LME49740 vs TL074, design considerations and stability - diyAudio
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Old 12th May 2013, 07:38 PM   #1
JanErik is offline JanErik  Finland
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Default LME49740 vs TL074, design considerations and stability

I recently spent an afternoon troubleshooting my sub filter... turns out the LME49740 would need output resistors to be stable, is that correct?
Yes, of course the power supplies are properly decoupled with 150 nF directly on each chips supply pins.

The problematic place seems to be the output stage that feeds the power amp, through shielded signal cables.
I had built the filter with TL072/TL074 on experiment board but the PCB was designed for two quads and I had only one TL074, so I used an LME49740 instead...

Think I burnt one of those sweet chips too :/
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Old 12th May 2013, 09:11 PM   #2
Xoc1 is offline Xoc1  United Kingdom
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The LME497X0 app note shows a 470R resistor on the output of a RIAA Preamp.
I have often used 100R in the past with other op amps, but I am not sure if that is the correct value with this one.
I would definitely fit a resistor on the output pin.
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Old 4th September 2013, 05:00 PM   #3
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hi
I have a crass-over board to used TL074. can I replace whit LM 49740AN?

thanks .
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Old 5th September 2013, 03:58 AM   #4
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Can you please supply a schematic. Burning a chip implies something even worse than stability issues.
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Old 8th September 2013, 11:24 PM   #5
jcx is offline jcx  United States
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swapping a 3-4 MHz GBW op amp with a 50+ MHz GBW op amp is asking for trouble - likely the pcb and decoupling aren't designed for the faster part - especially trying to use the pDIP package at low/unity gains you might see in fliter/crossover circuits

and the higher speed does increase sensitivity to C load, often requiring the added isolating series R - or ferrite bead

also it is often not so good to replace a fet input circuit with a bjt op amp - often the noise will be higher because the impedance of the parts in a circuit designed for a fet input op amp are too high to take advantage of the lower Vn spec of the bjt

Last edited by jcx; 8th September 2013 at 11:27 PM.
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Old 9th September 2013, 05:52 AM   #6
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Tanks Jcx

But I'm too old to cross-over the Jfet Use ...

Is it still not possible?

Thank you.
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