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PostPosted: 19 Mar 2018, 03:13 
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Joined: 19 Mar 2018, 02:57
Posts: 2
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I made a simple voltage follower on a breadboard with an audio op amp (LME49726) and I'm getting so much noise that the audio signal is barely intelligible at the output.
This is the datasheet of the op amp: 
http://www.ti.com/lit/ds/symlink/lme49726.pdf
Have a further understanding of op amp:
http://www.apogeeweb.net/article/60.html
The positive rail is at +3V and negative rail is Gnd. The audio signal is connected to the same Gnd as the op amp. I checked the connections and I'm not sure how else to troubleshoot because it's such a simple circuit...any suggestions?
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I went to one of my classmates for help, he thought i might need a breadboard, an op-amp cookbook, an O'scope, and some time working with real world signals.
For example, the circuit below isn't a mixer, but it demonstrates several concepts:
It uses rail-to-rail, 3V op-amps.
It uses a unipolar power supply, (0/3V, not +/- X volts).
It uses a 1.5 V Virtual Ground.
It has two "AC Coupling" stages, that block any DC offset and keep the signal centered at 1.5 V, ("Ground").
It includes several gain stages and filter stages.
......
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This image was something I found on a search...
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My ultimate circuit is a summing amplifier to mix different signals. I applied a VCC/2 bias at the positive op amp terminal and it works! The audio quality isn't as good as it could be though...how do you choose the input capacitor value? I am setting all resistors to the same value of 10k so it just adds the signals without any gain. What else can cause noise/distortion?
I am biasing the positive op amp terminal at VCC/2. Here's my quick sketch of the way I have it connected now.
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Not as sophisticated as his circuit lol...is this design too simplified? The first stage is a summing amplifier, which has an inverted output so I thought a second inverting stage would be needed to de-invert the signal...but it doesn't seem to make a difference in the sound! Why is that? I'm not sure if the second op amp is necessary.
Also, I don't want the signals V1 and V2 to be affected at all, so it's unity gain. Should the input signals also need to be biased at VCC/2 instead of being connected to Gnd? 
How do you choose the capacitor value? The 0.1 uF caps seem to be working fine...
Oh yeah by the way, the audio sounded better when I connected Gnd of the audio signal to VCC/2...
The peak to peak voltages fluctuate but the maximum of the first signal is about 1V peak-peak and the second signal maximum is about 200 mV peak-peak:
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How does it help to have the virtual ground be created by an op amp instead of biasing the positive terminal of the first op amp?
That's interesting that you can't hear the difference between an audio signal and its 180 inverted form!
Sorry for so many questions, thanks for your time.


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PostPosted: 27 Apr 2018, 00:21 
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Joined: 23 Feb 2017, 02:02
Posts: 243
Noise would be caused by either via virtual ground, poor ground wirering, poor power supply.
If any signal path is close to a switch mode or a transofmer it will also pick up noise.

Inverited signal would not cause noise


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PostPosted: 04 May 2018, 18:20 
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Joined: 26 Dec 2016, 03:46
Posts: 178
Location: Bayarea
Without looking deeply, you do have a lot of gain in the circuit, gain is 100K/(1.5K + 200) = 58.8. So any noise from the output of the ADMP401 will be amplified.

Yes, I would use non inverting opamp configuration. When you use inverting configuration like in your circuit, your input impedance seen by the mic is only 1.5K. Using non inverting configuration will give higher input impedance ( depending on your input termination resistor) and less loading of the mic. BUT that has nothing to do with your noise.

You have to make sure your layout is not picking up noise. Make sure the wiring on the -ve input of the opamp is as short as possible. It's easy to draw a schematic, building it is another story.


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