Electronics > Beginners

Positive Feedback in Amp?

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eev_carl:
Hi,

I'm building a bass guitar preamp and am starting with the input and output stages.  I have a pair of op amps on a single 1455 IC, both unity-gain followers.  The circuit is powered by a split power supply.  There are 0.1 caps on +V to GND and -V and GND.

After about 90 seconds of playing or not playing, a repeating sound gets increasingly loader.  You can hear it in the attached link to a WAV file.  I'm on US power.

https://www.bekwam.net/audio/simple_audio_buffer_noise.wav

I was wondering if someone could help me identify the problem.  I killed the lights and extra gear besides the power supply and amp.

Thanks,
Carl


Benta:
Where's your DC bias? The first amp is floating completely.

schmitt trigger:
The circuit is no preamp. With the feedback as shown, you have two unity gain stages in series.

In addition to the lack of bias.

eev_carl:

--- Quote from: schmitt trigger on September 15, 2018, 06:09:52 pm ---The circuit is no preamp.

--- End quote ---

I'm going to add a gain stage, a clipping stage, and a tone control stage.  I figured I'd start with the input and output.

eev_carl:
For the first stage, should I put a resistor to GND.  If so, what's a good value?  I was confused because most of the schematics I read that had a bias were single supply and needed to account for the negative input.

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