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Old 11th December 2012, 05:37 PM   #41
Struth is offline Struth  Canada
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Hi Guys

There are contradictions in the relevance of stable Vq.

One one hand, Self extols the virtues of stable Vq and even suggested this might be a new insight. Supporting that claim in a manner of interpretation are the distortion specs of his Trimodal amplifier. When the class-A mode is pushed hard and reverts to class-B - he calls this class-AB - THD is higher than for either class-A or class-B. In both latter cases, Vq is constant but for AB it is not.

The above might suggest a flaw with sliding bias methods as Vq will vary over the signal cycle. On the other hand, one of Self's conclusions about certain circuit behaviour is that the circuit portion turning 'off' has the greater impact on THD. This would imply that disallowance of turn-off would obviate the distortion, giving credence to the sliding bias, class-i et al techniques.

It would be nice to completely eliminate distortions, but that is not likely to happen in this universe. For my own listening and construction, moving the THD artifacts away from the power range most often used is sufficient to make THD within that used range vanishingly small. The methods I referred to in my books and elsewhere above, achieve this in a simple way. Most tend to move the amp to class-A, or at least greatly increase the class-A region. Look at my posts in the Lineup buffer thread to see a circuit app.

Never posted to the circlophone threads but read them to see if there was anything worth learning about. Same with the tringleton.

Have fun
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Old 12th December 2012, 04:21 AM   #42
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There may be a good explanation or reason, but I'm unclear on how Vqs changing while an AC signal is present ought to have any effect different than the presence of the AC signal? At any instantaneous moment the voltage present is Vqs +/- [AC signal]. There is some question as to if this is a voltage or a current, or a combination of the two I suppose... perhaps I am naive about how this works, entirely possible. But it seems unclear to me why it makes any difference.

As far as non switching output stages intuitively it seems to me that there are two choices as how to make it happen, maybe three:

- never apply an AC signal that drives "below" the Vq point.
- apply a signal that is more or less the inverse of the driving signal to each half of the output stage only when that signal is traveling "south" of Vq (selective inverse cancellation).

A trick is done in AM ham transmitters called the "3 diode limiter" that prevents "baselining" and permits positive modulation in excess of 100%. Baselining is when the modulation is sufficient to overcome the carrier amplitude and causes a cutoff, which results in briefly having zero power out, and the transient going off and coming back on causes severe IM products.

I'm not sure, since the Kendall Perry circuit isn't immediately obvious to me, but after reading the Linear Audio article once or twice, I got the sense that his method is more or less a "see-saw" sort of approach where the drive in one phase forces the current in the side headed toward cutoff back up before cutoff is reached. If this is done on a low(er) level circuit then only normal bias is required in the output stage since the signal sent never sends each half toward full cutoff... which if I have this part correct is far more elegant than trying to wrap around a loop to do this job. And, even if I have the Kendall-Perry idea all wrong, and I may well, that's how I would envision doing it.

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Old 12th December 2012, 04:38 PM   #43
Struth is offline Struth  Canada
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Hi Guys

Bear, your interpretation of the class-i circuit is correct. However, the idea that the correction for crossover elimination can be moved forward in the circuit is incorrect. Unless Vq is massive by way of operating the output stage in class-A, then crossover distortion will occur as the signal can turn off one side of the output or the other.

Most transmitters are SE so cutoff is related to one side of the signal being excess in amplitude. In this case bounding the signal in one direction keeps cutoff from occurring. Because the final output in most transmitters is class-C, the "saturation" side of the wave is less problematic.

There are two mechanisms involved at the crossover point and two distortions that result: The frst is actual crossover distortion caused by the nonconjugate nature of the transfer curves of the two halves of the output stage. The second is switch-off distortion, likely caused by charge storage in the devices being turned off. Pages 180-184 detail Self's study into this ((Audio Power Amplifier Design Handbook 5th ed.). Where thermal compensation to cancel Vbe variance under load and with temperature variations assures Vq will be maintained constant, the traditional discussion ends there. Direct tracking of Vq is not considered except for specific designs, such as class-A amplifiers where Vq corresponds to a very high Iq that presents the possibility of thermal runaway. It is obvious that such a direct-control approach of Vq can readily be applied to lower idle currents.

As stated previously, part of class-i's promise is to allow low idle currents and cool operation of the output stage without crossover distortion. Direct Vq control is the surest way to achieve this but the class-i engine may be too fragile (sensitive to component parameters) to achieve it in a practical fashion.

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Old 12th December 2012, 05:14 PM   #44
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Well the issue of crossover distortion is one of addition at the xover point. Eliminating the turn off introduces some issues it seems to me. The "Gm doubling" that I first saw published by Ben Duncan, which addresses that exact point where the output stage goes from two conducting devices to a single remains to be considered. Even if the amp without device turn off is "high bias, there is still a point in the transfer curve where there is a hand off.

