Hi
I have TL084a and there 4 amp in the case , I and use only 2 of them , what should I do with the inputs i dont use ?
Thanks

 

Ground the unused inputs (and outputs).

*CORRECTION* grounding outputs is a very bad idea as pointed out in following posts.

 

and outputs too????
I dont use bipolar power supply.
And you mean , short -unput and+ input and put them on -24vdc???

 

Bipolar Supply:
+V
GND (0V) <---connect unused inputs and outputs to here
-V

Single Supply:
+V
GND (0V) <---connect unused inputs and outputs to here

It doesn't matter if you don't use bipolar supply- You still have a GND at 0V in both cases. Short the + input, -input AND the op-amp output and connect all of them to ground. You also ground the output so it does not stay floating voltage and act like an antennae and pick up noise (that is the reason you ground the unused inputs. Unused outputs can do the same thing so you must also ground them.)

*CORRECTION* grounding outputs is a very bad idea as pointed out in following posts.

 

okey ,thanks

 

dont GND the output's
GND the inputs yes but not the output.

you never tie the outputs to GND, via a resistor maybe but even then it is bests to leave them floating


In fact the propper way to tie out unused OPAMPS is to wire it up as a buffer and tie the +VE to GND and leave the output floating

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Nothing is impossible.
Once a problem is realised, the rest is just details

 

Yeah, I wasnt sure about outputs ,
I did like you said , left them as buffers and I put +input on half Power supply .
Thanks to all

 

Just went through this myself, found it on the web... I was using LM324. Anyway, several webs give this as the best way:

Tie inverting (-) input to output. Ground the noninverting (+) input.

Search 'unused opamp' Works. My RGB LED Fader, based on a low frequency triangle, has been running 2 full days now, no heat or other problems.

 

Oh yeah, my bad about the output. Sorry. Forgot that it would cause some kind of short. I was assuming that you couldn't disable individual op-amps since its a quad-op-amp, but powering it down is best if possible (ground +V like styx said) And then wiring the inputs and outputs to be a buffer to deal with the floating output.

Makes sense.

 

NEVER connect the output of an opamp to ground! It will get very hot.
Connect the non-inverting input (+) to ground, then connect the inverting input (-) to the output so the opamp is a follower with ground as its input.

Attached Images

unused opamp.PNG (2.3 KB, 5 views)

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Uncle $crooge

 

Thank you , guys for your help!
I almost put all staf together and now thinking if I can put 600 -600 trans as a load for the buffers (TL084) just for the line level .

 

Why use an output transformer? It reduces frequency response and adds distortion.

A TL084 and most other opamps can't drive a load as low as 600 ohms. The minimum load on the datasheet is 2k ohms.
Dual NE5532 opamps can drive a 600 ohm load.

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Uncle $crooge

 

I need balanced output , and I dont need a wide frequency response, its just for voice system .
The signal for the buffer only -20 db , I think its Ok , what you think? ( I mean
600 -600 trans as a load before power amp)

 

-20dB is low so the opamp won't have trouble driving a 600 ohm load to only -20dB.

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Uncle $crooge

 

thanks,
i will see how it works