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Old 9th September 2009, 11:19 PM   #1
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Default LME49713 Opamp Problem

Hearing that my particular DAC chip would "like" a current feedback opamp in the output stage, I set about replacing the NE5532s with LME49713s. The LME49713 is only available in SO-8 single packages, and the DAC board uses DIP-8 dual, so I used Brown Dog 020302 adapters (2 single SO-8 to dual DIP-8). After soldering each pair, I would test the completed dual DIP-8 LME49713 in a Chu-Moy headphone amp.

Schematic is here, courtesy of our friend Tangent:

The problem is that one channel will sound much louder than the other. I made three dual LME49713s total, and with one the left channel sounds louder, with one the right channel sounds louder, and with the third, they sound about the same. With the stock opamp I've been using (an OPA2227 if it matters), I've never had a channel imbalance before. All the resistors are 1% metal films, and were hand-matched to at least 0.5% between channels.

Any ideas what might have caused this? This is the first time I've ever soldered surface mount components - maybe I damaged the chips with static or heat? Do they need matching? Is this Chu Moy simply not a good test bed for these opamps?

thanks in advance,
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Old 9th September 2009, 11:22 PM   #2
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oh - looking at Tangent's schematic again - I'm not using dual 9V rails, but dual 4.5V rails (one battery instead of two). The LME49713 requires +/-5V or more. Might this have something to do with it?

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Old 10th September 2009, 04:01 AM   #3
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The reduced supply is probably the least of your worries here. Notice that that LME49713 has a typical input bias current of 1.8uA. Your input bias resistor is 100k - this gives you a typical input offset voltage of 180mV. Since your gain is 10X, the output offset will be typically 1.8V though it could be more than 3X greater in the worst case. Thus I guess your 3 opamps are sticking against the rails to varying degrees. You'd need to change a few component values to adapt that Tangent schematic to your particular choice of opamp.
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Old 10th September 2009, 04:28 AM   #4
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Thanks for your response, abraxalito - it makes sense. Yikes - I hope I haven't damaged my headphones...

At any rate, as noted above I was only using the Chu Moy to test that the opamps worked. The power rails in the DAC are +/-15V, so this shouldn't be a problem. I'll just have to pop them in and see how they do.

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