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!
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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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............
........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............
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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
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
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.
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.
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.
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.
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.
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.
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My bad. Sorry, I missed out on the input 10K resistor and focused on the op-amp.
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.
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/Circuits/Audio/lf071_mic.htm
I suspect your problem is as much to do with amplifying, then attenuating, then amplifying again in the recorder etc...
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/Circuits/Audio/lf071_mic.htm
I suspect your problem is as much to do with amplifying, then attenuating, then amplifying again in the recorder etc...
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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.
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.
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.
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.
G²
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