Cheap as Chips OPA1688 Low-THD Muscle Amp

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Just try not to be negative.

-Chris

I am and I was quite positive. I made, what I concider useful, posts with measurements in this topic before. Apparently xk971 is struggles to understand those. This ended up in a second round of our conversation and some deleted posts. That's where I had enough. Again, I'm quite positive, just I have my reasons to write what I wrote.

I'm sorry, but being nice should go both was. It obviously didn't.
 
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No Problems.
Not exactly like Self's circuit. I prefer not to connect anything else to the input if possible. Anything injected into this point can't be compensated anywhere further along the line. Although it is not necessary a problem in every given design.
I just added a divider from the output with the same ratio as the feedback divider to create a voltage approx equal to the input common mode signal. Than I have a buffer made from 4 transistors. I think transistors can be replaced with one opamp if needed.
 
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Hi Shaq888,
Your recent posts are helpful and more than civil. I'm not participating in this thread, which is why I can moderate it. Therefore I do not have the full history and can only go on what I see. You will notice that I didn't give an infraction or any points either as I felt a comment would do what was needed.

Don't let negative people drag you into a "fight" online. Just stick to facts and ignore the noise. You'll feel better if you do. Thank you for helping other members.

-Chris
 
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Thanks for the links, <|00940|> - very helpful to get actual schematics. I use Danyuk’s cross feed circuit - a smart guy who is also an excellent communicator.

From second link above:
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Connecting another opamp with non-bootstrapped supply to the feedback divider pretty much defeats the purpose. The only reasons why it works OK is that the divider has quite low impedance. It would be better to add separate divider as I described earlier. This would also allow conveniently control its frequency response for maintaining stability.
 
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There is an asymmetric divider. There are two rails, positive and negative. There is a feedback divider. There are two bootstrap caps. Now, how those all can be arranged and connected to produce what you describe, I for one, cannot divine out of the few terse sentences. Like I said, a schematic is worth a thousand words. And 25 words is worth maybe 15 words when not detailed enough. Maybe I’m just a simple country bumpkin who doesn’t have an EE degree so need more hand holding than most. I have no issue with asking questions and asking for help when I don’t understand.
 
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Post 123:
I just added a divider from the output with the same ratio as the feedback divider to create a voltage approx equal to the input common mode signal. Than I have a buffer made from 4 transistors. I think transistors can be replaced with one opamp if needed.

Maybe you are just referring to the same ratio as the feedback. But I can use more details on how to connect the divider for the two rails.

Thank you.
 
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The feedback divider, r6 and r5 is fairly obvious in the schematics above (post 127). He is saying don't take the signal from the inverting input, but duplicate the divider and take from that.

I have the impression that Shaq888 is well schooled with possibly much experience. I'd encourage him to keep engaged.


There is an asymmetric divider. There are two rails, positive and negative. There is a feedback divider. There are two bootstrap caps. Now, how those all can be arranged and connected to produce what you describe, I for one, cannot divine out of the few terse sentences. Like I said, a schematic is worth a thousand words. And 25 words is worth maybe 15 words when not detailed enough. Maybe I’m just a simple country bumpkin who doesn’t have an EE degree so need more hand holding than most. I have no issue with asking questions and asking for help when I don’t understand.
 
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XRK: I think you would benefit from taking a step back and figuring out what you're trying to accomplish. Which problem are you trying to address? Once you have the answer to that question, you'll probably find the solution to be more straight forward.

At the moment, it seems you're pulling schematics out of a hat hoping that one of them will solve the problem that you have yet to define.

I suggest reading the first few paragraphs of the Electronic Design article you linked to earlier. You'll find a lot of good tips there. Perhaps you could try some of them on the bench. Perhaps you will find that you don't need the fancy circuitry. And if you do find that you need fancy circuitry, you can design accordingly. It's usually more productive to understand the problem rather than trying solutions with a shotgun approach.

At work we had a saying: Get fancy, get F...ed. Sometimes designers get caught trying to put 10 lbs of dung into a 5 lb bag. I'm thinking this may be one of those times.

Just saying...

Tom
 
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The schematic in post 127 was what I posted based on the Danyuk article cited by 00940, to which Shaq basically said it was wrong:
Connecting another opamp with non-bootstrapped supply to the feedback divider pretty much defeats the purpose. The only reasons why it works OK is that the divider has quite low impedance. It would be better to add separate divider as I described earlier. This would also allow conveniently control its frequency response for maintaining stability.

So, I am not smart enough to understand what part of the schematic to keep or change. Again, a schematic of what Shaq actually has in mind would be helpful.

It seems schematics were created to quickly and effectively show in one snapshot the topology, parts used, and values in an unambiguous manner. Prose on the other hand, can be open to interpretation unless accurate, complete and verbose. It’s possible of course to describe completely in words alone - this is done all the time in patents. But that’s a very distinct style of technical writing that is exceednly verbose so as to not be ambiguous. Another example of readable text that perfectly describes a circuit: the LTSpice .asc file.

If this is so obvious, would someone be kind enough to make a hand sketch or even edits to the existing Danyuk schematic?

He is saying don't take the signal from the inverting input, but duplicate the divider and take from that.

In this sentence, I have no idea what you mean by “duplicate the divider and take it from that”. What is referred to as “that”? And “don’t take the signal from the inverting input” of which opamp? Isn’t the second opamp a bad idea according to Shaq?

693355d1532364473-cheap-chips-opa1688-low-thd-muscle-amp-6737237f-1fec-4589-ae03-7aa603e20a7c-jpeg


So if there is no schematic to be shown, maybe it’s all elementary to you guys, so go on ahead and implement it your circuits. As everyone appears to be able to interpret schematics from a few lines of text. I will go on with my circuit as is then, given no further schematic on implementation of the bootstrap as conceived by Shaq.

Edit: plus mine is simple. As Tomchr said, don’t make it fancy just to make it fancy. I am not sure I need the bootstrap because honestly.

I suggest reading the first few paragraphs of the Electronic Design article you linked to earlier. You'll find a lot of good tips there. Perhaps you could try some of them on the bench. Perhaps you will find that you don't need the fancy circuitry. And if you do find that you need fancy circuitry, you can design accordingly. It's usually more productive to understand the problem rather than trying solutions with a shotgun approach.

I will read it again, it’s enjoyable as it comes with schematics.
 
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