Hello again,

So I want to harness audio from balanced audio outputs (XLR) and amplify it to 0-5 volts.

First of all lets take a look at how balanced cables work:

From what I read a few months ago , the way balanced cables noise-resistant is that they send the signal , along with a mirror of the signal (flipped on the x axis) which is sent out on the cable and on the reciever side sits a differential OP amp configuration which basically amplifies the diffrence in the signals therefore getting a gain of 2 and rejects the common mode noise on the wires is that correct?

So if I built something like this : http://www.electronics-tutorials.ws/opamp/opamp_5.html ( first picture)

And fed the hot and cold to both inputs then I can expect a nice clean signal on the output right?

Question 2: How would I controll the reference voltage of the op amp input?
Normally when I only have one input , I bias the non-inverting input between two 10k resistors and then I know that my ouput will be in the middle of the supply voltage , but what about in this case?


Thanks in advance for your input

 

What If I biased the ground of the XLR in between two 10k resistors, that would work wouldn't it?
Also , since we are expecting to get low noise then we could use a jfet op amp or should I go for a BJT op amp
Another edit: The audio doesn't have to be very precise as I'm going to be feeding it to a Vu meter (avr adc)
The reason I'm here and not fiddling with my breadboard already is because My only source of balanced audio is my sound card and I don't want to do something stupid.

 

Another edit: The audio doesn't have to be very precise as I'm going to be feeding it to a Vu meter (avr adc)

The simplest way goes like this: Get a double rail to rail opamp, use one half in buffer configuration to get 1/2 of the reference voltage for the ADC. Use second half for the amp itself, connect the xlr signals through capacitors to V1 and V2, connect the bottom end of R4 to the output of the reference buffer.
Coupling caps should be high enough so that you do get the bass response you require, also don´t forget bypass cap on the power supply pins.

I don´t think you need to care about the noise very much, as the 10 or 12 bit ADC in the avr will most likely be the limit anyway.

 

I see... but when you say "use the one half in buffer configuration do you mean biasing the non inverting between 2 equal resistors connected to vcc and gnd to get 1/2 vcc?

What would happen if I did what I mentioned above ? Connecting the ground terminal of the XLR inbetween two 10k resistors connected to vcc and gnd?

 

If you don´t use a buffer, then the parallel combination of the two 10K resistors will add to R4, thus changing its gain, thus allowing the noise to be not ballanced out and adding it to the output.
So maybe if you change R4 to 5K it could be ok.

 

So if I do it the way you say , just to make it clear I connect the non inverting rail betwen two equal value resistors to give me 2.5 volts on the output and then i connect R4 to the output of the buffer right?

 

yes that´s right

 

Ok nice thanks , I'll try that out then.

 

How about just using some audio transformers?

You can convert balanced to unbalanced and change voltage levels- easy.

 

I have never done it like that , how many transformers would I need for both left and right audio signal?

 

Theoretically, just one per signal.

Stereo=2

 

Here's a good source for mic and line level audio transformers:
http://www.edcorusa.com/c/6/matchers

 

those look to be pretty expensive

 

those look to be pretty expensive

Or get cheap ones from digikey?

 

Personally I wouldn´t use them, as the frequency response is quite bad, especially in the low frequency range.