If its a single supply then you need to bias pin 3 an 5 of the op amp to 1/2 supply voltage.
Op-amp problem Argent help plz
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20th April 2012 10:47 AM #1dr.power
Op-amp problem Argent help plz
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
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20th April 2012 10:59 AM #2dr pepper
0
Last edited by dr pepper; 20th April 2012 at 11:00 AM.
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20th April 2012 12:30 PM #3Jaguarjoe
First line of OP's story sez +/-6v supply.
An Electret needs DC to keep it's internal Jfet happy.0Recursion [ri kur shon] n. See recursion
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20th April 2012 02:08 PM #4audioguru
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.+1Last edited by audioguru; 20th April 2012 at 02:09 PM.
Uncle $crooge
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20th April 2012 03:37 PM #5unclejed613
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.
+1.................................................. .................................................. .
E=mc˛ (+/-1dB)
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20th April 2012 04:38 PM #6WTP Pepper
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.0
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21st April 2012 03:26 PM Thread Starter #7dr.power
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 again0
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21st April 2012 03:36 PM #8audioguru
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.+1Uncle $crooge
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21st April 2012 05:03 PM Thread Starter #9dr.power
I need it anyway, How can I control the input level of the op-amp externally? where to put the pot plz?
Originally Posted by audioguru
0
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21st April 2012 06:07 PM #10unclejed613
you mount the pot through the wall of the case like any other volume control
0.................................................. .................................................. .
E=mc˛ (+/-1dB)
"If a curl of smoke is seen, the technician will soon experience the cause of difficulty"........ CTX arcade game monitor service manual 1985
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