Simple non inverting op-amp circuit to amplify mic sound don't work

Thread Starter

Rusty Rifle

Joined Dec 18, 2019
14
Hi all
Considering this simple circuit
IMG_20211001_080959.jpg
When I connect it as shown the sound became amplified for 2 seconds then it stops. So I have to connect the jack to pin 3 instead of pin 1 ..
how to fix this
Thanks a lot.
 

Beau Schwabe

Joined Nov 7, 2019
109
You need a DC offset cap on the output 1uF should be enough.
Also the 330 Ohm might be too low, you could have clipping on the output.

Most important, you need a reference that is 3V to pin 3. This is easy to do, take two 100k resistors in series to form a voltage divider. Connect the middle to pin 3 and one outside leg to 6V and the other to GND.
 

LowQCab

Joined Nov 6, 2012
1,340
1)
Actually measure the "Bias-Voltage" at the Output of the Mic.
It should be exactly 1/2 of the Supply-Voltage.
The exact Resistance value required to achieve this Voltage is not important.

2)
Do not use an Electrolytic-Capacitor on the Input.
Use a 100nf (0.1uf) Ceramic-Capacitor.
No Capacitor is required if You have a Regulated-Supply-Voltage.

3)
Use a higher quality Op-Amp. such as a LM6142 (~$4.oo).

4) Use 2-stages of amplification, each with much less Gain.
The Gain will be multiplied by the second-stage.

5) 30X-Gain may not be enough Gain.
The amount of Gain that You will need will be determined by
the type of Input on your Amplifier or Mixer.

6)
The Non-Inverting-Input must be "Grounded" a Supply-Point
that is 1/2 of the Supply-Voltage.
.
.
.
 

Thread Starter

Rusty Rifle

Joined Dec 18, 2019
14
You need a DC offset cap on the output 1uF should be enough.
Also the 330 Ohm might be too low, you could have clipping on the output.

Most important, you need a reference that is 3V to pin 3. This is easy to do, take two 100k resistors in series to form a voltage divider. Connect the middle to pin 3 and one outside leg to 6V and the other to GND.
Thanks a lot
But it didn't work

6)
The Non-Inverting-Input must be "Grounded" a Supply-Point
that is 1/2 of the Supply-Voltage.
Thanks a lot
I can't understand this part
 

ericgibbs

Joined Jan 29, 2010
14,183
hi RR,
Look at this marked up version.
E
The 1uF must be added to the end of the 330R

Update:
If you want to improve the low frequency response change the 1uF on the 330R to 10uF.

EG 860.png
 

Attachments

Last edited:

LowQCab

Joined Nov 6, 2012
1,340
Thanks a lot
But it didn't work


Thanks a lot
I can't understand this part
I got that part wrong.
I should have said that both Inputs need to be kept at 1/2-Supply Voltage.

You would probably get better results by using it in an Inverting configuration.

The LM358 Output will not operate all the way to Ground unless You have
an extremely light Load on it,
and you're starting out with a ~10k Load via the Feedback-Resistor.
.
.
.
 

Thread Starter

Rusty Rifle

Joined Dec 18, 2019
14
hi RR,
Look at this marked up version.
E
The 1uF must be added to the end of the 330R

Update:
If you want to improve the low frequency response change the 1uF on the 330R to 10uF.

View attachment 249175
Thanks a lot I will try it♥

Thanks guys for all of you♥
I found this design and it worked perfectly ..
I think It was all about raising the input signal using voltage divider. I don't knowIMG_20211001_102545.jpg
 
Last edited:

Sensacell

Joined Jun 19, 2012
2,917
Your circuit has a 3Volt DC offset at the output. Some inputs won't care, some will.
Best to add another capacitor in series with the output to block the DC.
 

Audioguru again

Joined Oct 21, 2019
3,846
The lousy old LM358 is too noisy (hiss) to be a mic preamp and it produces crossover distortion. Also it has a poor response to high audio frequencies.
Use an audio opamp instead.
What will prevent the microphone from hearing the speakers and producing acoustical feedback howling?
 

Attachments

Thread Starter

Rusty Rifle

Joined Dec 18, 2019
14
The lousy old LM358 is too noisy (hiss) to be a mic preamp and it produces crossover distortion. Also it has a poor response to high audio frequencies.
Use an audio opamp instead.
What will prevent the microphone from hearing the speakers and producing acoustical feedback howling?
I think low pass filter will reduce the high frequency of this self feedback.
 

Beau Schwabe

Joined Nov 7, 2019
109
"What will prevent the microphone from hearing the speakers and producing acoustical feedback howling?" - Usually an adjustable phase shift at the feedback frequency. Keep in mind the frequency will change based on the distance between the speaker and the mic.
 

Audioguru again

Joined Oct 21, 2019
3,846
There will be many feedback frequencies.
A lowpass filter will "turn off" a little shrieking speaker. When the microphone cannot hear the speaker's sounds then you will also not hear the sounds.
Record the sounds and play them back later with the microphone turned off.
 

Audioguru again

Joined Oct 21, 2019
3,846
A lowpass filter cuts highs (produces muffled sounds) and a highpass filter cuts lows (produces tinny sounds) that you do not want. If you use a bandpass filter to reduce one feedback frequency then another frequency will cause feedback because all microphones and speakers have level peaks at a few frequencies.

A complicated and expensive digital feedback elimination circuit makes a model of frequencies and their phases then computes corrections.
 
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