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Topic: How to make a pre-amp for a electret? microphone (Read 17371 times) previous topic - next topic

nickgammon

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.
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polymorph

Yeah, my example circuit is rather optimistic with an attempted gain of 1000, and a very artificial continuous 1kHz tone.

C2 is meant to be a smoothing capacitor, so you'd not see much except a little DC.

That should really be a two stage amplifier. Gain no more than 100 per stage.
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polymorph

Even in LTSpice, 10mV peak input only got us 2.8V peak.
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MarkT



Yellow is on pin 3 of the op-amp (+ input) and blue is on pin 1 (output).



But we are using an inverting circuit where the opamp inputs are virtual ground, right?
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nickgammon

Not in the original circuit, I don't think (first post).
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oric_dan

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. What you are seeing in your waveforms is clipping due to the opAmps being DC-biased at ground potential.

Also, the ckt of #11 is pretty much ALL wrong.

1. when adding the pullup on R3, use 10K to 100K for both.

2. a gain of 1000X is too high on an LM358. It's GBP [gain bandwidth product] is only 1 Mhz, so at gain=1000, the BW is only 1000 Hz. The gain probably shouldn't be set to over 100X or so, for audio work.

3. the LPF [low pass filter] on the output kills the signal, as you observed. It has a low-pass F3db = 1/(2*pi*R5*C2) = just 16 Hz.

Low-pass on the very output is an extremely poor way to remove the noise in the system - ie, after the noise has been "amplified" by the ckt. Your waveforms show how much noise there is on the electret signal.

4. both in your ckt and #11, and 99% of the time with non-inverting opAmp ckts, you want a low-pass filter cap across the "feedback" R. Especially at higher gain, this both filters noise and prevents the ckt from oscillating. Calculate the cap using the same formula as above,

F3db = 1/(2 * pi * R2 * C) --> C = 1/(2 * pi * R2 * F3db)

For audio, C = 1/(2 * pi * 100K * 5000Hz) = 318 pF, so something between 270 and 470 pF should work. Try items 1 and 4, and you should have a much nicer waveform output.

nickgammon

Quote

4. both in your ckt and #11, and 99% of the time with non-inverting opAmp ckts, you want a low-pass filter cap across the "feedback" R.


R2 in my circuit, right?

BTW, in the screenshots I took the input (yellow) shows lots of noise, but it doesn't seem to be amplified. So am I correct in thinking this is a measuring artifact? The noise isn't really there? I was watching one of Dave Jones' videos where he showed that what looked like noise from a power supply wasn't really what it seemed.

http://www.youtube.com/watch?v=BFLZm4LbzQU
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oric_dan

Yeah, your R2, the cap goes in parallel with it, not to gnd or anything else. You should always use a cap there whenever you have a gain of about 10X or more. There should also be one across R2 in the inverting ckt of reply #11 - along with all the other fixes.

You're blowing up a small signal, so you'll always see a lot of noise on it, unless you take significant scope grounding precautions as talked about in the past. Also, try just connecting your probe gnd lead to the test pin of the probe, and see what the noise looks like at the same o'scope gain setting - it'll probably look about the same as when on the ckt [maybe]. You need to know what your test equipment is actually doing FIRST, as a yardstick for comparison.

A lot of the noise is also high-frequency, so the limited BW of your amp due to the high gain setting will filter it. Remember, BW = GBP/Gain.

nickgammon

With your suggested changes to my original circuit I now get:


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oric_dan

It's looks like the noise on the output is slightly less, as might be expected from dropping F3db from 10Khz down to 5 Khz. However, it looks like you didn't add the pullup R at the "+" terminal to move the DC bias. I forgot, when you do that you'll need to put a 1 uF cap in "series" with R1, the R to gnd off pin 2, in order to make the "DC gain" = 1, when the AC-gain = 101. At that point, the output waveform should be more symmetrical.

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

polymorph

I did mention that point about not trying to get that much gain in one stage. And my circuit was not meant to amplify more than one half of the incoming signal. And I did also mention that the RC network on the output was meant to smooth it to something approaching an envelope detector.

The intention in wiring the Op Amp so it is biased at ground was to make it act as a precision rectifier, only passing one half of the waveform. It does what I meant it to do, it just needs a preamp before with less gain in it and the preamp.
Steve Greenfield AE7HD
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nickgammon

I added the resistor but not the cap you forgot to mention. :P
Please post technical questions on the forum, not by personal message. Thanks!

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nickgammon

This is the reading with the probe tip grounded:


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polymorph


I added the resistor but not the cap you forgot to mention. :P


Ahem.

Quote
C2 is meant to be a smoothing capacitor, so you'd not see much except a little DC.


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.
Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
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polymorph

Steve Greenfield AE7HD
Drawing Schematics: tinyurl.com/23mo9pf - tinyurl.com/o97ysyx - https://tinyurl.com/Technote8
Multitasking: forum.arduino.cc/index.php?topic=223286.0
gammon.com.au/blink - gammon.com.au/serial - gammon.com.au/interrupts

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