Where to put the other lead?

[Electronman]

Hello,

I have designed an amplifier based upon LF411 OP-Amp. I have used a split power supply for it. I want to connect my MP3 player to it. Can somebody direct me the correct connection for the other pin of the output of the MP3 player? One pin of the output of the MP3 player goes to the input of the op-amp, But what about the other pin of the MP3 player? Do I need to put it to the ground of my split power supply or have to put it to the negative pin of the split power supply? What about the output of the Op-Amp which maybe I connect it to a very small speaker? Can you give me your idea and tell me the real reason please? Sorry I do not know how to google it, So any link would be very helpful too.

Thanks in advance

 

[dougy83]

The MP3 player will have 2 signal outputs (left & right) and a ground return. The ground connects to you circuit reference, which in your circuit is the ground. The audio signal connects to the opamp input, and would normally be capacitively coupled (as extra protection for the mp3 player) and referenced to ground.

Same for the output of your amp (not not normally capacitively coupled).

 

[Electronman]

I want to use one leads of L-R output of MP3 player.
Are you sure that I have to connect it to the ground of my circuit and not to negative lrad???! What's the reason then please?
If I do what you told me, then is my circuit able to swings from positive to negative While I guess my MP3 player does not so??? Please somebody enlighten me about it.

 

[Nigel Goodwin]

What are you trying to do?, I fail to see any use for an opamp on the output of an MP3 player?.

 

[Electronman]

Well I have made an amplifier with 741 and then just wanted to put my MP3 player to it to see the result in the output (the amplifier will be used as an Mic Amplifier), When I wanted to do so, then I face the problem I told in this thread. Sorry but can somebody direct me or give me some Kewwords to search in Google?

Besides, dougy83 if You are right (Which I think so, but do not know the reason), then We have to crate a Voltage divider using Resistors for the input/ output references when we design a Single ended circuit??! Sorry I seem to be confused.

 

[Reloadron]

As was mentioned the MP3 Player has a Left Channel and Right Channel audio output (think stereo) with a common (GND.). Your amp is a single channel amplifirer likely using the 741 you mentioned. Therefore you can connect either the right channel or the left channel and the common from the MP3 to your amp but you can't connect both channels (stereo) to your amp (monural).

That is how I see it anyway.

Ron

 

[dougy83]

We have to crate a Voltage divider using Resistors for the input/ output references when we design a Single ended circuit??! Sorry I seem to be confused.

um, yeah but don't worry, you're making me a bit confused also. What voltage divider are you referring to? There shouldn't be any extra reference voltage required as you have the ground reference.

Split rail means that you have a positive, a negative and a ground rail. The opamp should be referenced to the ground rail and supplied power from the positive and negative rails. The input and output must also be referenced the same ground rail (i.e. connect the plug shield to the ground rail), or you will get a DC offset (which wastes power).

 

[killivolt]

**broken link removed**

I went hunting to find out if I could locate something earlier and found this?

Poor design, typical or what.

 

[audioguru]

An opamp does not have enough output power to feed a speaker. A power amplifier is used to drive a speaker.
Do you want to use this mono amplifier to drive 32 ohms headphones?
Do you want this amplifier to have enough gain so that a microphone can feed it then switch to an attenuator so that your MP3 player can feed it?

 

[Electronman]

um, yeah but don't worry, you're making me a bit confused also. What voltage divider are you referring to? There shouldn't be any extra reference voltage required as you have the ground reference.

Split rail means that you have a positive, a negative and a ground rail. The opamp should be referenced to the ground rail and supplied power from the positive and negative rails. The input and output must also be referenced the same ground rail (i.e. connect the plug shield to the ground rail), or you will get a DC offset (which wastes power).

Well Take a look at these 2 circuits which one is single ended and the other is split supplied and then you would see what I told about the inputs and voltage dividers (wich was a conclusion from your post number 2).

https://www.circuitstoday.com/wp-content/uploads/2008/03/12-w-transistor-amplifier_circuit_ckt.JPG

https://www.elecfree.com/electronic/wp-content/uploads/2009/06/op-amp-audio-amplifier.gif

And So my main question was If We must ground the said leads (one of input/output leads) when use a dual supplied amplifier, then we must do so too for a single ended amplifier (by 2 same values resistors as voltage dividers), While the designers seem to not do so?

 

[Electronman]

**broken link removed**

I went hunting to find out if I could locate something earlier and found this?

Poor design, typical or what.

Thanks for the circuit,
Why it has just one Mike and a left and a right output? If the designer wanted to design a stereo Mike amplifier then He must use 2 mikes not just one? whats the usage of the above circuit?
Besides, those two 6.8nF Caps cause me to think of an integrator op-amp, But as this is not an integrator circuit then whats the benefit of those 2 caps there?

 

[killivolt]

A guy was hacking an old Lamp and using it as a parabolic mic.

This was the circuit he came up with, I just found it interesting because I have some equipment that uses monural for all the sound. Then my mixers can split it and I can amplify it from there if I choose.

It's really just for recording though. Anyhow I saw talk about stereo and monural and thought I would offer it up to look at.

 

[dougy83]

Maybe it's late, or maybe my head has loosened a bolt or two but I'm having trouble understanding your posts.

Well Take a look at these 2 circuits which one is single ended and the other is split supplied and then you would see what I told about the inputs and voltage dividers (wich was a conclusion from your post number 2).

The second circuit uses a reference created by using a voltage divider to make the operating point fall somewhere around the middle of the rail voltage. The dotted line might as well be solid as the bottom left op amp doesn't do much - looks like they're trying to make a differential input. Also note that the output is AC coupled, so it's not as important where the return lead of the speaker goes (you could also connect it to the positive rail - just remember to reverse the output cap as it's polarised).

The 1st circuit doesn't use the resistor dividers as it already has a reference well within the supply rails (i.e. ground). As the input is DC coupled (i.e. no ac-coupling capacitor), the input return lead is connected to ground.

https://www.circuitstoday.com/wp-content/uploads/2008/03/12-w-transistor-amplifier_circuit_ckt.JPG

https://www.elecfree.com/electronic/wp-content/uploads/2009/06/op-amp-audio-amplifier.gif

That first circuit is a steaming pile. Has any poor soul actually bothered to build it?

And So my main question was If We must ground the said leads (one of input/output leads) when use a dual supplied amplifier, then we must do so too for a single ended amplifier (by 2 same values resistors as voltage dividers), While the designers seem to not do so?

"single ended" is not the same as "single supply". for the dual supply amplifier you have a low impedance reference/operating point, so you can connect the input and output with 1 leg to ground. For the single supply amplifier, the reference is generally high impedance and therefore cannot be used as a return path without substantial buffering. Therefore it's common to connect the return lead to some low impedance point in the circuit and AC-couple the signal into and out of the amp. You'll notice the 1M resistors are used to set the 'bias' or reference/operating point for the amplifiers.

Hope this answers your questions.