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Opamp compatibility
Opamp compatibility
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Old 15th May 2009, 06:07 PM   #1
morfic is offline morfic  United States
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Default Opamp compatibility

If a headphone amp is built around this opamp, what could be the reason to say that this opamp is not going to work? (other than wanting to keep support possibilities down?)
What particular trait of the original MC33078P would require a compensation that would make drop in replacements difficult?

I am used to swapping pin compatible opamp in my portable amp w/o problem and the manufacturer even states most any pin compatible unity gain stable opamp should work, and they did.
From all the opamps i tried, the LME49710 was the most all around enjoyable (and i tried it after reading about the opamp in a thread here), so naturally i would like to try the LME49720 (not sure why a dual channel opamp is used per channel) in this desktop amp, but since i see no reason why they shouldn't be swapped out for each other am a little hesitant to find out the hard way why not

Thanks if anyone has the time and fun to give me some feedback on this,

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Old 15th May 2009, 07:00 PM   #2
satoru is offline satoru  United States
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There are many things where an OPAMP cannot be a direct replacement of an other.
Usually, a good designer will take care of a lot of them. I don't see major reason why LME49720 won't work in a typical OPAMP circuit with usual precautions.

For MC33078 and LME49720, the major difference would be load capacitance:

MC33078: 22° phase margin with 300 pF
LME49720: <100pF, no phase change, needs series R for >100pF

Watch for the load capacitance. Both OPAMPs may not be forgiving for capacitive loads larger than 100pF. A series resistor at the output can take care most of such situation. Insert a low pass filter at the input of LME49720 if there isn't one.

Good luck!

PS: I'm still studying, too. Please correct me for anything wrong here!
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Old 15th May 2009, 09:35 PM   #3
mjf is online now mjf  Austria
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what is the impedance of your headphones?
opamps like mc33078,ne5532.........etc.
can drive 600 ohm loads.......so the impedance should not be much lower than 600 ohm.
in my experience that opamps are only a substitute for a real headphoneamp.
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Old 15th May 2009, 09:54 PM   #4
morfic is offline morfic  United States
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thanks, i guess my real question should be if the capacitance is properly done for the MC33078, what would that very same circuit do for the LME49720?

The amp i talk about is the Little Dot MkV. Was it not a commercial product, i would probably know a little more about it.

The opamps are only part of the whole amp, it's not a opamp and nothing else type circuitry, it should drive any phone from 32Ohm to 600Ohm.

My question is really if a pin compatible chip with similar (but not identical) specs is used, should i expect at worst undesirable unenjoyable results, or could a circut actually be optimized in such a way that the slightly different opamp could cause damage to the circuit or phone attached?

I am expecting the former (undesirable at worst), but with the cost of both the amp and the headphone i would hate to realize it is the latter.

I would much rather abandon the amp and use the funds from the sale to build a PPAV2 than to own a paperweight

Thanks for your replys,

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