Hi all. I have a question about the following circuit:

I have a mic, 1 lead going to ground and the other going to a potential devider circuit to bias the mic output at a set voltage (in this case +/-6V). This feeds into a Non inerting op amp to amplifiy this voltage. The output from the op amp goes gets filtered by an RC bandpass filter. I now need to rectify this output and use a ripple capacitor to try and make a vary voltage dependant on the amplitude of the mics input. My question is can I use a 4 Diode full wave rectifier to rectify the wave?

Thanks, Phil.

 

I think we're going to need a bit more understanding of what you're trying to achieve here and the reason for the AC bias.

 

An electret mic sets its own voltage so a voltage divider is not used, just a single resistor supplies the current it needs, usually 0.5mA.
An active Precision Rectifier circuit using an opamp (the preamp opamp can be the active Precision Rectifier circuit) is used.

 

My question is can I use a 4 Diode full wave rectifier to rectify the wave?

Maybe, but don"t forget you"ll lose about 0.7 volts across each diode and thus about 1.4 volts off your signal. Unless the op amp delivers a lot of voltage gain, you might simply lose your signal altogether. You can cut the loss in half using a half wave rectifier, and a bit more using specialized schotky diodes, but it's still going to be of the same magnitude.

 

An electret mic sets its own voltage so a voltage divider is not used, just a single resistor supplies the current it needs, usually 0.5mA.
An active Precision Rectifier circuit using an opamp (the preamp opamp can be the active Precision Rectifier circuit) is used.

Still don't get it

1) Electret mikes use a DC voltage to excite them

2) Any audio is going to sound like brapppp! if you run it through diodes

 

An electret mic has a Jfet impedance converter inside that uses a few volts at 0.5mA.
I think the diodes are used to convert the audio to a DC voltage corresponding to the loudness.

 

http://www.uni-bonn.de/~uzs159/diode_tutorial/index.html

Using the opamp with the diodes can do away with the diode voltage drop problem(mostly) and give you much better response to millivolt level audio signals. Check into it, sounds like the perfect way to rectify microphone audio.

 

This might explain the rectifier circuit a bit better:
http://sound.westhost.com/appnotes/an001.htm

 

Ok, i have jumped the gun a little and am now struggling to even set up a basic op amp circuit. Any ideas why the following circuit doesnt work, it does nothing at all. Thanks, Phil

 

Without more information- the number of reasons that might cause the circuit not to work are almost infinite.

More details please. What voltages do you see at the various pins(AC and DC). Do you have an O'scope? That will be the only easy way to troubleshoot the small audio signal you are using as an input.

You have the inverting input DC biased at 6 volts. What is the DC bias at the non inverting input? You don't have any buffering between the opamp and the microphone. Is that how you want it?

<More info please>

 

I do have access to a scope yes, I used this to calculate that the mic is outputing about 30mV peek to peek. I would just like to build a simple op amp circuit to amplify this voltage so it can be heard through an 8Ohm speaker, or headphones. I would replace the Rf resister with a pot for variable gain once the basics were up and running. The potential devider is in place to use 6v as virtual ground, giving the op-amp a dual rail of +6v/-6v. I plan to also use the ouput to light 7leds to indicate the amplitude, but this is once its up and running. Thanks, Phil

Ps im afriad I dont know the impedence of the mic, its a 'standard' kareoke mic.

Also yes I could use a buffer, if need be, do you think this is the case?

I didnt have time to test the output on the scope, which would have been the good idea, however I could not here anything through the speaker or headphones, even when tapping the mic. Thanks.

 

Without some dc readings on the chip the possiblities still remain large. Maybe the opamp isn't actually on? You didn't say what voltages you saw there. etc etc etc.

We need something to work from. either schematics and pictures or lots of details. Usually when you go back and get those things, you find the problem is something silly, like forget to connect ground to the chip, or a wire has come loose somewhere.

 

Why bother using dual supply op amps when single supply chips are so plentiful? Your circuit doesn't seem to indicate any real reason for needing to have a negative output and the coupling caps involved in an audio amp generally take care of that anyway.

Oh, and simple op-amps don't have much output driving capabilities, you're probably going to need at least one buffer transistor for the output stage.

 

Without some dc readings on the chip the possiblities still remain large. Maybe the opamp isn't actually on? You didn't say what voltages you saw there. etc etc etc.

I double checked that the chip had power. Ill get some reading tonight when I get home.

We need something to work from. either schematics and pictures or lots of details. Usually when you go back and get those things, you find the problem is something silly, like forget to connect ground to the chip, or a wire has come loose somewhere.

I appriciate that, and am very gratefull for any help.

I thaught I would need to have dual rail as the mic will be outputing + and -, correct? with just having the positive part of the wave will this not sound bad?

Cheers, Phil.

 

You connected it wrong and missed many parts.

 

The circuit is a mic preamp and can feed a power amp through a coupling capacitor. Then the power amp can drive a speaker or headphones.
If the mic is a dynamic type (coil and magnet) then the 10k resistor biasing the electret mic is not needed.
An additional precision rectifier circuit can give a DC voltage depending on the sound level.

 

Thank you. Would you care to tell me what the cpas and resistors are doing in the circuit, as I have never used them like that when I was taught about op amps.

Thank you very much.

Phil.

 

I knew you would ask about my corrections.
The functions of the resistors and capacitors should be obvious.

 

Haha, am I that predictable? Is it ok to adjust the Rf resister to increase/decrease the gain without adjusting any of the other values?

Would a push-pull mosfet set up be ok to amplify the power to connect to a speaker?

Thanks, Phil.

 

Is it ok to adjust the Rf resister to increase/decrease the gain without adjusting any of the other values?

Yes, but its value must not be too low because the output of the opamp cannot drive less than 2k ohms.

Would a push-pull mosfet set up be ok to amplify the power to connect to a speaker?

Most audio power amplifiers operate the outputs as followers. It is easy to bias transistors as followers because the bases need to be only about 0.7V higher than the emitters for the transistors to be turned on.

But Mosfets need the gates to be 5V to 10V higher than the sources so they need a much higher supply voltage that creates a lot of heat.