# bal line driver

Discussion in 'Homework Help' started by mali13, Sep 1, 2012.

1. ### mali13 Thread Starter New Member

Jul 1, 2012
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Hi guys im new here. Can someone help me understand specifically how the whole circuit works? Like what each block does? I read some sites about bal line drivers and tried simulating the circuit but I cant understand the output.

why are there two outputs?
what kind of power supply does the circuit require? AC or DC?
and how does each block function?
I really need help :s
Thanks!

Jul 1, 2012
3
0
help anyone?

3. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
968
Balanced lines in the most basic sense are simply mirrored imaged signals One is the exact 180 degree opposite of the other. Both signals are then transmitted over the twisted pair line.

The action of EM fields will induce 'noise' onto the line carrying your signal. At the receiver end of the line you have a differential amp which ignores any signals common to both lines. Therefore, the differential amp ignores all the noise that is common to both lines and you get you signal back 'noise free'

The schematic is just two op amps which are arranged to give you a two signals 180 degrees out from each other...(mirror imaged)

4. ### mali13 Thread Starter New Member

Jul 1, 2012
3
0
can you please specify how each amplifier works? like a buffer or what?
And I also tried simulating the file. I shorted the two output capacitors and connected an oscilloscope. I got an AC waveform. Why is that?

5. ### Kermit2 AAC Fanatic!

Feb 5, 2010
3,854
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Look closely and you will see that one amp(A2@top) gets an input from A1 on its inverting pin. The other amp(A2@bottom) gets the same signal into its non-inverting input.

The two signals are then mirrored at the output.

You see an AC voltage between the two lines of double the amplitude on either line alone.

When you get to the reciever the amps will keep one signal as is and invert the other again, resulting in two signal in phase with each other. The NOISE on the other hand was already equally present on both wires and most importantly, was IN PHASE before it enters the reciever. When the reciever inverts one of the signals the noise signal becomes inverted with respect to the noise on the other line. The noise cancels itself out.

6. ### #12 Expert

Nov 30, 2010
16,705
7,358
A1 on the left side is a gain of 1 buffer amplifier. A1 in the center is producing a DC voltage to center the signal in the other 3 amplifiers.

Other people have already described the A2 amplifier stages.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
One caveat: The circuit doesn't work unless the outputs are each terminated in a low resistance near 50 ohms. A differential 100 ohm termination also doesn't work.
No, I didn't analyze it. I simulated it.