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Old 28th August 2014, 09:37 AM   #1
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Default Near Zero THD down to the threshold of hearing

Apart from using a well designed Class A configuration for the audio power amplifier output stages, does any one know of a Class AB configuration that offers almost complete freedom from crossover distortion across the normal range of hearing from normal listening levels right down to near the lowest threshold of hearing? Clearly any such design must offer near zero disturbance in passage through the crossover regions, at all signal levels, but especially at the lowest signal levels.
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Old 28th August 2014, 10:36 AM   #2
dadod is offline dadod  Croatia
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Originally Posted by AudioDad99 View Post
Apart from using a well designed Class A configuration for the audio power amplifier output stages, does any one know of a Class AB configuration that offers almost complete freedom from crossover distortion across the normal range of hearing from normal listening levels right down to near the lowest threshold of hearing? Clearly any such design must offer near zero disturbance in passage through the crossover regions, at all signal levels, but especially at the lowest signal levels.
Maybe something like this? ThermalTrak+TMC amp
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Old 28th August 2014, 11:15 AM   #3
udok is offline udok  Austria
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Class-AB designs have a bias current. Therefore at low hearing levels they are effectively in Class-A mode.
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Old 28th August 2014, 11:41 AM   #4
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That's a very wide question.

Better question is what you want to do, and is this an academic exploration?

A short answer is that there are a variety of methods by which engineers and designers attempt to optimize class AB designs, quite a large variety. There are examples that succeed in terms of different aspects. Quite a few have been thrown up the flagpole here.

Fwiw, it is essentially impossible to have a class AB amp without some sort of "disturbance" that can measured due to changing from class A to B and from the positive rail to the negative.

So, what's ur interest in this topic?
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Old 28th August 2014, 02:39 PM   #5
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Over my years as a professional electronics engineer in telecommunications, and even longer, since my lifelong fascination with with audio and radio began in my childhood, I have followed with great interest (and explored myself when possible) the arguments between audiophiles about the various merits of power amplifiers in various configurations such as: Class A vs Class B, Valve vs Solid State, then Class AB, (that in most cases was no more than Class B with a higher quiescent current setting and a transfer function around the cross over region more like the hind leg of a donkey than anything linear), then the growing popularity of MOSFETs and more recently CLASS D which is, I believe best suited to Rock Bands and PA Systems but is NOT sited to the true pursuit of High Quality Sound Reproduction, as expounded so well by the likes of James Moir, in his 1958 book of the same name, and others such as John Lindsey Hood, (Arthur?) Williamson, Doug Self, Ted Jordan and (John?) Baxandale here in the UK - with articles and prototype designs for amps and simple but musically satisfying loud speakers, published in The Wireless World Magazine. I am now particularly interested in any recent work done that seeks to control the THD, TID and IMD at the lowest output power levels, in power amps in that magical 'quiescent region' of the typical, 'so-called' Class AB power amplifier since the human ear has a dynamic range of 10^12 which far exceeds the measurement range of most audio test equipment and yet deserves much more attention. It is probably the sound between the notes and the way the sound 'decays to nothing' that really defines the realism of reproduced music and speech and not whether it is realistically loud and fully ranged from 20 Hz to 30KHz - "just like being right there in front of the orchestra"!
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Old 28th August 2014, 02:50 PM   #6
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Maybe something like this? ThermalTrak+TMC amp
Many thanks Dadod for this design information - it looks very interesting. Agood example of what we used to call a Darlington or Super Darlington configuration and is basically a series of two or more complementary emitter followers in the output stages. The THD figure at 1 watt seems to offer the possibility of sustained low THD at much lower output power levels - which was what I was seeking. However, the proof of the pudding is in the listening of course..
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Old 28th August 2014, 03:04 PM   #7
rayma is offline rayma  United States
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Originally Posted by AudioDad99 View Post
Apart from using a well designed Class A configuration for the audio power amplifier output stages, does any one know of a Class AB configuration that offers almost complete freedom from crossover distortion across the normal range of hearing from normal listening levels right down to near the lowest threshold of hearing? Clearly any such design must offer near zero disturbance in passage through the crossover regions, at all signal levels, but especially at the lowest signal levels.
The simplest way would be to load the output with a current source, to move the crossover distortion away from the zero crossing.
Preferably, this would keep the NPN transistor side turned on.
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Old 28th August 2014, 06:40 PM   #8
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There are a variety of approaches that you can find here.

You do know about "Hawksford" error correction?
Also, do you know about Halcro? (An amp mfr, but you should read the patents)
There are discussions of that amp here.

Search online and here for Syn08. He left this forum, but his website has an amp or two that might interest you also.

Then too there is an idea probably first started by the Japanese, where you float a pure class A amp between powerful class B sides. That might fit your bill?
Depending on the implementation, the class A amp might never be driven to saturation (bootstrapped).

All sorts of neat ideas. Someone could take all the compatible ones and design a super amp of some sort, I guess.

But, a whole lot on the listening side probably depends on the sensitivity of your speakers, unless ur of the club who feels that what goes on at and below the noise floor (acoustic noise floor in your room too!) matters much.

And what is your source going to be that does not add in noise of its own? And do we care if it is correlated or uncorrelated noise?

BEWARE - you have opened a large can of worms!
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Old 28th August 2014, 06:50 PM   #9
Bigun is offline Bigun  Canada
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Then too there is an idea probably first started by the Japanese, where you float a pure class A amp between powerful class B sides. That might fit your bill?
Depending on the implementation, the class A amp might never be driven to saturation (bootstrapped).
I remember reinventing that idea - I was informed that Graham Maynard developed something simple along these lines which he called the GEM amplifier - worth a search perhaps.
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Old 28th August 2014, 06:51 PM   #10
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Actually, now that I re-read ur post, there are two separate issues.

The first is ultra low distortion and noise.
The second is effects at the "zero crossing".

The Gm "doubling" or "halving" depending on how you wish to view it happens in class AB or B output stages. But it does not happen in Class A.

So you could decide on a Class A design to eliminate that issue.

However that says little about distortion or noise, it merely eliminates one source of possible distortion and noise.

Peeling the pesky onion anyone?
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