# Audio Amp Help - Basic

Discussion in 'The Projects Forum' started by RobD, Dec 14, 2013.

1. ### RobD Thread Starter New Member

Dec 14, 2013
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0
Heyo!

First time trying to use the forums here. Anywho, I've been toying around with learning op-amp audio amps and am having some trouble. I found this schematic online and whipped it up in multisim, but am only getting square waves through the output with a sine input. Is this because it is acting as a comparator? Would that be because there is no feedback resistor? Any help/advice is greatly appreciated!

2. ### ronv AAC Fanatic!

Nov 12, 2008
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Yes, that and it only has a single power supply.

3. ### shteii01 AAC Fanatic!

Feb 19, 2010
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That does not look like amplifier.

Forget the audio for now. Just do simple amplifier, say Gain of 2, put 1 volt in, get 2 volts out.

4. ### RobD Thread Starter New Member

Dec 14, 2013
12
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Ok, so if I put a feedback resistor between the output and the non-inverting input, it would then become an amplifier? As for the power, what would having a positive and negative power supply alter? Forgive me for being a little naive

5. ### #12 Expert

Nov 30, 2010
16,703
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Just the basic principle that waves go positive and negative. If the reference point of the amplifier is zero volts and the audio voltage goes negative, how is it going to output a negative voltage with no negative voltage available? You have to either give the amplifier a negative voltage supply or build a circuit to keep the audio centered between zero and the positive supply voltage, then make a circuit on the output to remove the DC from the audio.

6. ### RobD Thread Starter New Member

Dec 14, 2013
12
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That makes perfect sense, thanks for clearing up why negative power is needed! So if there is only positive power supplied with an AC input, could you bias the output with a negative DC power source to make the output after the op amp consisted both positive and negative voltages? And what do you mean by removing the DC from the output?

7. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,503
512
Two problems.

Problem 1.
You obviously do not understand how to setup amplifier. I suggest a quick read of the wiki. It is not the best, but it does give you the basic "meat and potatoes" of the setup.

Problem 2.
You input signal is centered on zero. For example, the sine wave would vertically go from -1 volt to +1 volt. Your power supplies are 0 volt (ground at Pin 4), and 9 volt (at Pin 7?). So. When your input is 0 to 1 volts, that part will be amplified and will be the output. When your input is any negative voltage (from -1 to 0 volts), this part will be blocked by the op amp and you will see a simple zero at the output. The two power supplies (0 volt and 9 volt) that you use only allow half of your input to be worked on and be outputted. If you want to see the negative half of your input, you need to have a negative power supply instead of zero. For example, you could setup -9 volt at Pin 4. This way you have two power supplies, -9 volts at Pin 4, +9 volts at Pin 7.

8. ### shteii01 AAC Fanatic!

Feb 19, 2010
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The dc bias moves the ac signal up or down.

Let say you have dc bias that is 0 volts. Your ac signal is from -1 to +1 volts. So the highest point of ac signal is +1 volt. The lowerst point of ac signal is -1 volt.

Now we apply dc bias of 1 volt. What happens to ac signal? It is shifted up by 1 volt. Now your ac signal is from 0 volt to +2 volts. The highest point of the ac signal is now 2 volts. The lowest point of ac signal is now 0 volts.

RobD likes this.
9. ### RobD Thread Starter New Member

Dec 14, 2013
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Ah, that is much simpler than I thought!

10. ### MikeML AAC Fanatic!

Oct 2, 2009
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All you guys failed to notice that the pictured amplifier is a LM386, and it is wired as shown on its data sheet.

If you used a real simulator, instead of that toy, you would see that the stupid function generator is overdriving the amplifier which as configured has a gain of 200.

Last edited: Dec 15, 2013
11. ### #12 Expert

Nov 30, 2010
16,703
7,338
I just looked again and still totally missed the LM386 label, but I saw the 741 label. That's probably why everybody thinks it's an LM741.