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Op-amp problem Argent help plz

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dr.power

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Hello guys,

I have got a question about the op-amp circuit I have designed to amplify the output signal of my electeret mike, the supply is +-6V.

I have gor an starnge problem with this circuit, when The 30k gain pot is set at say 1k to 2k The output is very ok but weak as expected. but when I rise the said pot from more than 2K to any other value I hear a noise/distortion at the output. When I add another op-amp satge (inverting one I used) at the input of the below circuit then the said distortion/noise would be gone even if the gain of the added stage happens to be more than 10.

Can you guys please tell me whats the problem?
I myself am suspect to the C111, I say maybe it would not be charged/discharged very well when I incrase the 30k gain pot to more than 2k. I am not sure if I am right, but have no other idea whats the said problem?!

Thanks for any help:eek:
 
If its a single supply then you need to bias pin 3 an 5 of the op amp to 1/2 supply voltage.
 
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First line of OP's story sez +/-6v supply.

An Electret needs DC to keep it's internal Jfet happy.
 
The maximum gain is 423 which is very high. Normal sound levels will cause the second opamp to severely clip the signal and background noises will be amplified.

Maybe your problem is caused by the first opamp oscillating caused by it trying to feed the capacitance of a shielded cable. Add a 100 ohm resistor in series with the cable to stop the oscillation.

C111 (22uF) with R3 (1k) produce a cutoff frequency of 7.3Hz. When the gain is high then low frequency traffic sounds will be amplified and might produce clipping.
 
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check for DC offset at pin 7 of the chip. even though TL072 input bias currents are tiny, across that 100K resistor, they can create a big offset voltage.
 
From your posted circuit, you appear to have a capacitor in series with the -Ve connection instead of what I assume was supposed to be across it.

Or maybe it's just your schematic diagram.
 
Ok thanks,

I solved the problem by lowering the gain..

Now another question is how can I set the input volume of the circuit when connected to the op-amp? I want to put the setup inside a case and want to have access to the input vlume when the setup is connecetd to another device like an mp3 plyer which have a higher input level. how can I do so? just by putting the 30k gain pot outside the setup or should I need a pot voltage dividing at the input of the first stage?

Thanks again
 
Your circuit is a microphone preamp. It is not needed when the input is from an MP3 player.
The input of the power amplifier (that drives a speaker) should have the volume control.
 
Your circuit is a microphone preamp. It is not needed when the input is from an MP3 player.
The input of the power amplifier (that drives a speaker) should have the volume control.

I need it anyway, How can I control the input level of the op-amp externally? where to put the pot plz?
 
you mount the pot through the wall of the case like any other volume control
 
Do you want to use a volume control at the input of your preamp to attenuate an MP3 signal?
Since the gain of the preamp is so high then you must turn down the volume control to almost zero.
 
he said something about using a mic also, i wonder if he's trying to make a karaoke box...... in which case he needs a mixer, and not just a mic preamp...
 
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