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Subject: very basic opamp amplifier

Date: 06/23/02 at 3:14 AM
Posted by: Corry Lazarowitz
E-mail: corry@unt.edu
Message Posted:

I just can't stay away from opamps it seems....
Anyways, what I am eventually trying to do (before next weekend) is create several mixers in a way that several microphones in a room will pick up a conversation, mix it togather, then be mixed with either a cell phone hands free speaker and sent to a computer (for an internet chat program) or mixed with a computers line out and sent to the cell phone mic. Also, all microphones will be mixed and sent to a tape recorder, and the cell phone speaker and the computer line out will be mixed and sent to amplified speakers (probably the computer lien in) Should just be a bunch of small mixers, no big deal or so I thought. I am generally cautious so I decided to test the basic circuit first. As the saying goes, a picture is worth a thousand words. So go to http://www.geocities.com/clazarowitz/mixer.html or I'll try and make it a link click here. If that just made a mess I'm sorry, It doesn't say if html code is possible here :)
So the first picture is a mock up of what I did. The 6.8K ohm resistor (I think that was the final value anyways) is to knock the 12VDC line down to about 2 volts to power the mic. Other than not having some caps, which to the best of my knowledge would just possibly cause some noise, whats wrong? I get nothing out of the opamp in the configuration! In fact they only way I get anything out of it is to short the inverting pin to the output pin. I get some good clean amplification, but I still need more :) Using a set of "Spin 90" amplified speakers with the volume all the way up, it is a little bit louder than the original sound from the source speakers. I included a blank picture there if you would like to draw in what I should be doing. Dont worry about the power input, I am running an old AT computer power supply into a 12VDC reg, with 2 capacitors to ground on both the input and the output, so that should be sending any power supply noise to ground. Before I did that I got some horrendous noise I assume was from the power supply's fan. If you do edit that picture send it back to me at corry@unt.edu thanks for the help in advance :)

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