsevier.com/.

Related links:
Distortion
in power amplifiers, Part 1: the sources of distorion | Part
2: the input stage | Part
3: the VAS stage | Part 4: the power
amplifier stages | Part 5: output stages
| Part 6: the remaining
distortions | Part
7: frequency compensation and real designs
Audio
design basics: Achieve low distortion and noise with Class-AB amps
Loudspeakers:
Effects of amplifiers and cables - Part 2
Introduction
to the Six Basic Audio Measurements - Part 2
Technical
Paper: Keeping Up with the Expanding Demands of High-Performance Audio

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