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Topic: How to make a pre-amp for a electret? microphone (Read 17381 times) previous topic - next topic

nickgammon

As instructed:

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BTW, did you measure the o'scope noise by tying the probe gnd to the probe tip?


I looped the ground around to the tip.

Interestingly, if I connect the probe tip to the Ground on the scope, I get more noise, not less:


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oric_dan

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This is the reading with the probe tip grounded:

You can see that that's basically the majority of noise you see superimposed on the electret signal. If you touch the probe gnd to the tip, you have a tiny pickup loop. If you touch the tip to the scope gnd, it's a bigger loop.

oric_dan

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Not meant to be a final circuit, just to illustrate that the circuit acts as a precision rectifier. As mentioned, with that ridiculous gain the frequency response is horrible.

Yeah, that's basically how the ckt works, but I'm not sure it's such a good idea to DC-bias an opAmp right to gnd, there might be some nasty nonlinearities as the feedback loop goes in and out of operation. Plus you might end up with serious clipping of the tiny signal being amplified. I should think even a rectifier app should have the DC-bias set close to gnd, but slightly above, so the opAmp stays in operation.

I was trying to get Nick to understand a little better about how to get a single-ended [non-bipolar] amp ckt to work as a general amplifier.

nickgammon

Yes, interesting. If I ground the tip and ground the ground wire, the noise drops slightly to around 5.6 mV.
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nickgammon


I was trying to get Nick to understand a little better about how to get a single-ended [non-bipolar] amp ckt to work as a general amplifier.


I'll add the extra cap tomorrow, I'm always keen to learn new things.
Please post technical questions on the forum, not by personal message. Thanks!

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nickgammon

Here's a slightly odd thing. I had the probe on 10x, and if I change it to 1x, the scope display doesn't change, but if I adjust the units, it now reads 600 µV as you expect as it is dividing the previous result by 10.

So whether you have 6 mV or 600 µV of noise depends on what setting you use. It's almost as if the noise is internal to the scope input circuitry.
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dc42


Nick, both your ckt and the one in reply #11, need a pullup R connected to the "+" input terminal (of the same value as the pulldown), in order to get the op Amps into the linear operating region.


The LM358 is not rail-to-rail output, the output range goes from close to 0V to around 3.5V when operating from a 5V supply. So I would not make the pullup resistor on the +input the same value as the pulldown, I would make it about twice as high, as I suggested in reply #5, so as to bias the input and output at about +1.8V. If there is noise on the power supply (e.g. because you are powering it from USB or a switched-mode power supply), then instead create a 2:1 voltage divider across the 5V supply to provide 1.8V, decouple it with a capacitor to ground, and take a 100K resistor from there to the +input.

Also, you must add the capacitor in series with R1, otherwise the amplifier will also be amplifying the DC bias and the output will be stuck at about 3.5V.
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oric_dan

Yeah, dc42 has a good point about how to bias the "+" input pin. Also, once you bias it into the linear [ie, symmetic] region of operation, and away from clipping the negative swings as at present, you may need to cut back on the AC-gain to keep it from clipping high input signals.

You can also play some more with the value of the cap across the feedback-R to see its effects - eg, check with larger values.

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I'll add the extra cap tomorrow, I'm always keen to learn new things.

You can learn several things at once, of course. One, how to get it working for general audio amplification to produce nice output with symmetrical swings, and once that's in hand, then how to fiddle the ckt to do rectification, envelope detection, etc. [good night now].

nickgammon


The LM358 is not rail-to-rail output, the output range goes from close to 0V to around 3.5V when operating from a 5V supply.


Well, I got that bit right:


It seemed to clip if the music was much louder, but I guess that since that isn't a rail-to-rail op-amp, you would expect that after about 3.6V.
Please post technical questions on the forum, not by personal message. Thanks!

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