AUDIO AMPLIFIER... urgent help needed

Thread Starter

marting

Joined Dec 3, 2010
10
I HAVE MADE A CIRCUIT TO MEET THE FOLLOWING SPECS

1. Av ≥ 2000 when driving a 100 ohm load, with a source resistance of 1 Kohm at 1 KHz. .

2. Rin ≥ 15 K with RL = 100 ohms at 1 KHz

3. Lower cutoff frequency ≤ 10 Hz with RL = 100 ohms

4. Upper cutoff frequency ≥ 25 KHz with RL = 100 ohms

5. Volume/gain control to vary the gain of the amplifier from ≈ 0 to 2000 with RL = 100 ohms. You will require a potentiometer/variable resistor to vary the gain. While varying the gain of the amplifier, the lower and upper cutoff frequencies at any intermediate gain should meet the above requirements.

6. Must be able to accept input either from a microphone or from music player through a male jack. Must be able to drive a speaker.

7. Nice loud output from the speaker with no audible distortion.

8. Supply voltage = ± 15 V (dual supply). You should design and build your own DC power supply. All the input sources should be AC coupled.

10. Output DC voltage ≤ 50 mV.

11. The output swing with a 100 ohm load ≥ 20 V pk-pk

The final output stage (the one that drives the speaker) must be a Class AB power amplifier.

HOWEVER, MY CIRCUIT IS NOOT MEETING ALL THE DEMANDS. I HAVE TO MAKE A COMPROMISE ON EITHER TO HAVE THE REQUIRED GAIN OR THE REQUIRED BANDWIDTH.

PLEASE EDIT MY CIRCUIT AND HELP ME OUT..

THANX
 

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beenthere

Joined Apr 20, 2004
15,819
Don't think you are anywhere close with the LM2904 - http://www.national.com/mpf/LM/LM2904.html#Overview

Some brief problems:
1. Can't read the resistor values, so overall gain is a mystery.
2. Rin is nowhere near 15K
3. No lower frequency cutoff.
4. Upper frequency is controlled by feedback - you don't have any.
5. No gain control
6. There is no mic preamp.
7. Read the data sheet to obtain the op amp output power.
8. Read the spec.
9. ???
10. This should be no problem - a 50 mv swing is possible.
11. Impossible - the op amp max supplies are +/- 16 VDC.

You have not got an AB output stage, just an op amp.
 
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Kermit2

Joined Feb 5, 2010
4,163
Concerning the answer to 11.


isn't the +15 V a 30 volt peak to peak? making something greater than 20 volt peak to peak possible?

+12.5 V for example - being 25V peak to peak
 

beenthere

Joined Apr 20, 2004
15,819
isn't the +15 V a 30 volt peak to peak? making something greater than 20 volt peak to peak possible?
I suppose that is a difference about how it is measured. In audio work where the output is AC, you need a positive 20 volt peak as well as a - 20 volt peak.

At any rate, the load is given as 100 ohms. The spec sheet says 40 ma is the 2904 max. That is a long way from 200 ma.
 
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Thread Starter

marting

Joined Dec 3, 2010
10
so where to start correcting?? which point should be achieved first??? how to set upper cutoff and lower cutoff? how to calculate Rin?
 

Thread Starter

marting

Joined Dec 3, 2010
10
The design of an amplifier is a bit involved. I have no idea where you are in your studies, so it is hard to offer adequate advice. It's your project, so it's really up to you.

You might find something useful at this link - http://www.osd.rutgers.edu/gs/09papers/Amp.pdf
if i could have done it all myself why would i ever ask for help??? the thing i would appreciate is somebody's kind assistance. that is what i am looking forword to!!
 

Kermit2

Joined Feb 5, 2010
4,163
If you need someone to tell you what input voltage you need in an amp with a gain of 2000 and you know the output needs to be 20 v pk-pk, then perhaps this project is beyond you at this point.
 

Audioguru

Joined Dec 20, 2007
11,251
An LM2904 opamp cannot drive a speaker and it cannot drive the 100 ohm load. You need a power amplifier to do that.

The LM2904 is the very first low power opamp that has lots of crossover distortion. Its low power causes its bandwidth to be the worst of any opamp.
 

Thread Starter

marting

Joined Dec 3, 2010
10
If you need someone to tell you what input voltage you need in an amp with a gain of 2000 and you know the output needs to be 20 v pk-pk, then perhaps this project is beyond you at this point.
i worried about the voltage input signal that might be provided by the music jack or any other source. there were some other queries as well that u did not answer
 

Thread Starter

marting

Joined Dec 3, 2010
10
An LM2904 opamp cannot drive a speaker and it cannot drive the 100 ohm load. You need a power amplifier to do that.

The LM2904 is the very first low power opamp that has lots of crossover distortion. Its low power causes its bandwidth to be the worst of any opamp.
what about LF356? is that feasible??
 

Kermit2

Joined Feb 5, 2010
4,163
i worried about the voltage input signal that might be provided by the music jack or any other source. there were some other queries as well that u did not answer

Your circuit should be able to take a signal with peaks of +5 millivolts and -5 millivolts, and deliver a signal to a 100 ohm load with peaks or +10 volts an -10 volts. It also needs to pass a signal of 8 V pk-pk through without clipping or distortion at your minimum gain level. That is, I believe, the specified output maximum of line level pro audio equipment. Commercial(consumer) equipment is 2 V pk-pk. Have no fear! someone will come along and correct that if I'm wrong. :)
 

Audioguru

Joined Dec 20, 2007
11,251
what about LF356? is that feasible??
Why don't you look at its datasheet?
It is an ordinary opamp, not a power amp. Its minimum load resistance is spec'd at 2000 ohms. Its max shorted ouput current is only 20mA to 25mA.
Like most other opamps.

To drive an 8 ohm speaker with only 0.5W the peak output current is 354mA.

I don't know why your teacher also wants a 100 ohm load. It is nothing but a fairly high current heater.
 

beenthere

Joined Apr 20, 2004
15,819

Kermit2

Joined Feb 5, 2010
4,163
I respectfully beg to differ with you.

p-p is defined as 2 times the peak. with a 10 volt peak you get 20 volt peak to peak.

Please don't just say I'm wrong. If there is a math equation for sine waves that has something OTHER than 2 x pk = peak to peak. Please post that. The op amp link you list gives the 2 x pk = peak to peak equation.

How is 20 volts positive and 20 volts negative; 20 volts peak to peak?

Answer: It's not. It's 40 volts peak to peak for a 20 volt peak symmetrical sine wave.
 

Audioguru

Joined Dec 20, 2007
11,251
I agree that the peak-to-peak output voltage is simply double the peak voltage.
20V peak-to-peak is 10V peak and an opamp needs a supply voltage of about plus and minus 12V to do it if its output current is low.
 
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