help with ear-phones

[parthshah08]

I'm trying to amplify my signal, and am giving direct input from an ear-phone, connected to my mobile. I'm giving this signal to a simple 741 op-amp amplifier circuit. When I stripped the wires of the ear phone by removing the left and right ear-plugs, I found that each of the sides had two metallic colored wires in it. Can anyone explain the significance of it. There is a copper-ish wire in each side(left & right) and metallic green and red accompanied by them resp...

 

[alec_t]

Does your mobile provide a mono output signal or a stereo one? If mono, then the two ear-phones may be wired in series.
Whether mono or stereo, if you strip the insulation from the ends of the two wires going to one of the ear-phones you should get a signal which you can amplify. Of course, for stereo amplification you would need two opamps.
BTW the 741 is not a particularly good opamp for audio amplification. Less noisy opamps are available.

 

[parthshah08]

Its stereo i guess. But each of the ear has two wires. So, which is the main one which gives the electric/audio signal? One may be ground i guess

 

[House0Fwax]

Most likely, the 'copperish' wires are the common ground and the 'metalic green and red' are left and right signal.

 

[parthshah08]

ok, thanks houseof wax.
And what should be a good op-amp if I want to amplify those signals??

 

[bountyhunter]

ok, thanks houseof wax.
And what should be a good op-amp if i want to amplify those signals??

You could try:

LF356, TLO84

 

[parthshah08]

Here it is...the circuit I made and gave input, direct music signals from my cell phone.

View attachment 61951

just changed R1 to 100 ohms, to get higher gain. On checking with multimeter I get 5mV AC input and 0.5V AC output (at pin 6)..
But still I'm not getting any output on the speaker.

 

[user_88]

Have you checked that you might have a DC voltage component on the output of your LM741 circuit.
If you take a 10 μF electrolytic capacitor and place it on the output node of you amplifier circuit, it might make a difference ... by blocking the DC part from your speaker. + side of the capacitor goes toward the higher voltage ... the LM741 output terminal.

 

[alec_t]

A 741 can supply only a few mA and has a highish impedance output. If you connect a low impedance speaker directly to its output you are in effect shorting the output, so it's not surprising that you hear nothing from the speaker. You need an amplifier, not an opamp, to drive a speaker. But the 741 could be a pre-amp for the amplifier.

 

[audioguru]

I betcha the lousy old 741 opamp did not have a plus 15V and minus 15V supply as shown in the schematic.
You measured 5mV AC input and 0.5V AC output which is a gain of only 100. But 100k/100 ohms makes a gain of 1000.
With a gain of 1000 then it cuts frequencies above 900Hz.
It can drive a 1k ohms load pretty well at low frequencies.

 

[parthshah08]

Damn!! nothing is working.. I checked on multi-meter. The input AC voltage with my cell-phone volume at full level is shaking from like 50mV to 150 mV. I need this signal to get detected by ADC0808, so I guess it needs to be pumped up till 3.5 V. I also tried with TL084 (the 10 gain circuit given in the datasheet) but got output is same as input.

 

[alec_t]

I also tried with TL084 (the 10 gain circuit given in the datasheet) but got output is same as input.

You will need a gain (probably adjustable) of at least ~ 30. What supply voltages are you using? What load are you driving with the opamp?

 

[audioguru]

Read post #9.
An opamp cannot drive a speaker. You need a little POWER AMP to drive a speaker.

 

[parthshah08]

You will need a gain (porbably adjustable) of at least ~ 30. What supply voltages are you using? What load are you driving with the opamp?

giving input from a cell phone as mentioned before. Am using supply voltage from a DC battery. Connected em in series and giving about 22Vcc. And as per me load is considered, I tried almost everything ( 100ohm, 1k, 10k) but same output.

 

[parthshah08]

Read post #9.
An opamp cannot drive a speaker. You need a little POWER AMP to drive a speaker.

As a matter of fact, I dont want to drive a speaker with the op-amp. Just want to amplify it to a level so that ADC0808 can detect it. Say want to take it to, like 3.5 V.
I've tried some circuits from google and data sheets, but not helping

 

[KJ6EAD]

If you're going to search for a design on the internet, look for audio pre-amps.

The Velleman K2572 is a stereo pre-amp kit that uses a TL072 op amp. You might have to change two resistors to set the gain but otherwise it's a ready solution.

 

[alec_t]

In the circuit you posted you have positive and negative supply rails and a ground rail. If you are using batteries in series to give 22V, is that the voltage from + to - or relative to ground? In other words, have you tapped off a ground somewhere between the two ends of the battery stack? Is you earphone signal wire connected to the ground rail or to the negative rail?

 

[audioguru]

Is you earphone signal wire connected to ....

The title of this thread is wrong.
There is no earphone load, the earphone output is the signal source.
The load is an ADC0808 analog to digital converter with an input resistance of 1k to 3k ohms.

 

[parthshah08]

If you're going to search for a design on the internet, look for audio pre-amps.

The Velleman K2572 is a stereo pre-amp kit that uses a TL072 op amp. You might have to change two resistors to set the gain but otherwise it's a ready solution.

I think I got, here something. Purchased an audio amp kit. It has a CD6283 IC. Amplifies the 100mV input mobile sound signal upto 7V.

 

[parthshah08]

In the circuit you posted you have positive and negative supply rails and a ground rail. If you are using batteries in series to give 22V, is that the voltage from + to - or relative to ground? In other words, have you tapped off a ground somewhere between the two ends of the battery stack? Is you earphone signal wire connected to the ground rail or to the negative rail?

yea, Indeed I've tapped a ground in between the connection of the batteries in series.
And don't go on the ear-phone part, as I've removed every clothing, and there remains only audio jack. Giving signal from a left channel to the input of op-amp and ground is to a common ground.