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Old 8th September 2003, 01:03 AM   #1
JoeBob is offline JoeBob  Canada
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Default Buffering problems.

I've got an inverting LM3875 setup, with an OPA627 as a unity gain buffer followed by a low pass filter feeding the LM3875.

Now, I've got a small problem of a sorts, when I turn on the amp I get DC at the output, even with a DC blocking cap in front of the LM, if I connect the LM all by itself with nothing at the input, I get 0V DC. But when I measure I don't measure any DC coming from the OPA627, so I'm guessing it's the OPA627 oscilating, again, just a guess.

But what makes it strange, at least strange to me, is that if I turn on my source before I turn the amp on, all is fine, even if the source (CD player) isn't playing anything yet.

Any ideas, I'm open for sugestions...
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Old 8th September 2003, 01:27 AM   #2
Pedja is offline Pedja  Serbia
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Do you have a pot before the OPA?
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Old 8th September 2003, 02:03 AM   #3
JoeBob is offline JoeBob  Canada
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No, no pot.
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Old 8th September 2003, 02:10 AM   #4
SY is offline SY  United States
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Can you post a schematic of your setup? Sounds like a floating DC reference or an oscillation.
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Old 8th September 2003, 02:20 AM   #5
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by JoeBob
No, no pot.
So put the pot or something in, or always turn on the source first. Otherwise OPA’s input sees too high (if you have too tall resistor from OPA’s input to ground) or (almost) infinitely high impedance (if you have nothing there). And yes, that leads to oscillation.

Again, as SY said, if you could post schematic it could clarify the thing…

Pedja
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Old 8th September 2003, 03:05 AM   #6
JoeBob is offline JoeBob  Canada
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here it is, excuse my poor drawing abilities...
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Old 8th September 2003, 07:01 AM   #7
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The first opamp: inverting input to output or have you drawn not right?
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Old 8th September 2003, 07:55 AM   #8
PMA is offline PMA  Europe
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Why do you use LPF (cap behind 700R)? OPA627 (BTW, FB should be connected to -input) does not oscillate in a circuit like this. Do not forget supply bypass capacitors (100nF//10uF) just at OPA627 pins no.4 and 7. But I guess that it is LM which oscillates, not the OPA627.
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Old 8th September 2003, 08:40 AM   #9
Nuuk is offline Nuuk  United Kingdom
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I always power up my OPA627 buffer before turning on the pSUs for the LM3875's.

DC offset on the speaker terminals is the same, with or without the buffer. In my case DC offsets are 32mV and 52 mV. I don't think you said what yours were.

Full details HERE .
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Old 8th September 2003, 09:37 AM   #10
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I think also that LP filter AFTER the first stage is a little odd. Normally you want to isolate from the nasty world the first thing you do.
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