Audio Amplifier Problem

[audioguru]

You have a single 9V battery to power the amplifier. The amplifier is biased so that its output rests at +4.5V so that the output can swing the same amount up to +6V or more and can swing the same amount down to +3V or less.

If you connect the speaker directly to the amplifier output and ground then the speaker has DC current in it which moves its cone forward or backward which heats it and heats the amplifier (wasting battery power) and causes the speaker to produce distortion.

So an output coupling capacitor is added between the +4.5V output of the amplifier and the speaker to block the DC. Then the speaker can swing the same amount up (positive) and can swing the same amount down (negative). It is normal for a speaker to be driven positive and negative.

Measure the speaker's resistance which will be only a little less than its impedance. An LM358 cannot drive a speaker, its datasheet shows a load that is 2000 ohms minimum .

Why are you amplifying 15kHz? Many speakers cannot produce a frequency that high.

The LM386 little power amplifier can drive an 8 ohm speaker to 6V p-p when powered from a 9V battery. It is already biased so that its output is at half the supply voltage and it already has negative feedback resistors for a voltage gain of 20 or 200. All its details are in its datasheet.

 

[Cason]

Maybe may I have this predict result ?

But may I ask that why the input signal contain negative voltage but I do not need to add a negative voltage supply (they are connect to same ground)?
Is that LM386 has the internal dc coupling ?

Thanks

 

[Nigel Goodwin]

I use the latest Internet Explorer 11 with Windows 7 Pro. I have no trouble seeing other attachments here or on any other website.

Hi AG,

Just tried IE11 here, it doesn't display that one image, yet Firefox shows it fine.

 

[Nigel Goodwin]

Maybe may I have this predict result ?
View attachment 84482
But may I ask that why the input signal contain negative voltage but I do not need to add a negative voltage supply (they are connect to same ground)?
Is that LM386 has the internal dc coupling ?

Thanks

AG posted example circuits in posts 13/14, why did you try and change it to something totally wrong that won't work? - just use his circuit .

An LM386 ISN'T an opamp (it's a power amp), you can't use it as an opamp.

 

[Cason]

AG posted example circuits in posts 13/14, why did you try and change it to something totally wrong that won't work? - just use his circuit .
An LM386 ISN'T an opamp (it's a power amp), you can't use it as an opamp.

Sorry, I am confusing with power amp and opamp. I go to purchase 1 to try it.
thanks

 

[audioguru]

Maybe may I have this predict result ?

Your schematic of an LM386 power amplifier shows that you did not read its datasheet and did not read what I wrote in post #21:
1) Its maximum allowed input is only +-0.4V because it already has negative feedback for a voltage gain of 20. If you use an audio taper volume control at its input then when it is set to halfway your +-1.5V signal is reduced to +-0.15V and the amplifier output is +-3V.
2) It is already biased so the resistors you added to pin 3 are not wanted.
3) It must have a series RC at its output pin to ground as shown on EVERY circuit in its datasheet.
4) Your 100pF capacitor coupling its output to a speaker is for radio frequencies, not sound frequencies. Use 470uF.
5) Like ALL electronic circuits, its power supply pin needs a bypass capacitor to ground. Use 100uF to 470uF.
6) Its datasheet shows that with a 9V supply and driving an 8 ohm speaker its maximum output is +-3V, not +-4.5V.

But may I ask that why the input signal contain negative voltage but I do not need to add a negative voltage supply (they are connect to same ground)?
Is that LM386 has the internal dc coupling ?

It is designed for an input signal that swings positive and negative. It is designed to use only one positive supply voltage.
ALL modern amplifiers use internal DC coupling.

 

[Cason]

Thanks AG.
The negative voltage and DC coupling problem make me feel very confusing before.
I am purchasing the component and tested the circuit in coming day.
Thanks

 

[audioguru]

Thanks AG.
The negative voltage and DC coupling problem make me feel very confusing before.
I am purchasing the component and tested the circuit in coming day.
Thanks

Most important: read the datasheet. Get it from www.google.com or from www.datasheetarchive.com .
Do not use it on a solderless breadboard. Use soldered parts on stripboard with a compact layout or on a pcb instead.

 

[Miles Prower]

Are there any op-amp suggest to increase the voltage to +-6V and 35-45mA?
And are that any method to decrease a little bit DC bias voltage only, that mean I have 4V floating and I would like to change to 1V DC bias ?

Thanks for help
Cason =}

You don't need to do that (attached).

What you want is at the bottom center of the schemo. For a 12V rail, I get 1.5W output: more than enough to drive a 3" speek.

 

[Cason]

Most important: read the datasheet. Get it from www.google.com or from www.datasheetarchive.com .
Do not use it on a solderless breadboard. Use soldered parts on stripboard with a compact layout or on a pcb instead.

Thanks, I will try to use the PCB. I am purchasing the components.

Moreover, I heard some friends using the transistor follower to provide extra current to the signal, using the resister and the BJT to change the gain and the current. Is this possible ? or just want to know BJT will cause more distortion?

 

[Nigel Goodwin]

Moreover, I heard some friends using the transistor follower to provide extra current to the signal, using the resister and the BJT to change the gain and the current. Is this possible ? or just want to know BJT will cause more distortion?

Again, post the circuit you're discussing, vague descriptions mean nothing.

 

[Cason]

The circuit was attached. Sor

 

[audioguru]

You show an opamp driving an NPN emitter-follower. It is a very hot heater that is hot even when it is just idling with no signal input. Its class-A transistor is conducting a high current all the time.
It poorly drives a speaker because the transistor pulls up the output but only simple resistors poorly pull down the output.

A audio amplifier IC (like an LM386) has TWO emitter-followers at its output. The NPN transistor pulls up the output and the PNP transistor pulls down the output. When there is no input signal then both output transistors are cool because they conduct only a low current. It operates in class-AB like this:

 

[Cason]

You show an opamp driving an NPN emitter-follower. It is a very hot heater that is hot even when it is just idling with no signal input. Its class-A transistor is conducting a high current all the time.
It poorly drives a speaker because the transistor pulls up the output but only simple resistors poorly pull down the output.
A audio amplifier IC (like an LM386) has TWO emitter-followers at its output. The NPN transistor pulls up the output and the PNP transistor pulls down the output. When there is no input signal then both output transistors are cool because they conduct only a low current. It operates in class-AB like this:

I have't consider the schematic inside the LM386 yet. After read the datasheet, yup, it show TWO emitter-followers at its output.
Thanks a lot