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Old 5th September 2014, 02:30 PM   #1
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Default OP-amp getting hot

I've build a heed canamp clone with +/-16.8V power supply. The OP-amp getting very hot when working and heatsink is needed.
I've try LM4562, LME49720HA, NE5532P, and each op-amp gets hot.
Is +/-16.8V too high for those op-amp? or there is another problem?
I've try OPA2107, OPA2132 and they didn't work, instead, producing bad noise.
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Old 5th September 2014, 03:01 PM   #2
Dfhaii is offline Dfhaii  United Kingdom
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Of those I only have experience with the NE5532. Those rails are within limits for it. They do run warm, but not very hot usually, have you checked whether it's oscillating, that's a good way to heat them up.
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Old 5th September 2014, 03:02 PM   #3
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Verify that your wiring is correct.
If it is and they are getting hot they are either bad or they are oscillating.

All of the types you posted are good up to +/- 18v with the exception of the NE5532 as it is good for up to +/- 22v.

Don't plug the opamp's in while your circuit is powered up as this can cause them to blow.
Make sure also that your power supply's filter caps are discharged to 0v as well before you swap out the IC's.

FWIW

jer
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Old 5th September 2014, 03:11 PM   #4
agdr is offline agdr  United States
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Quote:
Originally Posted by moses1202 View Post
The OP-amp getting very hot when working and heatsink is needed..
What is the impedance and sensitivity of the headhones you are powering (or the brand and model number)?
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Old 5th September 2014, 03:14 PM   #5
JMFahey is offline JMFahey  Argentina
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Do they still overheat if you add, say, 22 to 47 ohms in series with phones?

I suspect too low impedance/oscillation/both .
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Old 5th September 2014, 03:49 PM   #6
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I have no idea how to check the "oscillation" problem. The amp just works well and sounds good but I usually swap the OP-amp without unplug and discharging the power. My headphone impedance is around 32-40 ohms and sensitivity is about 116 db (but does it matter?).
This is the picture:
Click the image to open in full size.

Last edited by moses1202; 5th September 2014 at 03:52 PM.
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Old 5th September 2014, 04:13 PM   #7
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Off topic but I haven't seen that op amp package for decades. I did see a couple listed at Mouser and they were expensive. Anyway, you can still get finned clip on heat sinks for that package.

Do you have a zobel on the output? If the resistor is hot, or even warm, then it is oscillating. What is your quiescent current? Does the chip get hot just idling?

It is a good idea to lower the voltage when using op amps to drive low impedance cans. +/- 10 volts is plenty. You could get away with +/- 7 volts.
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Old 5th September 2014, 08:26 PM   #8
agdr is offline agdr  United States
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Quote:
Originally Posted by moses1202 View Post
My headphone impedance is around 32-40 ohms and sensitivity is about 116 db (but does it matter?).
32R at 116dB/mW would need just 0.02Vrms and 0.5mA to hit 95dB SPL. Those are pretty sensitive headphones. With headphones that sensitive you probably don't need a headphone amplifier, even with a cell phone or ipod.

With 16.8V rails you would be left with 16.8V - 0.02Vrms = 16.78V across the chip, or 16.78 * 0.0005 = 8mW per channel of chip power dissipation, plus the quiescent current dissipation which is around (16.8Vdc * 2 rails) * 8mA = 268mW per channel, or 536mW for both channels if it is a dual op amp, and if my quiescent current guess of 8mA per channel is somewhere in the ballpark.

So your load is producing only a tiny amount of dissipation in the chip due to the very sensitive headphones. But with 16.8V rails your quiescent (idle) current in the chip is taking up nearly all of the 550mW available or so with that TO-99 package. The LM158 data sheet here http://www.ti.com/lit/ds/symlink/lm158-n.pdf lists that 550mW number for the TO-99.

Fast Eddie D has your solution. You don't need such high voltage rails for your sensitive headphones. Cut your power rail voltage in half and the chip should cool right down.

Last edited by agdr; 5th September 2014 at 08:47 PM.
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Old 6th September 2014, 01:52 AM   #9
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Thanks for reply. I'll try to drop down the voltage by put some linear step-down IC (maybe).
Though it seems don't need to have an amp for my headphone but those amps give different taste for listening music. Some amp draws the female voice close to you while some other gives you a wide-range of stage.
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Old 6th September 2014, 02:13 AM   #10
davada is offline davada  Canada
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Quote:
Originally Posted by moses1202 View Post
I have no idea how to check the "oscillation" problem. The amp just works well and sounds good but I usually swap the OP-amp without unplug and discharging the power. My headphone impedance is around 32-40 ohms and sensitivity is about 116 db (but does it matter?).
This is the picture:
Click the image to open in full size.
How much of that load is the op amp output seeing? Those op amps are rated for minimum load of 600 ohms. 32 - 40 ohm will definitely heat them up and probably cause oscillation.
Of course I'm not at all familiar with the circuit used. can you post it.
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