Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've made a LM1458 Op-Amp based on this video.

  • it amplifies the input music but it produces noise and has low volume.

How can I remove the noise and how to increase the volume?

  • when i parallel 2A333J Mylar Capacitor or 104 ceramic capacitor in the input sound it removes the noise but produces whistle sound.

Does adding gain and bypass on LM1458 or/and decoupling can remove the noise? but how to add?

In case anyone could help me how to remove noise, i want to add base and treble adjustment but how?

Schematic Diagram:

schematic

simulate this circuit – Schematic created using CircuitLab

Update: I've made LM386 amplifier with gain and bass boost which is far better than LM1458 amplifier. Thanks for all the comments and suggestions.

share|improve this question
3  
Embrace what you've done so far as a learning experience and then blow off the hardware as a crap design and go here. – EM Fields Feb 26 at 9:44
1  
I agree with that, the main reason why this design doesn't work very well is because the LM1458 opamps are not suited to drive a 4 ohms speaker. Also the design is a kludge. You can make a far better audio amplifier by using an amplifier made for that purpose like the LM386. – FakeMoustache Feb 26 at 9:50
    
This is for school project. What ohms of speaker we need? and we need something louder that can use in classroom. Does using lm386 suitable for this? – Vanille Feb 26 at 10:01
3  
Since this is for a school project, society's assumption is that you'll use the exercise to learn something instead of asking to have everything handed to you on a silver platter while getting credit for someone else's work. I suggest that you study the data sheets you've already been handed in an effort to see whether you can glean from them what you need/want to know and, if you fail, come back with what you've not found and then ask for help. – EM Fields Feb 26 at 10:23
1  
I've upvoted your question since you actually provided a schematic and tried to fix the problems yourself first. We get a lot of questions here where there's no effort made, and it is nice to see someone at least try. – JRE Feb 26 at 10:33
up vote 11 down vote accepted
  1. I'm not going to watch a video of some wiener building a circuit then try to extract the schematic from a blurry screen shot of a video, and you shouldn't either. You need cleanly drawn schematics to work from, and a written explanation of the circuit function would be good, too.
  2. The 1458 is a dual version of the 741, commonly known as "that piece of crap that people should stop using because there are so many much better alternatives."
  3. The 741 cannot drive a 4 ohm speaker with any kind of power. The datasheet for the ua741 gives the output resisitance as 75 Ohms. This cannot really deliver any useful power to the speaker.
  4. Regardless of how much power the 1458 could put out, that jiggery pokery with R1,R2, and C1, C2 that you've connected the speaker to would make it impossible to get any real amount of power through the speaker.
  5. Your circuit diagram is hard to read. You generally start on the left with the input and work your way to the right for the output.
  6. Your circuit diagram makes no sense. There are so many non-sensical parts that I can only assume you've copied the circuit incorrectly from a fuzzy video.

In short:

If you built the circuit as you've drawn it, then it is a wonder that your circuit does anything at all. Expecting it to work correctly is just too much.


I would suggest you start with clear and correct circuits. Here are two simple circuits that will be much easier to build:

  1. Suggested circuit from the datasheet of the LM386 This is a simple IC that can directly drive a speaker. Use the first circuit from page 5 of the linked document: enter image description here
  2. This is a stereo amplifier that can drive two speakers and still has fewer parts than that mess built out the 1458. This amplifier can output 2.5Watts per channel, and can be built to provide 5Watts to one speaker. See the circuits given in the Tea2025 Datasheet for more information.
share|improve this answer
    
base on the pdf you have given. what amplifier is best? amplifier with bass boost, amplifier with 200 or 50 gain? by the way thank you for helping us. – Vanille Feb 26 at 10:20
    
what does broken lines means in bypass? – Vanille Feb 26 at 10:22
    
As mentioned, use the simplest with gain of 20. From any normal audio source (mp3 player, tape player, line out from stereo, etc.) that will be more than enough. If you find you need more, then going to gain of 200 involves adding a single capacitor from pin 1 to pin 8. The bypass capacitor in the gain 200 circuit is recommended but not required. – JRE Feb 26 at 10:23
    
The dashed line in the bypass capacitor parts means that they are recommended but not required. You can build the circuit without them. If it causes problems then you can add a capacitor to the indicated points. – JRE Feb 26 at 10:25
    
where should we put the input sound and ground? – Vanille Feb 26 at 10:28

Your circuit is probably not working well because you are trying to drive a speaker which requires maybe 0.5 W with an OpAmp designed for low output currents. To give you 0.5 W across a 4 Ω load, you need an output current of I = sqrt(0.5 W / 4 Ω) = 0.35 A -- too much for a small-signal, general-purpose OpAmp.

