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Old 8th January 2010, 01:38 PM   #1
johnr66 is offline johnr66  United States
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Default Evaluate first preamp

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!

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Old 9th January 2010, 12:27 PM   #2
johnr66 is offline johnr66  United States
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Anyone?

Okay, I want it to sound great! Suggestions?
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Old 9th January 2010, 10:37 PM   #3
mjf is offline mjf  Austria
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hello.
........with this kind of preamp i used tl072 for lower noise.
0.22uf is low - cuts off the deep tones..........0,47uf or more.....
perhaps a res with a few hundred ohms (220 or so.....reducing rf noise) in series with the input cap.
gain of the first stage is 11.............i use lower values here,10k and 1k or so.......
suggestion:second cap as big as input cap.
pot built in reverted........not good.
second stage :10k feedback res and
the other res to ground depends on your mic (perhaps 1k...........to 10k or so).
output cap perhaps 1uf...........and a load res to ground (100k........) is always a good idea for universal use of a preamp.
the power supply caps (0,22uf) should sit near by the pins of the opamp.
greetings............

Last edited by mjf; 9th January 2010 at 10:40 PM.
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Old 10th January 2010, 02:13 AM   #4
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After the coupling cap of the second stage op-amp, I have found that resistor values between 1K to 10K sounds best. In my designs I usually use carbon film 1/4W 2K or 3.3K. My experience with discrete metal thin film resistors is not very positive.

Last edited by rascal101; 10th January 2010 at 02:26 AM.
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Old 10th January 2010, 07:15 AM   #5
Mooly is offline Mooly  United Kingdom
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It's a workable design... just make sure 10k isn't too low an impedance for the mic to work into... 100 k may suit better... check the mic data sheets.
Double pole switch and two LED's... you could use an additional TLO62 (low current version) to provide a virtual earth supply with one opamp and use the second to make a simple window comparator to light LED when battery voltage too gets too low. Use a single switch and batteries in series... or design using opamps that swing rail to rail and use single 9 volt battery. I would add a small electroylitic of around 47uf across the rails as well as the 0.22uf
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Old 10th January 2010, 12:34 PM   #6
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The mic is isolated by the op-amp which acts as a buffer. Further to that the op-amp has very high input impedance so it should be no problem for the mic driving it.

For the op-amp, the min 1K load resistor translates to 9mA (with rail voltage at 9V). This is very much less than specified typ output current for the TL082 of about 50mA. So this is also not a problem for the op-amp.
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Old 10th January 2010, 01:35 PM   #7
johnr66 is offline johnr66  United States
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Thanks for the replies. Yesterday, I tinkered and found I don't really need a second stage, at least when using the TDA7267 power out chip to hear the output since it's gain is 32db. I'll have to hook it up to my cassette deck to see if it produces quieter noise levels than the decks own mic amps.

So far I'm not too excited about the noise of the TL072/82 chips. With no signal and electrical/RF noise removed, there is a faint, but noticeable white noise hiss from the speaker. I found a LM833 in my parts box and tried it. It makes half the noise! Looking at the spec sheets, the 833 does have quite a low noise figure in comparison. Looking at some of these new OP amps the audio wizards seem to love, the noise figure is rather high.

I guess I may need to look for a dedicated mic preamp chip or maybe go discreet for lowest possible noise? I was hoping to record faint sounds with minimal noise which requires me to turn up the levels on my deck to almost max and there is much more noise produced on to tape than the tape itself.
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Old 10th January 2010, 04:16 PM   #8
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by rascal101 View Post
The mic is isolated by the op-amp which acts as a buffer. Further to that the op-amp has very high input impedance so it should be no problem for the mic driving it.
The input impedance is the value of the 10k resistor shown on the circuit.
That is what the mic "sees".
The TL072/82 etc will only drive loads down to 2k or so at full swing.
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Old 10th January 2010, 04:23 PM   #9
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by johnr66 View Post
So far I'm not too excited about the noise of the TL072/82 chips. With no signal and electrical/RF noise removed, there is a faint, but noticeable white noise hiss from the speaker. I found a LM833 in my parts box and tried it. It makes half the noise! Looking at the spec sheets, the 833 does have quite a low noise figure in comparison. Looking at some of these new OP amps the audio wizards seem to love, the noise figure is rather high.
At the impedances you are using a bipolar opamp will generally give a lower noise figure. The LM833 is similar in concept to the NE5532.

Edit... another thought... cheap electret mics have a FET buffer built in and they are noisey to start with very often.
You may be trying to provide to much gain... to amplify (the first opamp stage) then attenuate and amplify again isn't good.

Last edited by Mooly; 10th January 2010 at 04:39 PM.
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Old 10th January 2010, 07:38 PM   #10
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Quote:
Originally Posted by Mooly View Post
The input impedance is the value of the 10k resistor shown on the circuit.
That is what the mic "sees".
The TL072/82 etc will only drive loads down to 2k or so at full swing.
My bad. Sorry, I missed out on the input 10K resistor and focused on the op-amp.
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