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Old 8th September 2003, 11:36 AM   #11
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Quote:
Originally posted by JoeBob
here it is, excuse my poor drawing abilities...
The drawing or the circuit around the 627 seems wrong to me.

Are you using an inverting or non-inverting circuit for the buffer?

If you are implementing a voltage follower, the output should go to the inverting input. The input filter connects only to the non-inverting input.

If you are reversing your 1st stage, connecting input filter and output to the inverting pin, then the non-inverting pin should be grounded.

Which is it?


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Old 8th September 2003, 01:10 PM   #12
Pedja is offline Pedja  Serbia
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In previous days I had a success simulating power on offsets, but the opamp buffered GC circuit (using OPA627 model) seems a bit weighty for my simulator. Using LM6172 model I did not saw anything to worry about (this is my report, not a wish to be responsible for damage…).

@ Nuuk:

Knowing that you are methodical and pedantic I wonder that you have omitted to indicate the inputs in the opamps.

@ Joe:

Make sure you are using the circuit as in the Nuuk’s schematic (OPA627’s minus input is up, plus input is down).

If you use the circuit as the high pas filter, IMHO it is better i.e. safer to put that high pass filtering cap to the OPA’s output.

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Old 8th September 2003, 01:14 PM   #13
JoeBob is offline JoeBob  Canada
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Ummm.... ya, the OPA part is drawn wrong, it's wired up as a non-inverting unity gain buffer, output tied to - input, sorry for that, was in a hurry when I drew it.
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Old 8th September 2003, 02:14 PM   #14
Pedja is offline Pedja  Serbia
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Joe, is the problem power on thump or you are having a constant DC at the output?

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Old 8th September 2003, 08:30 PM   #15
JoeBob is offline JoeBob  Canada
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Constant DC, well it goes positive then swings almost all the way negative. I get just a slight power off thump, barely noticable (this is when the source is turned on first).
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Old 8th September 2003, 09:08 PM   #16
Nuuk is offline Nuuk  United Kingdom
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Quote:
Constant DC, well it goes positive then swings almost all the way negative. I get just a slight power off thump, barely noticable (this is when the source is turned on first).
Spooky Joe! That's pretty much what I found when I started off with my OPA627 buffer! I got round it by using a different PSU!
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Old 8th September 2003, 10:42 PM   #17
JoeBob is offline JoeBob  Canada
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Actually, changing the PSU lead to such a problem, at least I think so. Originally I used just a 15V regulator, then I added a 24V regulator followed by the 15V regulator, so now I have a "pre-regulator". And now I've got the PSU working perfectly I notice this problem.

I don't know if before, during my testing, I always turned on the CD player first, or if it was the PSU causing the problem. But the problem is there now and I'd like to understand it, not jst fix it, I find it rather odd.

Because it's only when I turn it on, if I have the CD player on when I power up the amp, all is good, even if I turn off the CD player and change CDs...

Since these are active speakers, I think if I can't fix the problem what I'll do is just make them turn on when a signal is present, a signal-sensing turn-on switch, then it would always turn on after the CD player... Well, that's a last resort, and then I'd have to figure out how to do that...
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Old 9th September 2003, 03:24 AM   #18
JoeBob is offline JoeBob  Canada
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I've toyed with it for enough today. I guess I'll give it a rest until tomorrow.

But if anyone has any ideas for what eles I should try, I'm up for it, changing the PSU slightly, adding a resistor on the input before the cap and a whole lot of all around changing things hasn't fixed anything yet.
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Old 9th September 2003, 07:22 AM   #19
Nuuk is offline Nuuk  United Kingdom
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Joebob - what caps are you using in the buffer PSU?
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Old 9th September 2003, 11:40 AM   #20
JoeBob is offline JoeBob  Canada
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