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JoeBob 8th September 2003 02:03 AM

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...

Pedja 8th September 2003 02:27 AM

Do you have a pot before the OPA?

JoeBob 8th September 2003 03:03 AM

No, no pot.

SY 8th September 2003 03:10 AM

Can you post a schematic of your setup? Sounds like a floating DC reference or an oscillation.

Pedja 8th September 2003 03:20 AM


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…


JoeBob 8th September 2003 04:05 AM

2 Attachment(s)
here it is, excuse my poor drawing abilities...

peranders 8th September 2003 08:01 AM

The first opamp: inverting input to output or have you drawn not right?

PMA 8th September 2003 08:55 AM

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.

Nuuk 8th September 2003 09:40 AM

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 . ;)

peranders 8th September 2003 10:37 AM

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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