Microphone To Speaker Schematic

Thread Starter

crazyengineer

Joined Dec 29, 2010
156
Hello! One project I want to work on is to simply transmit sound from a microphone to a speaker. I know I have to add decoupling capacitors at the VCC pins of both Op amps. The microphone is modeled as vac in the schematic.Also the speaker is modeled as a 8 ohm resistor at end of the right op amp.



I just want to know whether there are a coupling things I should fix in this schematic? Can anyone recommend a good audio op amp to use? I have zero intention of using the LF324 that's in this schematic.
 

crutschow

Joined Mar 14, 2008
23,144
Your output amp won't work since there is no dc bias path for pin 5. Move C3 from its location and connect it between pin 7 and R6.

The resistors loading the mic seem rather low. The equivalent impedance the mic sees is only 1/3 of 1k or 333 ohms. I suspect that will significantly reduce the signal voltage from the mic.

Audioguru has some suggestions for good audio op amps but I don't recall what they are. If you search for some of his posts, you should find it.
 

Thread Starter

crazyengineer

Joined Dec 29, 2010
156
Your output amp won't work since there is no dc bias path for pin 5. Move C3 from its location and connect it between pin 7 and R6.

The resistors loading the mic seem rather low. The equivalent impedance the mic sees is only 1/3 of 1k or 333 ohms. I suspect that will significantly reduce the signal voltage from the mic.

Audioguru has some suggestions for good audio op amps but I don't recall what they are. If you search for some of his posts, you should find it.
Do you know an equation that will tell me the signal voltage?Do you think having a 10k resistor for the microphone will be fine?
 

Audioguru

Joined Dec 20, 2007
11,251
1) An electret mic uses only 0.5mA. Then R5 should be 18k (18 times higher) and R2 and R3 should be 180k each (180 times higher).
2) A lousy old LM324 is NEVER used as a mic preamp because its noise (hiss) level is too high, it has class-B crossover distortion and it performs poorly above only 2kHz. Most people can hear to 20kHz except gun shooters and acid rock "musicians".
3) I agree that the pin 5 input has no bias nor reference voltage then the opamp will not work.
4) An opamp does not have enough output current to drive a speaker. Use a power amp IC instead.
 
Last edited:

Audioguru

Joined Dec 20, 2007
11,251
Do you understand the problems and solutions I discussed with your defective design?
 
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Thread Starter

crazyengineer

Joined Dec 29, 2010
156
I am sorry that you posted an extremely simple circuit that has a few serious problems. Most come from little kids in India. Only little kids in India still use the lousy old LM324 quad opamp.
I am sorry that you are not a little kid in India (then why did you use the lousy ... ?) and that you are using Orcad that almost nobody uses anymore.

Do you understand the problems and solutions I discussed with your defective design?
Yes. I understand completely. I just need to find a power op amplifier to use.
 
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