Audio Amplifier Problem

[Cason]

Hi all,
I am trying to build a electronic circuit amplifier sound. In the beginning, I have construct a audio signal from 0V to 7.28V (0-4.3V is the DC-bias voltage), and I just connect to the speaker with series with a capacitor. But I found that the speaker is very low volume, so I measured different operation point, I found that the current is just about 1.5-2mV. Afterthat, I would like to increase the current to 40mA.

I have try to
Audio Signal -> LM358 with +9V supply ( working in voltage follower mode) ->Series Capacitor -> Speaker
Final , I just got the voltage between -+1.5V and 15.72mA current at two pole of speaker.
But I think it is also not enough parameter to drive the speaker, so LM358 may be not enough in this time. I have try to search on website, but I cannot find a approach one to give out a current output 35-45mA with just positive voltage input (because I have just one +9V voltage sources only).

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 =}

 

[Nigel Goodwin]

You need a power amplifier, not an opamp.

But most of your post makes no sense (for example what has DC bias got to do with anything?), and you don't really provide any details of what you're wanting to do.

For a single 9V supply something like an LM386 would probably be fine?.

 

[Cason]

You need a power amplifier, not an opamp.

But most of your post makes no sense (for example what has DC bias got to do with anything?), and you don't really provide any details of what you're wanting to do.

For a single 9V supply something like an LM386 would probably be fine?.

I am sorry that I am a beginner in the electronic, so maybe some concern or idea were worst.
In the starter, I have a audio signal from other devices. Since I want to convert the these signal to the signal I want (+-6V with 35-45mA), so I observed the audio signal from oscilloscope.

But I found that the signal contain about 4V DC bias, which input to LM358(+9V) will cause distortion. I know that capacitor can filter out the DC bias voltage, but the signal become +-1.5V which input to the LM358 will remain the positive side only. Is there any method to create negative voltage to LM358 ?

 

[ronsimpson]

Here C1 blocks the DC from the amplifier.
C2 blocks the DC from the load.
R4,R4 makes a DC that is 1/2 of the supply for the amplifier.

 

[Cason]

Here C1 blocks the DC from the amplifier.
C2 blocks the DC from the load.
R4,R4 makes a DC that is 1/2 of the supply for the amplifier.

Thanks ronsimpson.
After watching the schematic, may I have the following predicts results?
Input Signal: 4.3V-7.3V (Vpp=3V)
After C1, the signal become: +-1.5V
By passing through the LM358, the signal will become +-1.5*(R5/R1) ?

Sorry about my Op-amp circuitry knowledge, is the 1/2 voltage(+4.5V) by R3,R4 is the DC offset added to the signal after C1 ? or will this voltage have any other meaning ?

Moreover, what is the meaning of R2 ? It seems it not in the part of inverting amplifier

Thanks

 

[Nigel Goodwin]

R2 is the bias resistor for the electret microphone (as that's what the particular circuit is for).

If you check the 'stickies' at the top of the General Electronic Chat forum there are some basic opamp circuits (including a slightly better electret mike example).

 

[audioguru]

+-6V with 40mA is into a speaker that is 6V/40mA= 150 ohms. Nobody makes a speaker that is 150 ohms, most are only 8 ohms.
An LM358 has a minimum peak output of only 5mA but maybe your LM358 is stronger and produces 10mA.
10mA into 8 ohms is a peak voltage of 0.08V and a peak power of 0.0008 Watts which is almost NOTHING.

The first post talked about the opamp as a follower with a 3Vp-p input then later a microphone was mentioned?? Very confusing.

The schematic in post #4 cannot be seen?? Very confusing.

An LM386 power amplifier with a standard gain of 20 and a volume control with an audio taper set to halfway with an input of 3V p-p will have an output of about 6V p-p which is a little more than half a watt into an 8 ohm speaker like a cheap clock radio. As the 9V battery voltage runs down then the volume control must be reduced to avoid distortion.

 

[Nigel Goodwin]

The schematic in post #4 cannot be seen?? Very confusing.

It's OK here AG, it's basically a poorer inverting version of your electret mike circuit - and not really a very workable one (lack of decoupling, too low an input impedance etc.)

 

[audioguru]

The image properties says it is not available:

 

[Nigel Goodwin]

The image properties says it is not available:

That's not what I see, what browser and version are you using?, the file was a PNG one (as is the one here).

 

[audioguru]

That's not what I see, what browser and version are you using?, the file was a PNG one (as is the one here).

I use the latest Internet Explorer 11 with Windows 7 Pro. I have no trouble seeing other attachments here or on any other website.
The link to the image does nothing when I click on it which is probably what the "X" says.

 

[JimB]

Using IE8, I do not see Rons image.

Using Google chrome, I see it no problem.

JimB

 

[audioguru]

I am attaching my same schematic as before but with the wording in Ron's post:

 

[audioguru]

It worked fine.
Here is is as a thumbnail:

 

[audioguru]

It also worked fine.

 

[Experimentonomen]

Internet explorer sucks, its only purpose is to be able to get online to download a better browser such as firefox.

 

[ronsimpson]

Should I not use .PNG files?

 

[audioguru]

Internet explorer sucks, its only purpose is to be able to get online to download a better browser such as firefox.

All the other websites work perfectly with Internet Explorer. Only this site has this problem recently with it.

 

[audioguru]

Should I not use .PNG files?

PNG works fine for me and everyone except when posted by you.

 

[Cason]

I am sorry to make all confusing. Maybe I ask the question more simply.
After testing the speaker, I find that current(AC) = 40mA, Vp-p = 7V can make the speaker become the highest volume, because the speaker is a kind of smartphone speaker.

But the max output signal of my devices is current(AC) before capcitor = 1-3mA, Vmax = 8V, Vmin = 5V, Vp-p = 3V, f = 15kHz Sine wave
The max output signal of my devices is current(AC) after 0.10 uF capcitor = 1-3mA, Vmax = +1.5V, Vmin = -1.5V, Vp-p = 3V, f = 15kHz Sine wave
These signal cannot drive the speaker in the highest volume, so I need to convert the signal to that.

Since the original has a very high DC coupling voltage, if i direct input to opamp like lm358, chopping must occurs. secondly, lm358 do not have much current ouput.

But if i series with a capacitor, the signal will have negative voltage, that I dont know how to process negative voltage with single 9V battery.

So I am confusing with the negative voltage (adding the DC component back to signal ?) and do the power amp LM386 can have my design output?

thanks