"Optimum" Class B better than Class AB? - diyAudio
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Old 28th July 2001, 04:12 PM   #1
paulb is offline paulb  Canada
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In the August 2001 issue of AudioXpress, in a reply to a letter, Kenneth P. Miller referred to "optimum Class B" for a bipolar transistor output stage. It refers to a bias setting that produces the lowest distortion of anything other than a full class A bias.
He talks about added distortion as you increase bias beyond the optimum point (but still keep it less than full Class A), and "transconductance doubling" being the reason.

Has anyone else heard of this?

I know that Class A is the modus operandi for most of the amplifiers discussed in this forum, but for those of us building an AB amp it would be useful to have some verification and background on this issue. I can't see building my tri-amped system with 3 Class A amplifiers; natural gas is a much cheaper way to heat my house than electricity.

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Old 28th July 2001, 07:44 PM   #2
Geoff is offline Geoff  United Kingdom
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Paul

Yes, optimum class-B biasing will give lower distortion than a class-AB design. If you want more details, borrow a copy of Doug Self's 'Audio Power Amplifier Design Handbook'. Chapter 5 explains the mechanism by which the distortion is increased in class-AB working.

Alternatively, have a look at the JLH Class-AB article on my website( http://www.gmweb.btinternet.co.uk/index.htm ), particularly the subsequent 'Letters to the Editor', which covers exactly the same points though with different terminology. This article was written over thirty years ago and shows that there is nothing new in this 'discovery' despite the impression given in various more recent publications. JLH makes a very valid arguement against the minimum distortion class-B arrangement, preferring the benefits of class-A working at low power levels with class-AB biased designs.

Geoff

[Edited by Geoff on 07-28-2001 at 04:14 PM]
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Old 28th July 2001, 08:06 PM   #3
paulb is offline paulb  Canada
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Thanks, Geoff. It's interesting that Miller referred to "new research" for something that Hood knew about 30 years ago. I'll check out your site's articles.
I also e-mailed Rod Elliott (it's his 60W AB that I'm building) to see if he had any thoughts.
I've been hemming and hawing about buying the Self book. This is a good excuse to take the plunge.
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Old 30th July 2001, 08:30 PM   #4
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Most of the Bipolar AB amps I've built had a bias "sweet
spot" for measured distortion around 100 mA or so. This
is not always the best sounding bias figure, however.
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Old 24th March 2003, 01:16 AM   #5
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Thumbs up Class AB

...i prefer class AB myself....with the ballast resistor kept as small as possible to minimise gain-step distortion...
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Old 24th March 2003, 06:37 AM   #6
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'Optimum class B' is certainly not class B, it is low bias class AB.
Just because someone as knowedgeable Doulas Self gives it such a silly name, that doesn't make it so....
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Old 24th March 2003, 03:38 PM   #7
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Quote:
Originally posted by pmkap
'Optimum class B' is certainly not class B, it is low bias class AB.
Just because someone as knowedgeable Doulas Self gives it such a silly name, that doesn't make it so....

Actually, 'Self gave very good reasons for this distinction.....an unbiased SEPP stage infact operates in Class-C, as each of the complementary members are driven foward-active for slightly less than 180 degrees........and can only be, (if we follow this urgument through to its logical conclusion), considered to operate in class-B if each member is biased to conduct for as close to 180 degrees as practicable...

Class-AB bias can then be described with some precision, as that condition in which the SEPP is biased into class-A up to some arbitrary power output before one or other of the members in the SEPP arrangement cuts off...
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Old 24th March 2003, 05:38 PM   #8
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It is well understood that a low bias AB usually gives better specs than a higher bias, but it is also seems that listeners generally prefer the sound of higher bias.

This conclusion is supported by several instances of single-blind comparisons of the same amplifier between low and higher bias.
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Old 24th March 2003, 06:02 PM   #9
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Default The Bias issue...

Quote:
It is well understood that a low bias AB usually gives better specs than a higher bias, but it is also seems that listeners generally prefer the sound of higher bias.
I agreed!!... maybe that's because with the high bias point the open loop output impedance is lower...and the amp becomes less afected with the EMF from the speaker!!!
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Jorge
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Old 24th March 2003, 06:05 PM   #10
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The reasoning is clear. In the A region of class AB, you have two active devices and hence a gm doubling, whereas the gain drops when class B mode is entered. Class AB with very high bias may still sound better because for normal listening levels, it is all A, and at higher levels the effect is so small it gets swamped.

What I haven't seen yet is a formula how optimum AB is really defined as a function of emitter resistance and load impedance.
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