Now, conceptually one might think of the non-switching amplifier's output stage as two SE amps, where they are more or less biased in class B, but there is a soft limiter on the negative going AC drive signal so that there is no hard transition and the SE output stage is never driven off completely. But the two transfer curves still must be made to meet.

It seems to me that perhaps the question revolves around the sonic deficits or lack thereof for the switching artifacts vs. non-switching linearity at the xover point(s).

Or if there is a way to skin both cats, for the best of all worlds.

I don't have that answer, but am interested in that question.

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Old 12th December 2012, 07:07 PM   #45
Struth is offline Struth  Canada
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Hi Guys

In class-A the issues of crossover and switch-off distortion are completely eliminated; those deficiencies are only present in class-B and -AB.

The audibility of these distortions has been well documented elsewhere... maybe even in St. Elsewhere?

Gm doubling is itself a myth. The issue is the reduction of gm either side of the mutual conductance zone. Even here, Leech dismisses it completely suggesting that the gm for the _whole_ output stage depends primarily on the load, with the result that gm is essentially the reciprocal of the load resistance. See his article on his site.

Oliver introduced the gm-doubling term in the 1960s, and explained a way to minimise its effect by setting Vq equal to 26mV, then adjusting Re accordingly for a given idle current. This is basically the approach Cordell follows, and one ends up with large Re values as a result. The problem is that Re sets the output impedance of the amplifier, influencing damping and high frequency response.

Gain variation near the crossover point is the main issue, and this is based on a combination of factors already listed above.

One possible approach is to simply add two current sources in parallel with the driver devices, combined with a direct-control current-set circuit. The latter locks Vq while the CSs provide a minimum current through the 'off' device base to keep that output from actually going completely 'off'. This is essentially what class-i is trying to do, but they do it in a more "clever" way.

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Old 12th December 2012, 08:54 PM   #46
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Kevin, Elvee has made some considerable contributions here over a pretty good period of time. He's put his circuits up to scrutiny. I don't know him personally either and we're not internet buds either.

Self and Cordell, as you know are participants here.

It seems like you're pretty good at this EE stuff. If you want to test yourself, jump into the Opamp Open Design thread and see how you fare. Likely ok, but expect to be challenged, I'd reckon. Or John Curl's Blowtorch thread, if you want more to chew.

It's easy to sit on the sidelines and complain or snipe, much harder to play the game.

For me, I am just interested in anything and everything that anyone can bring to the table, right or wrong, since that causes people to think and discuss, and that brings illumination and sometimes understanding.

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Old 13th December 2012, 04:39 AM   #47
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Mods, sorry for a little OT here...

Kevin, as I said before if you want to talk privately, do it privately, as in PM or email. Since you keep bringing it up, my email suggested (among other things) that your product for guitar amps, having made certain claims in strong language, was rather an unknown quantity and that you might want to put up some clips that illustrated that it worked as featured... whatever. It's rather irrelevant.

As far as your books, I never heard of you before you posted here and I followed your link to your website. Never heard of you, your company or your books before that. Your books appear to focus on guitar amps, not low distortion high end hi-fi design. No reason that I can see for thinking otherwise.

It is funny how different people read other people differently, in person and in print. I never detected any edge at all to Elvee's posts. Do keep in mind he is European, and his first language is likely [I]not[I] English? He seems to have answered lots of people and come up with some neat designs - regardless of their original source. After all almost nothing has not been done before, right? I don't need to defend Elvee, as I said before.

As far as my own posts, I try to be positive and ask questions when I don't know the answers. I also try to ask questions when i do know the answers, depending on who is participating and what the audience is...

You might find that Curl does much of the same, only he actually knows a bunch more than I do... as do a whole lot of folks on here. Which is why this is a high class forum, btw.

How about Wurcer, he only came up with the 797? Maybe other stuff he doesn't take credit for in public... that's in the past too.

And as far a their contributions, Paul McCartney hasn't written a good song in a long time, but he IS still a Beatle.

You might just buy the copy of Linear Audio, or subscribe?
Jan Didden is on here too...

Myself, my business? I am semi-retired right now. Not bitter at all, maybe a tad frustrated at my private personal challenges, but that's private, eh?

I understand that it is difficult to know how one's writing comes across. So one might find ways of getting independent feedback, if one so desires? Just a thought.

_-_-bear

PS. it is important to also keep in mind that this is a DIY site, and the level ranges from absolute noob through world class... it's not a graduate PhD seminar. (although topics may reach that level)
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Old 13th December 2012, 11:07 AM   #48
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Struth View Post
Hi Guys

"Unity reference" is of course, yourself, Elvee, as you set yourself upon your golden pedestal above the rest of the world.

For myself, I am content to be a part of the Earthbound. Karl Marx had great ideas; it was the implementation by others that gave it a black eye. As for keeping company like Self, Cordell, Leech, Blomley, Baxandall... I like this crowd a lot better than where you seem to be seated.
Well, well...

Let us try to forget personal attacks and concentrate on technical matters.
For me, a glass half empty is the same as a glass half full, but others may differ.

