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Old 14th July 2012, 05:00 AM   #1
ZLyzen is offline ZLyzen  United States
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Default Question on Potentiometer placement in lme49720/lm4780 amplifier arrangement

I have built a small amp based around a lm4780ta and it's great and everything I love it, and so on. However, I do have one issue that's been bugging me and I'm wondering if anyone has any advice.

I have a small preamp built-in using lme49720 op amps. The signal path is as follows.
Input (rca/3.5mm) >
dual gang 100k pot (log) >
lme49720 with ~1.5 gain >
lme49720 with bass control >
head phone output and to second potentiometer (10k log)>
lm4780

so basically the signal after the bass control goes both to a headphone jack and to another potentiometer. My thinking here (when designing this) was I could (sorta) have a separate control for speaker volume, and when the lm4780 is muted I can have something to control to volume to the phones. However in retrospect only the first potentiometer is needed.

So now that all of that is known, here is the problem. When I turn up the second potentiometer anywhere past ~75% no matter where the bass control is at or no matter how loud the input signal is, there is immediate and abrupt static. However the first potentiometer can be turned up to its maximum with minimal of no noise at all. is the problem that it is a 10k pot? since this will allow more current to flow from the op amp right to ground since I'm using the potentiometers as voltage dividers? In the future should I have another op amp stage before the lm4780 or will this not make a difference?

Any insight would be great! thanks!
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Old 14th July 2012, 06:25 AM   #2
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I would be more confident with my reply if you could post a schematic. Nonetheless, you may have an impedance issue. The bass control output may need to see a higher impedance. Another op amp as buffer ought to solve that problem. As an alternative, if it's not too much trouble, you could try swapping the 10k and 100k pots. All potentiometers are voltage dividers, so the problem isn't the pot per se.
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Old 17th July 2012, 04:00 PM   #3
ZLyzen is offline ZLyzen  United States
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hmm looking at this more, I'm noticing that this problem is most profound when the lm4780 is running hot, and I believe that since this particular problem pot is near the heatsink in my very small enclosure that it is derating how much power that pot can handle.
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