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Old 10th June 2006, 03:09 AM   #1
alexcd is offline alexcd  United States
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Default LM4702 triple darlington output

Why hasn't everyone decided to go triple-darlington with the LM4702? You could put as many output transistors as you wanted to maximize the available voltage. Sure, you'd probably drop a bundle on transformers and caps but why not go for the gold with this chip? Just curious. I have a schematic in mind. There must be some con's to this. Let's hear them.
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Old 10th June 2006, 10:21 AM   #2
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The allure of the chipamp is typically the lower cost, higher integration, and short feedback path.

I'm not entirely confident most are looking to just increase output wattage as much as possible either, "more" is not always better, particularly if you have to lose hearing to realize the benefit of the design.

That doesn't make it a bad idea though, but it also drifts away from the chipamp forum idea, because this isn't then going to be a chipamp, it's just a chip driver.
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Old 10th June 2006, 10:40 PM   #3
tf1216 is offline tf1216  United States
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As I see it this isn't a typical chip amp because it lacks a power output stage. I'm simply adding a much MUCH larger current gain stage. So I may be guilty of being a wattage hog but I'm sure a lot of DIY'ers building large passive speakers are interested in at least a part chip amp. A little work is done for you. Good work National Semi once again.
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Old 10th June 2006, 10:41 PM   #4
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That was supposed to be under my name
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Old 12th June 2006, 01:28 AM   #5
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Hi,

In your own words, go for the gold and do not let them get in your way with their entropy downed ideas.

Peace,

Mark
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Old 12th June 2006, 04:26 AM   #6
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Quote:
Originally posted by tf1216
As I see it this isn't a typical chip amp because it lacks a power output stage. I'm simply adding a much MUCH larger current gain stage. So I may be guilty of being a wattage hog but I'm sure a lot of DIY'ers building large passive speakers are interested in at least a part chip amp. A little work is done for you. Good work National Semi once again.

It isn't a chipamp, it's a chip voltage gain stage, essentially a high voltage tolerance opamp. "Everyone" in the chipamp forum isn't doing it because it's not a chipamp and you would not see a non-chipamp in the chipamp forum.

Is it a "bit" like a chipamp? Yes, but lots of other things are a bit like one too, even all the way to using old school discrete parts for everything.

So what it has in common: amp
What it doesn't: the word chip in front of "amp".


THEN there is the real chipamp, what we use in this forum. You asked why not everyone and the aswer was given. That doesn't make a driver topology evil or bad, but if you ask why CHIPAMP builders aren't building this instead, it's because that's not what they want or at least not on-topic in this forum.

I suppose the more relevant question with a larger current amp is this: Given the larger expense, is the LM4702 appropriate for that class of amp? It might end up sounding better, or worse, or about the same but louder, and yet in the end only you have to be happy with it.

Finding amps with separate current gain output stages is the rule, not the exception... except of course, in the chipamp forum. Do you see what I mean?

What you propose isn't unique at all except quite specifically in a chipamp forum. To a certain extent it is the opposite of a chipamp.
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Old 12th June 2006, 04:59 AM   #7
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Default Mike Gergan Thread Starter

Hi,

Go to Mike Gergan's thread starter on Google and find Glennb

and check out his home page at

www.werple.net.au/~gnb/audio/lm4702.html and

the class A driver vs. chipamp issue will be clear.


Mark
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Old 12th June 2006, 01:41 PM   #8
alexcd is offline alexcd  United States
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Perhaps this isn't the correct forum to talk about a driver CHIP, but there are plenty of threads here about the LM4702 (all with output transistors or FET's.) So what are a few more components? If you want to be technical, it's not a chip amp because it isn't the whole amp in a single IC. I can share my ideas somewhere else then.
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Old 12th June 2006, 05:50 PM   #9
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Default Chip Amp Fourm OK

Hi,

Feel free to use one alternative Chip Amp fourm for the LM4702

as made by Mike Gergen thread starter. Bring any and all of your

electronics ideas to this tread, since we try to be open minded

people. Good luck with your design theories.

Mark
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Old 12th June 2006, 08:30 PM   #10
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Well, this thread has been a huge waste of time so far. Thanks for the ramblings, "!".

Back on topic:

A quick back of the envelope calculation regarding current needed to drive the three darlingtons leaves me a little short. I may be doing some math wrong (or assuming the wrong Hfe for your darlingtons) but I don't think the ~5mA the LM4702 puts out will be enough to drive three output darlingtons.

I do like the idea, so to make sure we are on the same page, what rails are you using, what output power do you want, and what is the Hfe of your darlingtons?

I think a pretty simple two stage voltage follower with multiple output pairs would do the same thing without taxing the chip as much, but using the darlingtons would indeed be a little more simple. I think it is worth exploring.

David



PS As to the appropriate forum for these questions, since the device is a chip/ss amp hybrid, I would think either forum would be fair to post in. Why is this even an issue???
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