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-   -   What's wrong with this typicial preamp circuit? (http://www.diyaudio.com/forums/analog-line-level/221055-whats-wrong-typicial-preamp-circuit.html)

kaneneville 6th October 2012 06:00 PM

What's wrong with this typicial preamp circuit?
 
2 Attachment(s)
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.

Mooly 6th October 2012 06:40 PM

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 ?

powerflux 6th October 2012 06:53 PM

your spice-sim schematic doens't match your 1st circuit.

Mooly 6th October 2012 07:00 PM

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.

kaneneville 7th October 2012 01:50 AM

2 Attachment(s)
Quote:

Originally Posted by Mooly (http://www.diyaudio.com/forums/showthread.php?p=3192023#post3192023)
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?

kaneneville 7th October 2012 01:51 AM

Quote:

Originally Posted by Mooly (http://www.diyaudio.com/forums/showthread.php?p=3192039#post3192039)
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?

kaneneville 7th October 2012 02:41 AM

Quote:

Originally Posted by kaneneville (http://www.diyaudio.com/forums/showthread.php?p=3192378#post3192378)
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).

rcw666 7th October 2012 02:45 AM

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).
rcw

balerit 7th October 2012 03:10 AM

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.
Mac

The Book Worm - DIY Audio Ebooks and Manuals - Speakers, Moscode Amplifiers, Electronics, Map Reading, Free Capacitor Intro.

kaneneville 7th October 2012 03:48 AM

Quote:

Originally Posted by balerit (http://www.diyaudio.com/forums/showthread.php?p=3192422#post3192422)
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.
Mac

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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