I am searching a good design low noise pre amp. I found this.


I want to take audio signal from PC sound card..
What is the maximum average output voltage from Sound card ???

Finally i want to built M250


Some thing like this.
PC sound card--->> Preamp----->> M250(power amplifier).

Ur reviews about this preamplifier.
How can i improve this?

Best OPAMP for this type of application.


Thanks.

 

Hi Ayne,
Your preamp has a voltage gain of only 4. Why bother?
The opamp has an input offset voltage of up to 6mV which is amplified to 12mV. It would cause crackling sound when the volume control is turned. Block the DC with a 1uF metalized plastic coupling capacitor feeding each volume control.

The M250 is a very simple design and is the first audio amplifier project that I have seen that doesn't have any spec's. It is about the same design as is used in amplifier projects at www.sound.westhost.com/index2.html and www.ampslab.com/projects.htm . Its power into an 8 ohm speaker might be only about 150W RMS at clipping.

The TL072 dual opamp is good enough as a line-level preamp for the simple power amplifier.

__________________
Uncle $crooge

 

I am assuming that
Sound card maximum output voltage = 200mV Vp
Power Amplifier input sensitivity into 8 Ohm = 1.2 V(RMS)



Now what u say about that pre amplifier.
what are the exact Maximum average voltage of PC Sound card???

Thanks.

 

Now your preamp has an input impedance of only 1k ohms which is much too low. It should be 10k or 100k.
You still didn't add a coupling capacitor ahead of the volume control.
The input impedance of the second opamp is also only 1k ohms which is much too low to be fed from a 10k volume control.

Every sound card has a different amount of output level. Check the spec's of yours.

__________________
Uncle $crooge

 

Changes:
Input impedence is changed to 10K.
Coupling capacitor added.. 4.7uF(non polar).

Need some more changes???



Thanks.

 

Considering the title of this thread is "High Quality Audio Preamp", so far it's only consisted of really poorly designed basic opamp circuits.

Why use two opamps anyway? - all you're wanting to do is give a small boost to the output from a sound card, which is already low impedance and at a fairly high level. So you can feed the sound card directly into the volume control, then an opamp to give the gain, feeding the power amp. It's up to you if you want inverting or not, but in either case make sure the input impedance of the opamp is at least five times that of the volume control.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The coupling capacitor for stopping DC in the volume control is supposed to feed the volume control. Yours is after the volume control.

__________________
Uncle $crooge

 

Capacitor has been changed.

It should give flat gain from 30 to 22KHz..

How can i improve this??

Thank u Audioguru.

After achiving best design i will go to it's PCB.

Thanks.

 

It would be difficult not to easily exceed that with a modern opamp (not that TL07x's are particularly modern - but still an impressive chip for the money).

Like I said above, why do you want two opamps?, and R3 is too low to be fed from a 10K pot.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

The 4.7uF capacitor feeding the 10k pot in parallel with the 10k R3 has a -3dB cutoff frequency of 6.8Hz.
The 2.2uF capacitor feeding the 100k R8 (the input of the power amplifier?) has a -3dB cutoff frequency of 0.7Hz. Therefore the circuit's low frequency response is flat down to about 35Hz.

The opamps have very low voltage gain so their low level high frequency response is flat up to about 200kHz and their high level high frequency response is flat up to about 100kHz.

__________________
Uncle $crooge

 

Now i am using single OPAMP. as NIGEL said. Thanks Nigel.

What u say about design.
It should be Low noise, Flat gain.


Thanks.

 

Like audioguru said before, feed the pot through a capacitor - because if there's any DC level on the incoming signal the pot will be noisey when you turn it.

There's no problem getting low noise and wide frequency response, it's a very easy job with an opamp.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

There will be built in capacitor at the output of sound card. If i also add a capacitor in series of input signal(before POT) then total capacitance will decrease.. and low frequency responce will be low.... I think.

What u say??

 

As long as you are 100% sure that it will always be used fed from a capacitor coupled source, then there's no problem. Even with an external capacitor it won't reduce your low frequency response, because you size it accordingly!.

Normally every line output, and every line input, will include a blocking capacitor.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Finnaly capacitor added..
It's all right or need some more improvements??

When Audioguru say that it's all right and ready for PCB then i will make it's PCB..
I am waiting his suggestions and recomendations.. and Nigel's Also.