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Old 22nd November 2012, 01:05 AM   #21
ilimzn is offline ilimzn  Croatia
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Quote:
Originally Posted by wahab View Post
They did squeeze as much gain as possible from the input stage
by keeping it undegenerated but the following stage has only
a symbolic gain to add to the chain , hence about 54dB OLG
at most , although with a 100Khz bandwith.
Not really, the input stage has limited gain because of it's collector resistor, which can be bootstrapped from the next (driver) stage increasing it's gain (and linearity) quite a bit. I did a lot od experiments with the A1 circuit. Also it could have used a current mirror similar to the circuit described here, to drive the next stage but the gain would be quite unpredictable (dependent on following stage impedance which is nonlinear), unless a fixed resistive load was added in parallel.
Making it a follower is a simple matter of increasing feedback till it's unity gain, several circuit variations are possible including some shown in the class i article(s).

Since the scheme is based on keeping a constant voltage across the two series connected 'output' resistors from each half of the circuit, one side turns off once Iout >= Iq, simply because the voltage drop appears at least with it's quiescent magnitude (or more) across only the resistor from the half providing the output current, which switches off the other half (the 'wingspan' diagrams clearly show that). Considering there is a diff. front end with some gain in there, the switch-off can be even more abrupt than a regular class AB. With some trickery I'm sure it might be possible to implement some sort of non-switch-off biasing scheme (or possibly hyperbolic operation)... maybe making the output resistors non-linear with current (schottky diodes, maybe).
I have also explored a variant of the A1 circuit with separate AC and DC feedback in order to have the DC feedback keep the constant voltage across output resistors hence fixing idle current, but having AC feedback from the output node, to keep output impedance low (this makes the output resistors 'invisible' hence loses the effect of swamping out nonlinear output resistance by adding an actual resistor of a larger value in series). This would probably be a necessity in a variant with non-linear output resistors to achieve non-switch-off operation of the output stage.
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Old 22nd November 2012, 02:28 AM   #22
Struth is offline Struth  Canada
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Hi Guys

I would think that to make the A1 circuit a unity buffer would require either bootstrapping the front-end, adding current sources, or cascoding, to eliminate or reduce Early effect and common-mode distortions of the front-end.

Maintaining constant Vq in an output stage is simple. One of the purposes of class-i is to allow low idle currents while eliminating crossover distortion. Again, this can be done in aconventional amp using already developed means.

ilimzn, your mods to the A1 are pretty nice.

Have fun
Kevin O'Connor
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Old 22nd November 2012, 04:03 AM   #23
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Quote:
Originally Posted by Struth View Post
Hi Guys
<snip>

Maintaining constant Vq in an output stage is simple. One of the purposes of class-i is to allow low idle currents while eliminating crossover distortion. Again, this can be done in aconventional amp using already developed means.

<snip>

Have fun
Kevin O'Connor

There has been considerable discussion, from time to time, about non-switching output stages. Thus far it has not appeared (or I have not seen or read) any means that either really does keep the outputs from switching off, or else is generally satisfactory or without serious deficits of one sort or another.

So, if you've got one/them, at least I am interested to learn what it/they is/are!

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Old 23rd November 2012, 02:55 AM   #24
wahab is offline wahab  Algeria
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Quote:
Originally Posted by ilimzn View Post
snip
Thank for your explanations Ilimzn , they are quite documented , as usual.

Personnaly i m fond for symmetrical differentials topology so no wonder
that this circuit get my attention , specialy because of its dual NFB loop ,
but it appeared through simulations that it is not very good perfs wise.

As for non switching behaviours , the only one i did take a look at
is JVC s Super A since there exist a discrete version of the circuitry
in their AX5 model before it was replaced by an integrated version
using their in house VC5022 wich according to JVC is the same circuit
as its discrete sibling.

Overall it works more or less in simulations but surely that the IC version
provide a better reproducibility.

Anyway , Edmond Stuart has provided quite a lot of efforts to explore
the non switching circuitry elsewhere in this site and as far as i know
it seems that integrated dedicated circuits as the VC5022 and Pioneer s
PA0016 are the way to go as they allow for reliable implementations.

Below is the schematic of the discrete version used in the JVC AX5 for whom
is interested by theses circuits.
Attached Images
File Type: gif SUPER A TEF SCH.gif (13.6 KB, 427 views)
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Old 23rd November 2012, 02:03 PM   #25
Bigun is offline Bigun  Canada
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Originally Posted by bear View Post
I have not seen or read any means that either really does keep the outputs from switching off, or else is generally satisfactory or without serious deficits of one sort or another.r
I would propose Elvee's Circlophone.

With a well designed Class AB output, how many people can really hear any cross-over distortion when it's properly biassed ? - it seems to me that the Super A and other non-switching scheme's from 1980's (?) never became a dominant technology - does anybody make one still...
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Old 23rd November 2012, 06:35 PM   #26
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I was hoping that Struth was going to make some suggestions...

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Old 23rd November 2012, 11:22 PM   #27
Struth is offline Struth  Canada
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Hi Guys

Bear, maybe you do need my books? Back in 1997, these things were published.

As far as audibility of crossover distortion goes, in most listening situations with an amp biased at the class-B point (lowest THD according to Self), all the crossover artifacts are right there competing well with the signal. For a CFP, ideal idle is 15-20mA; with EF is is 115-120mA - both with a single pair of outputs. These current ranges correspond to audio powers of about 3mW and 115mW. The effective THD is quite a bit higher at these power levels than for full output, as THD is a ratio of signal to distortion+noise.

Self thought this was an important enough issue to devise his crossover-displacement method. This moves the crossover artifacts to a higher power level, hopefully above the normal listening level.

Apart from requiring matched devices to work well, the Super-A circuit and others at the time floundered more because of brand loyalties, packaging, etc rather than performance deficiencies of the methods.

Have fun
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Old 24th November 2012, 12:26 AM   #28
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I thought your books are on guitar stuff, like ur biz...

Please be specific, which things were published by whom in 1997?

I see you are a disciple of Doug Self, who, btw is on this forum... which is fine. I have his book.

Well, I guess then that all amps with distortion below some level, like that of the xover distortion, for example, sound the same...?

Whatever, I'm not looking to argue, I am interested in ideas that revolve around non-switching amplifiers (other than standard class A). The idea that I or anyone else is fully expert and knowing of all aspects of these topologies or designs isn't going to be true. That includes some of the luminaries on here. I do not count myself as being in that league, and am happy to make mistakes and learn. So, if you have something to add, without being condescending, I am sure that I and others would be more than pleased to hear it.

_-_-bear
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Old 24th November 2012, 09:57 AM   #29
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Old 24th November 2012, 11:23 AM   #30
Bonsai is offline Bonsai  Taiwan
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Yes, the MF A1 shows more than a passing similarity than KCP class I.

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