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Old 10th January 2010, 07:58 PM   #11
johnr66 is offline johnr66  United States
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When I listened to the noise, I had removed the mic from the source. I did not know that the cheap condenser mics had a built in buffer. Is it built inside the little round mic element itself? If so, I wonder if there are higher quality elements to replace them with? I might even put the elements and preamp into its own case.
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Old 11th January 2010, 06:19 AM   #12
Mooly is online now Mooly  United Kingdom
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Electret condensor mics
Electret microphone - Wikipedia, the free encyclopedia

Maybe something like this may be more suitable,
ECM Mic Preamplifier

And read the last line here,
http://www.zen22142.zen.co.uk/Circui.../lf071_mic.htm

I suspect your problem is as much to do with amplifying, then attenuating, then amplifying again in the recorder etc...

Last edited by Mooly; 11th January 2010 at 06:25 AM.
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Old 11th January 2010, 05:21 PM   #13
johnr66 is offline johnr66  United States
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Thanks for the Links, Mooly. So much to learn.

I don't understand your last line about amplification, attenuation and amplification. The opamp output would connect to the line in of the deck. I would not feed it to the mic inputs, nor would I use the power amp IC in the final circuit, that was just for a listening test. If the preamp is quieter than the deck's own mic preamp, I will proceed with the design.
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Old 11th January 2010, 06:05 PM   #14
Mooly is online now Mooly  United Kingdom
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Quote:
Originally Posted by johnr66 View Post
Thanks for the Links, Mooly. So much to learn.

I don't understand your last line about amplification, attenuation and amplification. The opamp output would connect to the line in of the deck. I would not feed it to the mic inputs, nor would I use the power amp IC in the final circuit, that was just for a listening test. If the preamp is quieter than the deck's own mic preamp, I will proceed with the design.
If you look at your circuit at the start of the thread you amplify with the first opamp, then attenuate with the level pot, then amplify again with the second opamp... so already the signal you want has been through two gain stages and had it's signal to noise ratio worsened by more than a single gain stage.
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Old 12th January 2010, 12:11 AM   #15
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by Mooly View Post
If you look at your circuit at the start of the thread you amplify with the first opamp, then attenuate with the level pot, then amplify again with the second opamp... so already the signal you want has been through two gain stages and had it's signal to noise ratio worsened by more than a single gain stage.
That was just a starting point. Later in this thread I mentioned that I had dropped the second op amp stage. I was getting too much gain from it. I'll test what I have and make adjustments and compare with the deck.
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Old 12th January 2010, 01:18 AM   #16
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Quote:
Originally Posted by johnr66 View Post
This is my first op amp design. It will be a portable mic amp to line level out. The mics are the low cost condenser tape recorder type which have their own battery inside. I'm using the TL082 opamp since I can get it at any old rat shack. I'm not after golden ear audio, just a good preamp that is stable and low noise.

Power: I'm using 2x9v for power. I will use high brightness LEDs and 10k dropping resistors for them to lower current draw.

Input: Not much to say here other than the shunt cap is to reduce RF noise entering amp.

First stage: I set gain to 10

Level control: I see I botched that. It is flip-flopped. The wiper arm goes on the input of the second stage. As shown, it would short the first stage with level turned down and there would be a low frequency roll-off as the level is turned down due to the Z of the circuit. I'll fix that.

Second stage: Not sure of the gain I will need yet.

Output: Just have a coupling cap. Should I have a load resistance?

So that's it. I still need to determine the proper value of capacitors, not difficult, just some calculations. Let me know if on track or clueless in this design. Thanks!

Click the image to open in full size.
You have your level control kind of inside-out. Swap the top and wiper connections. I've found over the years that 40-60 dB of gain is good at least for low impedance mics. Your first stage has 19.6 dB (gain of 11) You might want to bump that up a bit so your second stage doesn't have to make up too much more. Keep in mind that pre-processed audio levels can be ALL OVER THE PLACE. You can NOT have too much headroom. if it were mine I'd run the first amp at a gain of 30dB which is 31.6 followed by your level pot and a gain of 6 (15.5dB).

Resistor (Johnson) noise is higher for larger resistances. I would make the 10 K of your '-' input first stage 750 and the feedback resistor 22K for a gain of 30.3 (29.6dB). The second stage would be 2K2 to ground and a feedback resistor of 11K for a gain of 6 (15.6dB) for a maximum of 45dB gain.

TL072 is a lower noise version of the TL082 and there are better opamps than that. I would consider the venerable 5532 rather than a FET amp. If you use a socket you can try different amps.

G
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