[Need Help] Circuit optimization

[d3ck]

Here is my circuit : https://i.imgur.com/lKVvEWB.png

The original idea was mixing 2 audio using 741 with 3 band equalizer. But I change the IC to 386, adding mic as input, 6 band equalizer, power amp to make sure there is enough supply to the speaker, also others enhancement. Any idea to make the circuit works better, with less noise to make it sound better? Or any other idea that I can add to this project. Thanks.

 

[tpetar]

Very scary circuit.

 

[FvM]

Although the circuit is surely not working, it raises some questions:
- How is LM386 different from a regular OP like 741? Which circuits are suitable for it?
- What's the working principle of a multiband equalizer? How can we design it with OPs?
- What's a reasonable impedance level for audio circuits? Where do we need power amplifiers in the audio signal chain?

 

[d3ck]

The circuit work with 741. Already build it using 741, but there's a lot of noise, not sure about 386, but its also should work. That's why LM386 been choose, to reduce the noise produced. Multiband equalizer should able to alter the frequency balance of an audio signal. From what I learn, power amp able to increase the output power, decrease crossover distortion, which increase the quality on the sound produced. From the design, power amp can support up to 9 Watt 8 Ohm speaker.

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Edit : I found that LM386 is not suitable for the design. Gonna redesign it

 

[FvM]

Some circuit ideas for multi-band equalizers: https://www.edaboard.com/threads/132500/

 

[crutschow]

An LM386 is an audio amplifier and is not a replacement for an op amp. Audio amps are used where you need a power output such as driving headphones or speakers.

A 741 is an old, noisy op amp design and not suitable for low noise audio amp circuits. Instead use a low noise op amp such as a TL074, MC33174, or MC34074.

 

[d3ck]

Yup. I misunderstand about the IC. Im thinking about to use NE5532

 

[crutschow]

Yup. I misunderstand about the IC. Im thinking about to use NE5532

Yes, that's also a good, low-noise audio op amp.

 

[betwixt]

I fully agree with earlier critisism of the design but I would be interested to hear d3ck's reasoning behind the two transistors stage. I see this kind of thing quite often and I'm curious as to why so many people do it when to me it seems totally counter-intutive.

Is it something taught at schools these days?

Brian.

 

[d3ck]

I fully agree with earlier critisism of the design but I would be interested to hear d3ck's reasoning behind the two transistors stage. I see this kind of thing quite often and I'm curious as to why so many people do it when to me it seems totally counter-intutive.

Is it something taught at schools these days?

Brian.

Idea is to increase the output power so that it will supply enough to the load (make speaker louder?). Class AB push-pull power amplifier been choose because it can lower the crossover distortion.

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EDIT : Q1 and Q2 should be difference type. My bad

 

[FvM]

Idea is to increase the output power so that it will supply enough to the load (make speaker louder?). Class AB push-pull power amplifier been choose because it can lower the crossover distortion.

But the discrete class AB buffer isn't driving a speaker in your circuit. As shown, it's only adding distortions and possibly reducing the maximum output voltage.

If used as an output stage, a class AB buffer might be helpful sometimes. But it should be better used in a feedback structure. And it surely can't compete with available monolithic audio amps or power OPs.

 

[betwixt]

d3ck, as a general rule, the input stages should be optimized for low noise and the output stages optimized for high power. FvM sees the same problem I was concerned about, you have, amongst other issues, placed a high power amplifier design with severely restricted output current and potential cross-over distortion in the middle of the amplifier stages and followed it it with a low power amplifier.

I suggest you change to low noise amplifiers in all the stages and remove the two transistor stage completely. You also need to look at DC conditions around the circuit which at the moment will prevent the amplifier stages working properly. Although the LM386 may be suitable as the output power amplifier, you will have to follow the manufacturers guidlines for connecting it. As it is, it will not work.

Brian.