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can you have a look at my eagle board layout - the Guru's mic amp

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The DC output of the opamp has a max output of 1.2V to 7.8V when its supply is 9.0V. The output DC voltage is 4.5V without a signal.
If the opamp's output is capacitor coupled to an ordinary diode that feeds a filter capacitor and a load resistor to ground then the max output to the encoder is only +2.65V.

But since the encoder needs an input of 5V then a DC amplifier must be used. A TL072 dual opamp can be used and its second opamp can be a DC amplifier. Its input must not be less than about +3V so level shifting must be used.
 
Right i see so I will probably have to use an amp of some sort to bring the voltage up.

I have ordered three TL071's so could I used a second one of those?

By the sounds of it it might be easier to use a relay lol

Thanks
 
sooo, its built finally and ive just tryed to test it. ive got 9v on the board, and I put the scope on 500mv and can see some movement in the signal from the mic on pin 3 of the TL071

I appear to be getting 0v out of the opamp on pin 6 even when blowing into the mic.

any ideas what the problem could be or what to look at next?

thanks!
 

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sooo, its built finally and ive just tryed to test it. ive got 9v on the board, and I put the scope on 500mv and can see some movement in the signal from the mic on pin 3 of the TL071

I appear to be getting 0v out of the opamp on pin 6 even when blowing into the mic.

any ideas what the problem could be or what to look at next?
Pin 6 of the opamp should be at half the supply voltage with audio causing it to fluctuate above and below a couple of volts.

Did you correct the polarity of the two electrolytic capacitors that were backwards?
Do you have the opamp plugged in backwards?
 
Doesnt seem to be anything coming out on pin 6 :( the

I think i built the circuit correctly, not sure if you can see from the picture? I followed Bonuk's layout design and as far as i can tell its ok.

when i connect to the output pin, after the 33uf polyester cap, im getting a capacitive voltage, ramping down from about 3v to 0v
 

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The pcb is correct.

Maybe you connected the electret mic backwards. The ground pin also connects to its metal case.
Maybe the load at the output is less than about 2000 ohms.

Is pin 6 of the opamp about +4.5V?
 
The voltages might be too low if the 1k resistor is 10k by mistake.
 
As a Newbie here....I have to say...:

"I have never ever seen the kind of help offered by a Forum" anywhere on the NET.

From helping a person understand the basics to a fully fledged design. And help every step of the way.

Exceptional people.

I like this place.
 
its an amazing place with such helpfull people.

big thanks to the Guru and Bonuk!

I've learnt more from being helped on here than I have in donkeys years of messing around on my own.

Heres a link to a little vid of the op amp (you probably wont apprieciate the music lol) in action with the mic next to the speaker :) **broken link removed**

now on with the next challenge of tripping my encoder :)
 
when i probe on the output pin of the op amp, i get a signal at about 3.1v, after the series 33nf cap i get it a proper ac signal around 0v

would someone care to explain how the cap is doing this and appears to be droping 3v?

thanks :)
 
how does that happen though? Does dc just make it become fully charged and stop passing signal?

so the op amp puts out dc, but in effect the cap is converting it to ac as it goes negative on the discharge cycle?

am i anywhere near correct?

thanks
 
The output of the opamp is a positive DC voltage that is modulated more positive and less positive by the signal.
The capacitor charges to the DC voltage so the average output voltage from the capacitor is 0V which is modulated up (positive) and down (negative).






9negative).
 
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