What's wrong with this typicial preamp circuit? - diyAudio
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Old 6th October 2012, 06:00 PM   #1
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Default What's wrong with this typicial preamp circuit?

I am a EE student but I am a newbie of audio design.
I just simulated this typical preamp circuit but the frequency response looks bad.

my idea is connect this preamp to a tone control circuit, then to the LM3875 power output.

attached the result of Pspice simulation.
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Old 6th October 2012, 06:40 PM   #2
Mooly is online now Mooly  United Kingdom
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The main problem is simple... the choice of opamp. The 741 is utterly and totally outclassed performance wise and has been for the last 35 years or so But its great for learning.

Your first circuit has a voltage gain of 100 and that means the gain of the 741 rolls off very early. The second circuit has a gain of 10 and uses a very decent OPA134. You could use that for all these circuits.

Your second opamp circuit also has another problem that the first one doesn't. Can you see what it is ?
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Old 6th October 2012, 06:53 PM   #3
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your spice-sim schematic doens't match your 1st circuit.
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Old 6th October 2012, 07:00 PM   #4
Mooly is online now Mooly  United Kingdom
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I just spotted something else. Normally when talking of poor response and the 741 we mean at HF but you have a problem at LF too. The input coupling caps are far to small in relation to the 1K input impedance. Try a 10uf instead of the 0.1uf.
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Old 7th October 2012, 01:50 AM   #5
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Originally Posted by Mooly View Post
The main problem is simple... the choice of opamp. The 741 is utterly and totally outclassed performance wise and has been for the last 35 years or so But its great for learning.

Your first circuit has a voltage gain of 100 and that means the gain of the 741 rolls off very early. The second circuit has a gain of 10 and uses a very decent OPA134. You could use that for all these circuits.

Your second opamp circuit also has another problem that the first one doesn't. Can you see what it is ?
oh Im sorry I didn't wrote the topic clear. The LM741 circuit and the tone adjust circuit are taken from some audio website. I changed the opamp to the OPA134. Now I change the coupling capcitor to 100uF and the Low freq response is better. How do I determine the suitable coupling cap without simulation?
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Old 7th October 2012, 01:51 AM   #6
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Quote:
Originally Posted by Mooly View Post
I just spotted something else. Normally when talking of poor response and the 741 we mean at HF but you have a problem at LF too. The input coupling caps are far to small in relation to the 1K input impedance. Try a 10uf instead of the 0.1uf.
Thank you. how do u know the input impedance is 1K?
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Old 7th October 2012, 02:41 AM   #7
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Thank you. how do u know the input impedance is 1K?
oh I am so dumb. It is just an active high pass op amp circuit and Vout/Vin = 10k/(1k +1/jwC).
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Old 7th October 2012, 02:45 AM   #8
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The input impedance is 1k which is rather low for most purposes, you don't say what it is intended to amplify.

A major difficulty with this circuit is that at the usual 47-100k input impedance general purpose pre amp the series resistor contributes a large amount of noise.

To calculate an input capacitor just transpose, r = 1/(2pifC).
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Old 7th October 2012, 03:10 AM   #9
balerit is offline balerit  South Africa
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The input cap should be 2uF2, i use tantalum here. Input impedance should be 10K or 47K for phono. Use NE5532 opamp, this is an industry standard.
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Old 7th October 2012, 03:48 AM   #10
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Originally Posted by balerit View Post
The input cap should be 2uF2, i use tantalum here. Input impedance should be 10K or 47K for phono. Use NE5532 opamp, this is an industry standard.
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The Book Worm - DIY Audio Ebooks and Manuals - Speakers, Moscode Amplifiers, Electronics, Map Reading, Free Capacitor Intro.
Yeah the response is very similar to using 1k and 100uF. Fc = 1/2*pi*R*C is so right. I am going to use this to amplify signal from a Bluetooth module btw.
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