fvnktion

Hello,

Im trying to build a functional circuit for mixing 2 audio signals with a single supply opamp @ 3.7V. Ive gotten a simple working circuit for dual supply, but am not able to get a single supply circuit working? I only get popping sound at the output of the opamp.

dual supply testing
-I have tested the mixing circuit with a dual supply lm348 +-5V unity gain no problems.

single supply testing
-I have purchased a single supply opamp TLC084 and tested it with a wave generator up to 20khz no problems @ 200mvpp and unity gain no problems

-When i use the same circuit and instead of inputing the wave i input the audio signal ~200mvpp. The audio is only amplified in for very small periods and sounds noisy and poppy from the speaker.



questions
1 - I do not know why the audio is signal is not continuosly being amplified as the wave generator worked great through 20 khz??

2 - Is there some incompatibilty with this single supply opamp tlc084 that i am using, would there be a better option for my application?

Thanks for the help, trying to get a fix on all this fun analog stuff

audioguru

You forgot to attach your schematic.
The minimum supply voltage for a TLC084 is 4.5V. Yours is only 3.7V.
Its inputs must be biased properly at about half the supply voltage when it has a single supply.

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Uncle $crooge

fvnktion

Thanks for the reply Audio.

Attached is a simplistic setup of the opamp with only one input.

Im currently using 5 volts to try to get the opamp functioning.

Can you explain how i might do this to get it working correctly? It sounds like the input signal is too close to the -Rail or gnd?? Will i need to add a dc level to the signal?

Thanks

Attached Images

audio circuit.jpg (112.9 KB, 26 views)

audioguru

Opamps don't work if their inputs are at the negative supply voltage which is ground in your circuit. The inputs must be biased at about half the supply voltage like this:

Attached Images

single-supply opamp.PNG (8.7 KB, 25 views)

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Uncle $crooge

fvnktion

Thanks for the help! I think im going back to the books on opamps as there is so much i dont know and cant remember!

In case anyone is interested i found another great resource for single supply op amp design by TI.

Attached Files

single supply designs.pdf (162.2 KB, 13 views)

audioguru

I don't believe it. Texas Instruments show a single supply non-inverting opamp without any input bias voltage.

Attached Images

single supply opamp.PNG (10.7 KB, 21 views)

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Uncle $crooge

Nigel Goodwin

Datasheets and application notes (from ANY manufacturer) are well known for mistakes, and they never seem to correct them?.

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http://www.winpicprog.co.uk

Hero999

The value of the biasing resistor should also be equivalent to R1 and R2 in parallel.

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