Even with low-performance, low-current opamps like the 741 and its derivatives, it is possible to build a somewhat capable design, though. One trick is to boost the output stage by adding external transistors.

It will likely be easier to use a ready-made IC designed for loudspeaker amplification, but for the sake of learning, I'm posting a small experiment I've tried out a while ago.

The trick that can be learned is: What have the designers of ready-made loudspeaker ICs had in mind when they built their designs? What's the difference between an OpAmp and a power amplifier? They share a very similar cicuit symbol, so what's actually going on here?

If you look into commercially available audio gear, you might be surprised how often you find simple, general-purpose OpAmps, even inside of mid- to high-grade equipment.

Some clues and background info about boosting small-signal OpAmps can be found in an application note, originally published by National Semiconductor (now TI), and written by Jim Williams. (His name is not in the application note, but there's evidence ;-) Note that he even has some designs that use vacuum tubes to boost the OpAmps.

Here's a circuit I've tried a while ago, quickly drawn on the back of an envelope: OpAmp boosted with BJTs

And here's what it does: It takes the OpAmp's output (741, half a 4559, whatvever), puts it through a complementary BJT output stage (Q1, Q2), and uses the BJTs' output signal as the input for its feedback, thereby eliminating any non-linearities and unknown properties (amplification = ?, linearity = ?) of the discrete output stage. Also, two additional transistors Q3 and Q4 are connected across the 22 Ω output resistors, provide an over-current protection that kicks in around Iout = 0.7 V / 22 Ω = 32 mA and protects the output transistors Q1 and Q2.

Output transistors are BC548/BC558. This design works well as a headphone amplifier. For loudspeakers, use bigger transistors for larger output currents and adjust the current sense resistors accordingly.

Note how the general idea looks very much like a basic, non-inverting OpAmp configuration, except for the additional BJT stage between the OpAmp's output pin and the circuit's output ("out") connected to the feedback resistor (22k).

The voltage of any signal taken from the input potentiometer (at the OpAmp's non-inverting input) will get amplified by a factor of A = 1 + R1 / R2. Q1 and Q2 just help to deliver more current while the OpAmp tries to maintain the desired voltage - and this is A Jolly Good ThingTM, quoting Mike Engelhardt, because for a speaker amp, you need both voltage and current, because power = voltage * current.

You may want to ask now just how much current the output stage is going to give you. Answer: The amp acts like a voltage source, and it will try to give you as much current as it can, just as long as Q3 or Q4 don't start to limit the show. Hence, the current is determined by the load connected across "out" and can be calculated once you know the load resistance RL: Iout = Uout / RL. Approximate a speaker's impedance using ZL = RL.

share|improve this answer

Trying to get amplifier circuits to work right is a very complex thing. It helps if you look into the concept of filters that pass the signal you want and block the parts you dont want. The fact your circuit has sound output is great. There are good suggestions here, the circuit drawn on the envelope looks good, but there is no output connector drawn; Am assuming its in the left center between the 22ohm resistors in series?

Picking the capacitor values even for the LM386 has to be done carefully, and you need an oscilloscope to see the signals in the stages of your design.

Add Bass and Treble: you must filter the input, or set the level to a known value then put it across filtering to pass or block frequency ranges, after the filtered input, then your bass/treble circuit, then you amplify, but make sure your amplifier will not pass anything above the audio range.

you could use some known Audio power amp circuit to drive the speaker, but make it 3 sections: normalize your input then apply the bass/treble, then amplify

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.