Hi,

I am working on a project where I am designing an audio amplifier. The amplifier I decided to use uses differential inputs. (a IN+ and IN- signal)

Since I need differential inputs, I decided to easiest way was to use an inverting amplifier to get the IN- signal. Everything seems to work great, my audio is getting amplified, and sound is good. However, there a problem. My inverting amplifier is outputing a high pitched noise. I've narrowed it down to my inverting amplifier. I removed my inverting amplifier, and just powered on the audio amplifier, and the high pitched noise is not there. I put back the inverting amplifier, and the noise is there. I can tell you the IN+ signal does not have the noise. Only the IN- signal. Attached is a schematic of my inverting amplifier, and my audio amplifer I am using. Any help would be appreciated, thanks.

The V+ for the LM358 is +5V.

 

If you have O'scope then you can measuring the noise where did it started.
The direction as:
pin 6 of U3-b → two sides of C79 → Pin 1 of U3-a → Pin 3 op U3-a
or
Pin 3 op U3-a ← Pin 1 of U3-a ← two sides of C79 ← pin 6 of U3-b

Or you may using the mV meter and in series with 0.1uf to measure it.

 

hi,
It maybe a 'noisy' AVCC voltage.? I would suggest you decouple the 10K divider at the non inverting input to 0V, say a 100nF

E

 

hi,
It maybe a 'noisy' AVCC voltage.? I would suggest you decouple the 10K divider at the non inverting input to 0V, say a 100nF

E

Thanks for the suggestion. Adding a cap eliminated the noise problem.