Quote:
As for gm-doubling - Oliver's term, also a great engineer in the early days of ICs - it is misnamed.
I more or less agree: that is the reason why I placed the sign ~ in front of doubling.

Actual doubling could only happen in extreme overbias situations.

What is generally observed is an increase of the apparent transconductance when the stage is overbiased.

This can be illustrated with this example. It is a CFP because it makes the phenomenon very clear-cut and easy to observe.

First, the situation with underbias: there is an increase of the output resistance (1/gm) in the crossover region, leading to Xover distortion.

Second, the stage is properly biased and the THD is minimal. Note that it is almost pure second harmonic, meaning it is caused mainly by the poor complementarity between devices, not by the biasing level.

And finally, the overbiased case: the THD has increased again, and if we look at the currents waveform, we see that in the part where they overlap, there is a change in slope: both output devices now contribute to the output signal, and each reduces its contribution by an inherent feedback mechanism, but the effect of the "~doubling" (see caveat above) remains, as the THD shows.

I have used a CFP because it shows very neatly the changes in slope, but the effect is present in any stage, even if it is somewhat blurred.

Why would one want to overbias an output stage, knowing that it leads to increases Xover distortion?

There could be a number of reasons.

-Although the THD in under/ overbias situations is comparable, the underbias one is much more objectionable from an audibility perspective, and since the bias setting is not very accurate, one prefers to remain on the safe side, even if it leads to increased distortion.
-Some people advocate a class AB, with a strong and wide A region, to move Xover problems towards a higher listening levels. I am not sure it is a good idea, but I do not judge the validity of one philosophy over the other, I simply examine the consequences.
-When the stage is optimallly biased, the output resistance is not flat for the Xover region, it has "ripples", which some feel are more detrimental than a continuously too low resistance. Here again, it is a matter of taste.
Attached Images
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File Type: png Xover2.png (131.1 KB, 260 views)
File Type: png Xover3.png (131.2 KB, 257 views)
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Old 13th December 2012, 04:43 PM   #49
Struth is offline Struth  Canada
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Hi Guys

Elvee, to me it looks like your graphs and explanation target internal gm variations as at least one source if not the major source of crossover distortion. The optimal bias point, which Oliver tried to specify, does minimise some of the negative result of the variation but the variation is always present.

The reason this comes to mind is twofold: First, a common fix for large signal nonlinearity (LSN) is to parallel more output devices. This has multiple effects itself: reducing beta droop, reducing the current through each device, reducing output impedance, etc, all of which widen and flatten the gain wobble of the output stage. There is also a potential to increase the "internal" gm of the stage depending on how you treat the idle current.

The second reason is when you consider the class-A situation. Internal gm is high as the current is high, so internal resistance is low - the "slope" I believe you are referring to. Yet, this condition could be interpreted in one way as being waaaaay overbiased, but the side effects of such a condition are not present until you drive the circuit out of class-A into -AB, as Self demonstrated. Again, this points to switch-off issues. Under-, over and optimal-B bias all have switch-off characteristics but the narrowness of gain wobble varies, along with the THD profile.

Bear, the portion of email you referred to was not what made me wonder about you.

As far as my history and views go, I've been involved with audio since 1974 and what you call "high end" since 1982. Most of my work in electronics has been designing for other companies, with the guitar amp and book biz just being the most visible and direct marketed part of what I do. My books go way beyond what is historic in the MI world and pulls in ideas from other disciplines of electronics - part of why the books are industry standards and have influenced every builder in MI. I don't consider circuit ideas to be "exclusive" to one end of the business or the other, so there are "highend" ideas in the guitar amp books that will be part of and expanded upon in the new range of hifi books we are working on.

Our site is one of the oldest on the web, but we keep it simple to load even for text readers. Our philosophy is that all of this should be fun. Whether you create music or simply listen to it; whether you build amps or simply read about them; none of this is critical were the bomb to drop, so as an entertainment it should be enjoyable. Our books, writings, web posts and site all convey this message. People who use our technologies and believe in the same things have created forums in support, such as the PowerScaling.com forum, set up by Mickey Corrieri of Soultone Amps. It is not my forum but I contribute to it and fully support it. Our technologies have been incorporated into all kinds of products and brands, so we must be doing something right. We never make a claim that our product cannot fully exceed.

I joined this forum because it seems to be relatively free of immature flaming - instead it has intellectual flaming, haha - and there is lots of talent here. I've made lots of contributions here since I've joined, as it is part of who I am and what I do - learning and sharing and trying to eek out a living. I've learned a lot, as well, and have seen sides of "heroes" that take away some of the shine but not always the glint.

Have fun
Kevin O'Connor
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Last edited by Struth; 13th December 2012 at 05:04 PM.
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Old 13th December 2012, 04:59 PM   #50
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Multiple output devices or not, keeping the total idle current equal, you will get the same gm wobble at the crossover region.

PS: And that makes nine (for those who know what I mean).
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