help in a mic preamp

[wakeke]

**broken link removed**
tried this on a breadboard but got only noise...we're using a 2-lead electret condenser mic 60Hz and the out is a headphone of a computer....not sure about the gain...somewhat from 3~4 gain levels based on simulation...tnx..
what would be the prob?? sometimes can't hear the noise anymore, maybe the connections are loose on the breadboard..not so sure.

 

[wakeke]

sorry..forgot to put the in...the electret mic is connected before the cap (left most part) and the ground...

 

[audioguru]

You have R1 connected to ground so the electret mic has no power. R1 should connect to the positive supply through an RC filter.

R3 and R4 both do the same thing and are not needed since pin 3 should be connected to ground.

The opamp is a very noisy and low bandwidth LM358 instead of a low noise wideband audio opamp.

The opamp is an inverting circuit type that has an input impedance of 1k which is too low. It should be a non-inverting circuit type that has a high input impedance.

All IC circuits need supply bypass capacitors. This one should have two 100uF capacitors.

The preamp will work with a single 9V battery if you bias the opamp at half the supply voltage.

 

[wakeke]

Thank you audioguru...but I'm not so sure if I followed your suggestions correctly..this is the new circuit with a new opamp....tried it in simulation & there's no change in the output signal...input & output signal are equal no matter the value of the pot...I'm not knowledgeable about bypass caps, sorry, so I just put them in my understanding...

Thank you for your reply...

**broken link removed**

 

[Hero999]

Is this supposed to be an inverting or non-inverting amplifier?

The gain will be 1 in it's current configuration.

The inputs have a very high impedance so R5 and R2 can be ignored.

The circuit in your first post would give a gain between 0 and 100, it's correct but the +input (pin 3) should go straight to 0V or the signal will be biased around 4.5V instead of 0V.

 

[flat5]

You don't have the power for the mic correct.
The 10k resistor should go from the + supply to the mic.
Your mic is not getting any power.
You could try a >20k from +vcc to the 10k to the mic.
Bypass the junction of the two resistors with C3 (to ground).

 

[wakeke]

@Hero999...originally it's an inverting amp..yes, the gain became 1 using the second circuit...
@ flat5....you're referring to the first circuit or the 2nd?...
I'm going to try your suggestions(on the first circuit)...thanks to you both..will be posting soon...

 

[wakeke]

The 1st circuit: **broken link removed**

basing from the 1st circuit:
**broken link removed**

changed the opamp to NE5532 (it's said that it's better and LM358 is noisier as said by audioguru)
changed the value of C1 to 100uF and added the 47kohm from a + supply (upper part)

from simulation:
the Vin (assumed signal that would be coming from the electret mic): 1Vpp 60Hz
the Vout (output signals): 50% pot = 50.354V, 100% = approx. 100V

the gain is about 100 ryt??

we're not sure abt the change of values...like C1, etc....are these all right? there was a big change in output when I tried this...(we're actually trying an alt. based from the 1st after the 1st circuit was posted w/ NE5532,Vcc=24V, R1=5M R2=100, R3=R4=10k, R5=1M, C2=22uF; these were made just to attain high gain)

But this latest circuit posted would be alright right??
Thanks very much for your reply...

 

[audioguru]

You don't seem to know anything about a simple opamp circuit.
An electret mic makes an output of 1V p-p if it has an explosion beside it. A voice makes about 0.02V p-p. That is why a preamp is used to amplify it to about 2V p-p with a gain of 2V/0.02V= 100.

You saved your schematic as a fuzzy JPEG file type instead of as a very clear GIF or PNG file type so I couldn't remove the fuzz.
This is a non-inverting electret mic preamp circuit:

 

[flat5]

You almost have the mic correct. You still have it connected to ground through the 10k resistor! Connect it to +9vdc, like pin 6 of the op amp.

Why the 10k resistor on the + input? Connect pin 3 to ground.
If you need to add a resistor to correct offset, try 100k.
If you don't understand what I'm talking about, try Google.
I'll let the experts make the other comments
I just see the simple stuff.

Oh, and don't expect 1 volt output from the mic. Scream into it and you might get 0.05 volts.

 

[elnino86]

hI Wakake,im suggesting you using MAX4465 or TS971,they got better performance on this.

Im not designing they yet,maybe a few weeks from now.

 

[audioguru]

hI Wakake,im suggesting you using MAX4465 or TS971,they got better performance on this.

The Maxim IC is tiny (for a hearing aid), is very noisy, has high distortion and a bandwidth to only 1kHz when its gain is 200.
The ST Micro IC is a good mic preamp opamp like many others.

 

[wakeke]

I'm very sorry....we're just learning opamps...but thank you all very much for your help...
@elnino86: will look into it, not sure if we got those in our area...

 

[elnino86]

The Maxim IC is tiny (for a hearing aid), is very noisy, has high distortion and a bandwidth to only 1kHz when its gain is 200.
The ST Micro IC is a good mic preamp opamp like many others.

Thank for the info Audioguru.Will check on this.

 

[Hero999]

You saved your schematic as a fuzzy JPEG file type instead of as a very clear GIF or PNG file type so I couldn't remove the fuzz.
This is a non-inverting electret mic preamp circuit:

The JPG looks pretty clear to me (JPGs aren't always fuzzy, you can use 100% quality which is fuzz-free but they take up more space), perhaps your new implant is failing.

 

[audioguru]

I'm very sorry....we're just learning opamps...

Do you believe an opamp that has a dual 9V supply will have an output of 100V p-p??
Do you see that your inverting opamp has an input impedance of only 100 ohms or 1k ohms which shorts the signal from the mic, and that my non-inverting opamp has an input impedance of 100k ohms and the mic works perfectly??

The NE5532 will work perfectly with a single 9V battery. Do you know how to bias the opamp in my circuit so it works with a single 9V battery??

 

[wakeke]

The NE5532 will work perfectly with a single 9V battery. Do you know how to bias the opamp in my circuit so it works with a single 9V battery??

no sir, don't know how to bias it to work with a single 9V battery...
but the opamp can work fine even if the supply is not a single 9V batt., right?
what's the difference of the two?...thnx!

 

[Nigel Goodwin]

no sir, don't know how to bias it to work with a single 9V battery...
but the opamp can work fine even if the supply is not a single 9V batt., right?
what's the difference of the two?...thnx!

Look at the sticky at the top of the General Electronics Chat forum.

https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

 

[wakeke]

Look at the sticky at the top of the General Electronics Chat forum.

https://www.electro-tech-online.com/threads/basic-opamp-circuits.35258/

Thank you very much

 

[wakeke]

I'm a little confused on the supply voltages (+-) connected on the electrolytic caps (on the left side)..or are they the one being supplied to the mic from the positive supply? ..and red colored voltages are the resulting biases when the high supply is applied?...